## MATLAB: Vectorisation is a double-edged sword

May 22nd, 2015

Update: 2nd July 2015 The code in github has moved on a little since this post was written so I changed the link in the text below to the exact commit that produced the results discussed here.

Imagine that you are a very new MATLAB programmer and you have to create an N x N matrix called A where A(i,j) = i+j
Your first attempt at a solution might look like this

```N=2000
% Generate a N-by-N matrix where A(i,j) = i + j;
for ii = 1:N
for jj = 1:N
A(ii,jj) = ii + jj;
end
end```

On my current machine (Macbook Pro bought in early 2015), the above loop takes 2.03 seconds. You might think that this is a long time for something so simple and complain that MATLAB is slow. The person you complain to points out that you should preallocate your array before assigning to it.

```N=2000
% Generate a N-by-N matrix where A(i,j) = i + j;
A=zeros(N,N);
for ii = 1:N
for jj = 1:N
A(ii,jj) = ii + jj;
end
end```

This now takes 0.049 seconds on my machine – a speed up of over 41 times! MATLAB suddenly doesn’t seem so slow after all.

Word gets around about your problem, however, and seasoned MATLAB-ers see that nested loop, make a funny face twitch and start muttering ‘vectorise, vectorise, vectorise’. Emails soon pile in with vectorised solutions

```% Method 1: MESHGRID.
[X, Y] = meshgrid(1:N, 1:N);
A = X + Y;```

This takes 0.025 seconds on my machine — a healthy speed-up on the loop-with-preallocation solution. You have to understand the meshgrid command, however, in order to understand what’s going on here. It’s still clear (to me at least) what its doing but not as clear as the nice,obvious double loop. Call me old fashioned but I like loops…I understand them.

```% Method 2: Matrix multiplication.
A = (1:N).' * ones(1, N) + ones(N, 1) * (1:N);```

This one is MUCH harder to read but you don’t worry about it too much because at 0.032 seconds it’s slower than meshgrid.

```% Method 3: REPMAT.
A = repmat(1:N, N, 1) + repmat((1:N).', 1, N);```

This one appears to be interesting! At 0.009 seconds, it’s the fastest so far – by a healthy amount!

```% Method 4: CUMSUM.
A = cumsum(ones(N)) + cumsum(ones(N), 2);```

Coming in at 0.052 seconds, this cumsum solution is slower than the preallocated loop.

```% Method 5: BSXFUN.
A = bsxfun(@plus, 1:N, (1:N).');```

Ahhh, bsxfun or ‘The Widow-maker function’ as I sometimes refer to it. Responsible for some of the fastest and most unreadable vectorised MATLAB code I’ve ever written. In this case, it brings execution time down to 0.0045 seconds.

Whenever I see something that can be vectorised with a repmat, I try to figure out if I can rewrite it as a bsxfun. The result is usually horrible to look at so I tend to keep the original loop commented out above it as an explanation! This particular example isn’t too bad but bsxfun can quickly get hairy.

Conclusion

Loops in MATLAB aren’t anywhere near as bad as they used to be thanks to advances in JIT compilation but it can often pay, speed-wise, to switch to vectorisation. The price you often pay for this speed-up is that vectorised code can become very difficult to read.

If you’d like the code I ran to get the timings above, it’s on github (link refers to the exact commit used for this post) . Here’s the output from the run I referred to in this post.

```Original loop time is 2.025441
Preallocate and loop is 0.048643
Meshgrid time is 0.025277
Matmul time is 0.032069
Repmat time is 0.009030
Cumsum time is 0.051966
bsxfun time is 0.004527```
• MATLAB Version: 2015a
• Early 2015 Macbook Pro with 16Gb RAM
• CPU: 2.8Ghz quad core i7 Haswell

This post is based on a demonstration given by Mathwork’s Ken Deeley during a recent session at The University of Sheffield.

## MATLAB: Repeated multiplication is faster than integer powers

February 26th, 2014

Consider the following code

```function testpow()
%function to compare integer powers with repeated multiplication

rng default
a=rand(1,10000000)*10;

disp('speed of ^4 using pow')
tic;test4p = a.^4;toc
disp('speed of ^4 using multiply')
tic;test4m = a.*a.*a.*a;toc

disp('maximum difference in results');
max_diff = max(abs(test4p-test4m))

end```

On running it on my late 2013 Macbook Air with a Haswell Intel i5 using MATLAB 2013b, I get the following results

```speed of ^4 using pow
Elapsed time is 0.527485 seconds.
speed of ^4 using multiply
Elapsed time is 0.025474 seconds.
maximum difference in results
max_diff =
1.8190e-12```

In this case (derived from a real-world case), using repeated multiplication is around twenty times faster than using integer powers in MATLAB. This leads to some questions:-

• Why the huge speed difference?
• Would a similar speed difference be seen in other systems–R, Python, Julia etc?
• Would we see the same speed difference on other operating systems/CPUs?
• Are there any numerical reasons why using repeated multiplication instead of integer powers is a bad idea?

## Generate a vector of powers more quickly using cumprod in MATLAB

August 5th, 2013

While working on someone’s MATLAB code today there came a point when it was necessary to generate a vector of powers.  For example, [a a^2 a^3….a^10000] where a=0.999

```a=0.9999;
y=a.^(1:10000);```

This isn’t the only way one could form such a vector and I was curious whether or not an alternative method might be faster. On my current machine we have:

```>> tic;y=a.^(1:10000);toc
Elapsed time is 0.001526 seconds.
>> tic;y=a.^(1:10000);toc
Elapsed time is 0.001529 seconds.
>> tic;y=a.^(1:10000);toc
Elapsed time is 0.001716 seconds.```

Let’s look at the last result in the vector y

```>> y(end)
ans =
0.367861046432970```

So, 0.0015-ish seconds to beat.

```>> tic;y1=cumprod(ones(1,10000)*a);toc
Elapsed time is 0.000075 seconds.
>> tic;y1=cumprod(ones(1,10000)*a);toc
Elapsed time is 0.000075 seconds.
>> tic;y1=cumprod(ones(1,10000)*a);toc
Elapsed time is 0.000075 seconds.```

soundly beaten! More than a factor of 20 in fact. Let’s check out that last result

```>> y1(end)
ans =
0.367861046432969```

Only a difference in the 15th decimal place–I’m happy with that. What I’m wondering now, however, is will my faster method ever cause me grief?

This is only an academic exercise since this is not exactly a hot spot in the code!

## Matrix multiplication speed-up trick on MATLAB

April 23rd, 2013

I was recently working on some MATLAB code with Manchester University’s David McCormick.  Buried deep within this code was a function that was called many,many times…taking up a significant amount of overall run time.  We managed to speed up an important part of this function by almost a factor of two (on his machine) simply by inserting two brackets….a new personal record in overall application performance improvement per number of keystrokes.

The code in question is hugely complex, but the trick we used is really very simple.  Consider the following MATLAB code

```>> a=rand(4000);
>> c=12.3;
>> tic;res1=c*a*a';toc
Elapsed time is 1.472930 seconds.```

With the insertion of just two brackets, this runs quite a bit faster on my Ivy Bridge quad-core desktop.

```>> tic;res2=c*(a*a');toc
Elapsed time is 0.907086 seconds.```

So, what’s going on? Well, we think that in the first version of the code, MATLAB first calculates c*a to form a temporary matrix (let’s call it temp here) and then goes on to find temp*a’.  However, in the second version, we think that MATLAB calculates a*a’ first and in doing so it takes advantage of the fact that the result of multiplying a matrix by its transpose will be symmetric which is where we get the speedup.

Another demonstration of this phenomena can be seen as follows

```>> a=rand(4000);
>> b=rand(4000);
>> tic;a*a';toc
Elapsed time is 0.887524 seconds.
>> tic;a*b;toc
Elapsed time is 1.473208 seconds.
>> tic;b*b';toc
Elapsed time is 0.966085 seconds.```

Note that the symmetric matrix-matrix multiplications are faster than the general, non-symmetric one.

## Faster GPU Random Number Generators in MATLAB 2012b

March 10th, 2013

Ever since I took a look at GPU accelerating simple Monte Carlo Simulations using MATLAB, I’ve been disappointed with the performance of its GPU random number generator. In MATLAB 2012a, for example, it’s not much faster than the CPU implementation on my GPU hardware.  Consider the following code

```function gpuRandTest2012a(n)

mydev=gpuDevice();
disp('CPU - Mersenne Twister');
tic
CPU = rand(n);
toc

sg = parallel.gpu.RandStream('mrg32k3a','Seed',1);
parallel.gpu.RandStream.setGlobalStream(sg);
disp('GPU - mrg32k3a');
tic
Rg = parallel.gpu.GPUArray.rand(n);
wait(mydev);
toc```

Running this on MATLAB 2012a on my laptop gives me the following typical times (If you try this out yourself, the first run will always be slower for various reasons I’ll not go into here)

```>> gpuRandTest2012a(10000)
CPU - Mersenne Twister
Elapsed time is 1.330505 seconds.
GPU - mrg32k3a
Elapsed time is 1.006842 seconds.```

Running the same code on MATLAB 2012b, however, gives a very pleasant surprise with typical run times looking like this

```CPU - Mersenne Twister
Elapsed time is 1.590764 seconds.
GPU - mrg32k3a
Elapsed time is 0.185686 seconds.```

So, generation of random numbers using the GPU is now over 7 times faster than CPU generation on my laptop hardware–a significant improvment on the previous implementation.

New generators in 2012b

The MATLAB developers went a little further in 2012b though.  Not only have they significantly improved performance of the mrg32k3a combined multiple recursive generator, they have also implemented two new GPU random number generators based on the Random123 library.  Here are the timings for the generation of 100 million random numbers in MATLAB 2012b

```CPU - Mersenne Twister
Elapsed time is 1.370252 seconds.
GPU - mrg32k3a
Elapsed time is 0.186152 seconds.
GPU - Threefry4x64-20
Elapsed time is 0.145144 seconds.
GPU - Philox4x32-10
Elapsed time is 0.129030 seconds.```

Bear in mind that I am running this on the relatively weak GPU of my laptop!  If anyone runs it on something stronger, I’d love to hear of your results.

• Laptop model: Dell XPS L702X
• CPU: Intel Core i7-2630QM @2Ghz software overclockable to 2.9Ghz. 4 physical cores but total 8 virtual cores due to Hyperthreading.
• GPU: GeForce GT 555M with 144 CUDA Cores.  Graphics clock: 590Mhz.  Processor Clock:1180 Mhz. 3072 Mb DDR3 Memeory
• RAM: 8 Gb
• OS: Windows 7 Home Premium 64 bit.
• MATLAB: 2012a/2012b

## Using the Portland PGI Compiler for MATLAB mex files in Windows #1

June 14th, 2012

I recently got access to a shiny new (new to me at least) set of compilers, The Portland PGI compiler suite which comes with a great set of technologies to play with including AVX vector support, CUDA for x86 and GPU pragma-based acceleration.  So naturally, it wasn’t long before I wondered if I could use the PGI suite as compilers for MATLAB mex files.  The bad news is that The Mathworks don’t support the PGI Compilers out of the box but that leads to the good news…I get to dig down and figure out how to add support for unsupported compilers.

In what follows I made use of MATLAB 2012a on 64bit Windows 7 with Version 12.5 of the PGI Portland Compiler Suite.

In order to set up a C mex-compiler in MATLAB you execute the following

`mex -setup`

which causes MATLAB to execute a Perl script at C:\Program Files\MATLAB\R2012a\bin\mexsetup.pm.  This script scans the directory C:\Program Files\MATLAB\R2012a\bin\win64\mexopts looking for Perl scripts with the extension .stp and running whatever it finds. Each .stp file looks for a particular compiler.  After all .stp files have been executed, a list of compilers found gets returned to the user. When the user chooses a compiler, the corresponding .bat file gets copied to the directory returned by MATLAB’s prefdir function. This sets up the compiler for use.  All of this is nicely documented in the mexsetup.pm file itself.

So, I’ve had my first crack at this and the results are the following two files.

These are crude, and there’s probably lots missing/wrong but they seem to work.  Copy them to C:\Program Files\MATLAB\R2012a\bin\win64\mexopts. The location of the compiler is hard-coded in pgi.stp so you’ll need to change the following line if your compiler location differs from mine

`my \$default_location = "C:\\Program Files\\PGI\\win64\\12.5\\bin";`

Now, when you do mex -setup, you should get an entry PGI Workstation 12.5 64bit 12.5 in C:\Program Files\PGI\win64\12.5\bin which you can select as normal.

An example compilation and some details.

Let’s compile the following very simple mex file, mex_sin.c, using the PGI compiler which does little more than take an elementwise sine of the input matrix.

```#include <math.h>
#include "mex.h"

void mexFunction( int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[] )
{
double *in,*out;
double dist,a,b;
int rows,cols,outsize;
int i,j,k;

/*Get pointers to input matrix*/
in = mxGetPr(prhs[0]);
/*Get rows and columns of input*/
rows = mxGetM(prhs[0]);
cols = mxGetN(prhs[0]);

/* Create output matrix */
outsize = rows*cols;
plhs[0] = mxCreateDoubleMatrix(rows, cols, mxREAL);
/* Assign pointer to the output */
out = mxGetPr(plhs[0]);

for(i=0;i<outsize;i++){
out[i] = sin(in[i]);
}

}```

Compile using the -v switch to get verbose information about the compilation

`mex sin_mex.c -v`

You’ll see that the compiled mex file is actually a renamed .dll file that was compiled and linked with the following flags

```pgcc -c -Bdynamic  -Minfo -fast
pgcc --Mmakedll=export_all  -L"C:\Program Files\MATLAB\R2012a\extern\lib\win64\microsoft" libmx.lib libmex.lib libmat.lib```

The switch –Mmakedll=export_all is actually not supported by PGI which makes this whole setup doubly unsupported! However, I couldn’t find a way to export the required symbols without modifying the mex source code so I lived with it.  Maybe I’ll figure out a better way in the future.  Let’s try the new function out

```>> a=[1 2 3];
>> mex_sin(a)
Invalid MEX-file 'C:\Work\mex_sin.mexw64': The specified module could not be found.```

The reason for the error message is that a required PGI .dll file, pgc.dll, is not on my system path so I need to do the following in MATLAB.

`setenv('PATH', [getenv('PATH') ';C:\Program Files\PGI\win64\12.5\bin\']);`

This fixes things

```>> mex_sin(a)
ans =
0.8415    0.9093    0.1411```

Performance

I took a quick look at the performance of this mex function using my quad-core, Sandy Bridge laptop. I highly doubted that I was going to beat MATLAB’s built in sin function (which is highly optimised and multithreaded) with so little work and I was right:

```>> a=rand(1,100000000);
>> tic;mex_sin(a);toc
Elapsed time is 1.320855 seconds.
>> tic;sin(a);toc
Elapsed time is 0.486369 seconds.```

That’s not really a fair comparison though since I am purposely leaving mutithreading out of the PGI mex equation for now.  It’s a much fairer comparison to compare the exact same mex file using different compilers so let’s do that.  I created three different compiled mex routines from the source code above using the three compilers installed on my laptop and performed a very crude time test as follows

```>> a=rand(1,100000000);
>> tic;mex_sin_pgi(a);toc              %PGI 12.5 run 1
Elapsed time is 1.317122 seconds.
>> tic;mex_sin_pgi(a);toc              %PGI 12.5 run 2
Elapsed time is 1.338271 seconds.

>> tic;mex_sin_vs(a);toc               %Visual Studio 2008 run 1
Elapsed time is 1.459463 seconds.
>> tic;mex_sin_vs(a);toc
Elapsed time is 1.446947 seconds.      %Visual Studio 2008 run 2

>> tic;mex_sin_intel(a);toc             %Intel Compiler 12.0 run 1
Elapsed time is 0.907018 seconds.
>> tic;mex_sin_intel(a);toc             %Intel Compiler 12.0 run 2
Elapsed time is 0.860218 seconds.```

PGI did a little better than Visual Studio 2008 but was beaten by Intel. I’m hoping that I’ll be able to get more performance out of the PGI compiler as I learn more about the compilation flags.

Getting PGI to make use of SSE extensions

Part of the output of the mex sin_mex.c -v compilation command is the following notice

```mexFunction:
23, Loop not vectorized: data dependency```

This notice is a result of the -Minfo compilation switch and indicates that the PGI compiler can’t determine if the in and out arrays overlap or not.  If they don’t overlap then it would be safe to unroll the loop and make use of SSE or AVX instructions to make better use of my Sandy Bridge processor.  This should hopefully speed things up a little.

As the programmer, I am sure that the two arrays don’t overlap so I need to give the compiler a hand.  One way to do this would be to modify the pgi.dat file to include the compilation switch -Msafeptr which tells the compiler that arrays never overlap anywhere.  This might not be a good idea since it may not always be true so I decided to be more cautious and make use of  the restrict keyword.  That is, I changed the mex source code so that

`double *in,*out;`

becomes

`double * restrict in,* restrict out;`

Now when I compile using the PGI compiler, the notice from -Mifno becomes

```mexFunction:
23, Generated 3 alternate versions of the loop
Generated vector sse code for the loop
Generated a prefetch instruction for the loop```

which demonstrates that the compiler is much happier! So, what did this do for performance?

```>> tic;mex_sin_pgi(a);toc
Elapsed time is 1.450002 seconds.
>> tic;mex_sin_pgi(a);toc
Elapsed time is 1.460536 seconds.```

This is slower than when SSE instructions weren’t being used which isn’t what I was expecting at all! If anyone has any insight into what’s going on here, I’d love to hear from you.

Future Work

I’m happy that I’ve got this compiler working in MATLAB but there is a lot to do including:

• Tidy up the pgi.dat and pgi.stp files so that they look and act more professionally.
• Figure out the best set of compiler switches to use– it is almost certain that what I’m using now is sub-optimal since I am new to the PGI compiler.
• Get OpenMP support working.  I tried using the -Mconcur compilation flag which auto-parallelised the loop but it crashed MATLAB when I ran it. This needs investigating
• Get PGI accelerator support working so I can offload work to the GPU.
• Figure out why the SSE version of this function is slower than the non-SSE version
• Figure out how to determine whether or not the compiler is emitting AVX instructions.  The documentation suggests that if the compiler is called on a Sandy Bridge machine, and if vectorisation is possible then it will produce AVX instructions but AVX is not mentioned in the output of -Minfo.  Nothing changes if you explicity set the target to Sandy Bridge with the compiler switch tp sandybridge64.

Look out for more articles on this in the future.

Related WalkingRandomly Articles

My setup

• Laptop model: Dell XPS L702X
• CPU: Intel Core i7-2630QM @2Ghz software overclockable to 2.9Ghz. 4 physical cores but total 8 virtual cores due to Hyperthreading.
• GPU: GeForce GT 555M with 144 CUDA Cores.  Graphics clock: 590Mhz.  Processor Clock:1180 Mhz. 3072 Mb DDR3 Memeory
• RAM: 8 Gb
• OS: Windows 7 Home Premium 64 bit.
• MATLAB: 2012a
• PGI Compiler: 12.5

## Optimising a correlated asset calculation on MATLAB #3 – Using the GPU via Jacket

February 15th, 2012

This article is the third part of a series where I look at rewriting a particular piece of MATLAB code using various techniques.  The introduction to the series is here and the introduction to the larger series of GPU articles for MATLAB is here.

Last time I used The Mathwork’s Parallel Computing Toolbox in order to modify a simple correlated asset calculation to run on my laptop’s GPU rather than its CPU.  Performance was not as good as I had hoped and I never managed to get my laptop’s GPU (an NVIDIA GT555M) to beat the CPU using functions from the parallel computing toolbox.  Transferring the code to a much more powerful Tesla GPU card resulted in a four times speed-up compared to the CPU in my laptop.

This time I’ll take a look at AccelerEyes’ Jacket, a third party GPU solution for MATLAB.

Attempt 1 – Make as few modifications as possible

I started off just as I did for the parallel computing toolbox GPU port; by taking the best CPU-only code from part 1 (optimised_corr2.m) and changing a load of data-types from double to gdouble in order to get the calculation to run on my laptop’s GPU using Jacket v1.8.1 and MATLAB 2010b.  I also switched to using the GPU versions of various functions such as grandn instead of randn for example.  Functions such as cumprod needed no  modifications at all since they are nicely overloaded; if the argument to cumprod is of type double then the calculation happens on the CPU whereas if it is gdouble then it happens on the GPU.

Now, one thing that I don’t like about Jacket’s implementation is that many of the functions return single precision numbers by default.  For example, if you do

`a=grand(1,10)`

then you end up with 10 numbers of type gsingle.  To get double precision numbers you have to do

`grandn(1,10,'double')`

Now you may be thinking ‘what’s the big deal? – it’s just a bit of syntax so get over it’ and I guess that would be a fair comment.  Supporting single precision also allows users of legacy GPUs to get in on the GPU-accelerated action which is a good thing. The problem as I see it is that almost everything else in MATLAB uses double precision numbers as the default and so it’s easy to get caught out.  I would much rather see functions such as grand return double precision by default with the option to use single precision if required–just like almost every other MATLAB function out there.

The result of my ‘port’ is GPU_jacket_corr1.m

One thing to note in this version, along with all subsequent Jacket versions, is the following command that appears at the very end of the program.

`gsync;`

This is very important if you want to get meaningful timing results since it ensures that all GPU computations have finished before execution continues. See the Jacket documentation on gysnc and this blog post on timing Jacket code for more details.

The other thing I’ll mention is that, in this version, I do this:

```Corr = [1 0.4; 0.4 1];
UpperTriangle=gdouble(chol(Corr));```

```Corr = gdouble([1 0.4; 0.4 1]);
UpperTriangle=chol(Corr);```

In other words, I do the cholesky decomposition on the CPU and move the results to the GPU rather than doing the calculation on the GPU.  This is mainly because I don’t have access to a Jacket DLA license but it’s probably the best thing to do anyway since such a small decomposition probably won’t happen any faster on the GPU.

So, how does it perform.  I ran it three times with the now usual parameters of 100000 simulations done in blocks of 125 (see the CPU version for how I came to choose 125)

```>> tic;GPU_jacket_corr1;toc
Elapsed time is 40.691888 seconds.
>> tic;GPU_jacket_corr1;toc
Elapsed time is 32.096796 seconds.
>> tic;GPU_jacket_corr1;toc
Elapsed time is 32.039982 seconds.```

Just like the Parallel Computing Toolbox, the first run is slower because of GPU warmup overheads. Also, just like the PCT, performance stinks! It’s clearly not enough, in this case at least, to blindly throw in a few gdoubles and hope for the best. It is worth noting, however, that this case is nowhere near as bad as the 900+ seconds we saw in the similar parallel computing toolbox version.

Jacket has punished me for being stupid (or optimistic if you want to be polite to me) but not as much as the PCT did.

Attempt 2 – Convert from a script to a function

When working with the Parallel Computing Toolbox I demonstrated that a switch from a script to a function yielded some speed-up.  This wasn’t the case with the Jacket version of the code.  The functional version, GPU_jacket_corr2.m, showed no discernable speed improvement compared to the script.

```%Warm up run performed previously
>> tic;GPU_jacket_corr2(100000,125);toc
Elapsed time is 32.230638 seconds.
>> tic;GPU_jacket_corr2(100000,125);toc
Elapsed time is 32.179734 seconds.
>> tic;GPU_jacket_corr2(100000,125);toc
Elapsed time is 32.114864 seconds.```

Attempt 3 – One big matrix multiply!

The original version of this calculation performs thousands of very small matrix multiplications and last time we saw that switching to a single, large matrix multiplication brought significant speed improvements on the GPU.  Modifying the code to do this with Jacket is a very similar process to doing it for the PCT so I’ll omit the details and just present the code, GPU_jacket_corr3.m

```%Warm up runs performed previously
>> tic;GPU_jacket_corr3(100000,125);toc
Elapsed time is 2.041111 seconds.
>> tic;GPU_jacket_corr3(100000,125);toc
Elapsed time is 2.025450 seconds.
>> tic;GPU_jacket_corr3(100000,125);toc
Elapsed time is 2.032390 seconds.```

Now that’s more like it! Finally we have a GPU version that runs faster than the CPU on my laptop. We can do better, however, since the block size of 125 was chosen especially for my CPU. With this Jacket version bigger is better and we get much more speed-up by switching to a block size of 25000 (I run out of memory on the GPU if I try even bigger block sizes).

```%Warm up runs performed previously
>> tic;GPU_jacket_corr3(100000,25000);toc
Elapsed time is 0.749945 seconds.
>> tic;GPU_jacket_corr3(100000,25000);toc
Elapsed time is 0.749333 seconds.
>> tic;GPU_jacket_corr3(100000,25000);toc
Elapsed time is 0.749556 seconds.```

Now this is exciting! My laptop GPU with Jacket 1.8.1 is faster than a high-end Tesla card using the parallel computing toolbox for this calculation. My excitement was short lived, however, when I looked at the resulting distribution. The random number generator in Jacket 1.8.1 gave a completely different distribution when compared to generators from other sources (I tried two CPU generators from The Mathworks and one from The Numerical Algorithms Group).  The only difference in the code that generated the results below is the random number generator used.

• The results shown in these plots were for only 20,000 simulations rather than the 100,000 I’ve been using elsewhere in this post. I found this bug in the development stage of these posts where I was using a smaller number of simulations.
• Jacket 1.8.1 is using Jackets old grandn function with the ‘double’ option set
• MATLAB #1 is using MATLAB’s randn using the Comb Recursive algorithm on the CPU
• MATLAB #2 is using MATLAB’s randn using the default Mersenne Twister on the CPU
• NAG is using a Wichman-Hill generator

I sent my code to AccelerEye’s customer support who confirmed that this seemed to be a bug in their random number generator (an in-house Mersenne Twister implementation).  Less than a week later they offered me a new preview of Jacket from their Nightly builds where the old RNG had been replaced with the Mersenne Twister implementation produced by NVidia and I’m happy to confirm that not only does this fix the results for my code but it goes even faster!  Superb customer service!

This new RNG is now the default in version 2.0 of Jacket.  Here’s the distribution I get for 20,000 simulations (to stay in line with the plots shown above).

Switching back to 100,000 simulations to stay in line with all of the other benchmarks in this series gives the following times on my laptop’s GPU

```%Warm up runs performed previously
>> tic;prices=GPU_jacket_corr3(100000,25000);toc
Elapsed time is 0.696891 seconds.
>> tic;prices=GPU_jacket_corr3(100000,25000);toc
Elapsed time is 0.697596 seconds.
>> tic;prices=GPU_jacket_corr3(100000,25000);toc
Elapsed time is 0.697312 seconds.```

This is almost 5 times faster than the 3.42 seconds managed by the best CPU version from part 1. I sent my code to AccelerEyes and asked them to run it on a more powerful GPU, a Tesla C2075, and these are the results they sent back

```Elapsed time is 5.246249 seconds. %warm up run
Elapsed time is 0.158165 seconds.
Elapsed time is 0.156529 seconds.
Elapsed time is 0.156522 seconds.
Elapsed time is 0.156501 seconds.```

So, the Tesla is 4.45 times faster than my laptop’s GPU for this application and a very useful 21.85 times faster than my laptop’s CPU.

Results Summary

• Best CPU Result on laptop (i7-2630GM)with pure MATLAB code – 3.42 seconds
• Best GPU Result with PCT on laptop (GT555M) – 4.04 seconds
• Best GPU Result with PCT on Tesla M2070 – 0.85 seconds
• Best GPU Result with Jacket on laptop (GT555M) – 0.7 seconds
• Best GPU Result with Jacket on Tesla M2075 – 0.16 seconds

Test System Specification

• Laptop model: Dell XPS L702X
• CPU:Intel Core i7-2630QM @2Ghz software overclockable to 2.9Ghz. 4 physical cores but total 8 virtual cores due to Hyperthreading.
• GPU:GeForce GT 555M with 144 CUDA Cores.  Graphics clock: 590Mhz.  Processor Clock:1180 Mhz. 3072 Mb DDR3 Memeory
• RAM: 8 Gb
• OS: Windows 7 Home Premium 64 bit.
• MATLAB: 2011b

Acknowledgements

Thanks to Yong Woong Lee of the Manchester Business School as well as various employees at AccelerEyes for useful discussions and advice.  Any mistakes that remain are all my own.

## Optimising a correlated asset calculation on MATLAB #2 – Using the GPU via the PCT

February 9th, 2012

This article is the second part of a series where I look at rewriting a particular piece of MATLAB code using various techniques.  The introduction to the series is here and the introduction to the larger series of GPU articles for MATLAB on WalkingRandomly is here.

Attempt 1 – Make as few modifications as possible

I took my best CPU-only code from last time (optimised_corr2.m) and changed a load of data-types from double to gpuArray in order to get the calculation to run on my laptop’s GPU using the parallel computing toolbox in MATLAB 2010b.  I also switched to using the GPU versions of various functions such as parallel.gpu.GPUArray.randn instead of randn for example.  Functions such as cumprod needed no  modifications at all since they are nicely overloaded; if the argument to cumprod is of type double then the calculation happens on the CPU whereas if it is gpuArray then it happens on the GPU.

The above work took about a minute to do which isn’t bad for a CUDA ‘porting’ effort!  The result, which I’ve called GPU_PCT_corr1.m is available for you to download and try out.

How about performance?  Let’s do a quick tic and toc using my laptop’s NVIDIA GT 555M GPU.

```>> tic;GPU_PCT_corr1;toc
Elapsed time is 950.573743 seconds.```

The CPU version of this code took only 3.42 seconds which means that this GPU version is over 277 times slower! Something has gone horribly, horribly wrong!

Attempt 2 – Switch from script to function

In general functions should be faster than scripts in MATLAB because more automatic optimisations are performed on functions.  I didn’t see any difference in the CPU version of this code (see optimised_corr3.m from part 1 for a CPU function version) and so left it as a script (partly so I had an excuse to discuss it here if I am honest).  This GPU-version, however, benefits noticeably from conversion to a function.  To do this, add the following line to the top of GPU_PCT_corr1.m

`function [SimulPrices] = GPU_PTC_corr2( n,sub_size)`

Next, you need to delete the following two lines

```n=100000;                       %Number of simulations
sub_size = 125;```

Finally, add the following to the end of our new function

`end`

That’s pretty much all I did to get GPU_PCT_corr2.m. Let’s see how that performs using the same parameters as our script (100,000 simulations in blocks of 125).  I used script_vs_func.m to run both twice after a quick warm-up iteration and the results were:

```Warm up
Elapsed time is 1.195806 seconds.
Main event
script
Elapsed time is 950.399920 seconds.
function
Elapsed time is 938.238956 seconds.
script
Elapsed time is 959.420186 seconds.
function
Elapsed time is 939.716443 seconds.```

So, switching to a function has saved us a few seconds but performance is still very bad!

Attempt 3 – One big matrix multiply!

So far all I have done is take a program that works OK on a CPU, and run it exactly as-is on the GPU in the hope that something magical would happen to make it go faster.  Of course, GPUs and CPUs are very different beasts with differing sets of strengths and weaknesses so it is rather naive to think that this might actually work.  What we need to do is to play to the GPUs strengths more and the way to do this is to focus on this piece of code.

```for i=1:sub_size
CorrWiener(:,:,i)=parallel.gpu.GPUArray.randn(T-1,2)*UpperTriangle;
end```

Here, we are performing lots of small matrix multiplications and, as mentioned in part 1, we might hope to get better performance by performing just one large matrix multiplication instead. To do this we can change the above code to

```%Generate correlated random numbers
%using one big multiplication
randoms = parallel.gpu.GPUArray.randn(sub_size*(T-1),2);
CorrWiener = randoms*UpperTriangle;
CorrWiener = reshape(CorrWiener,(T-1),sub_size,2);
%CorrWiener = permute(CorrWiener,[1 3 2]); %Can't do this on the GPU in 2011b or below

%poor man's permute since GPU permute if not available in 2011b
CorrWiener_final = parallel.gpu.GPUArray.zeros(T-1,2,sub_size);
for s = 1:2
CorrWiener_final(:, s, :) = CorrWiener(:, :, s);
end```

The reshape and permute are necessary to get the matrix in the form needed later on. Sadly, MATLAB 2011b doesn’t support permute on GPUArrays and so I had to use the ‘poor mans permute’ instead.

The result of the above is contained in GPU_PCT_corr3.m so let’s see how that does in a fresh instance of MATLAB.

```>> tic;GPU_PCT_corr3(100000,125);toc
Elapsed time is 16.666352 seconds.
>> tic;GPU_PCT_corr3(100000,125);toc
Elapsed time is 8.725997 seconds.
>> tic;GPU_PCT_corr3(100000,125);toc
Elapsed time is 8.778124 seconds.```

The first thing to note is that performance is MUCH better so we appear to be on the right track. The next thing to note is that the first evaluation is much slower than all subsequent ones. This is totally expected and is due to various start-up overheads.

Recall that 125 in the above function calls refers to the block size of our monte-carlo simulation. We are doing 100,000 simulations in blocks of 125– a number chosen because I determined empirically that this was the best choice on my CPU. It turns out we are better off using much larger block sizes on the GPU:

```>> tic;GPU_PCT_corr3(100000,250);toc
Elapsed time is 6.052939 seconds.
>> tic;GPU_PCT_corr3(100000,500);toc
Elapsed time is 4.916741 seconds.
>> tic;GPU_PCT_corr3(100000,1000);toc
Elapsed time is 4.404133 seconds.
>> tic;GPU_PCT_corr3(100000,2000);toc
Elapsed time is 4.223403 seconds.
>> tic;GPU_PCT_corr3(100000,5000);toc
Elapsed time is 4.069734 seconds.
>> tic;GPU_PCT_corr3(100000,10000);toc
Elapsed time is 4.039446 seconds.
>> tic;GPU_PCT_corr3(100000,20000);toc
Elapsed time is 4.068248 seconds.
>> tic;GPU_PCT_corr3(100000,25000);toc
Elapsed time is 4.099588 seconds.```

The above, rather crude, test suggests that block sizes of 10,000 are the best choice on my laptop’s GPU. Sadly, however, it’s STILL slower than the 3.42 seconds I managed on the i7 CPU and represents the best I’ve managed using pure MATLAB code. The profiler tells me that the vast majority of the GPU execution time is spent in the cumprod line and in random number generation (over 40% each).

Trying a better GPU
Of course now that I have code that runs on a GPU I could just throw it at a better GPU and see how that does. I have access to MATLAB 2011b on a Tesla M2070 hooked up to a Linux machine so I ran the code on that. I tried various block sizes and the best time was 0.8489 seconds with the call GPU_PCT_corr3(100000,20000) which is just over 4 times faster than my laptop’s CPU.

Can you do better using just the GPU functionality provided in the Parallel Computing Toolbox (so no bespoke CUDA kernels or Jacket just yet)? I’ll be looking at how AccelerEyes’ Jacket myself in the next post.

Results so far

• Best CPU Result on laptop (i7-2630GM)with pure MATLAB code – 3.42 seconds
• Best GPU Result with PCT on laptop (GT555M) – 4.04 seconds
• Best GPU Result with PCT on Tesla M2070 – 0.85 seconds

Test System Specification

• Laptop model: Dell XPS L702X
• CPU: Intel Core i7-2630QM @2Ghz software overclockable to 2.9Ghz. 4 physical cores but total 8 virtual cores due to Hyperthreading.
• GPU: GeForce GT 555M with 144 CUDA Cores.  Graphics clock: 590Mhz.  Processor Clock:1180 Mhz. 3072 Mb DDR3 Memeory
• RAM: 8 Gb
• OS: Windows 7 Home Premium 64 bit.
• MATLAB: 2011b

Acknowledgements

Thanks to Yong Woong Lee of the Manchester Business School as well as various employees at The Mathworks for useful discussions and advice.  Any mistakes that remain are all my own :)

## Optimising a correlated asset calculation on MATLAB #1 – Vectorisation on the CPU

February 6th, 2012

Recently, I’ve been working with members of The Manchester Business School to help optimise their MATLAB code. We’ve had some great successes using techniques such as vectorisation, mex files and The NAG Toolbox for MATLAB (among others) combined with the raw computational power of Manchester’s Condor Pool (which I help run along with providing applications support).

A couple of months ago, I had the idea of taking a standard calculation in computational finance and seeing how fast I could make it run on MATLAB using various techniques. I’d then write these up and publish here for comment.

So, I asked one of my collaborators, Yong Woong Lee, a doctoral researcher in the Manchester Business School, if he could furnish me with a very simple piece computational finance code. I asked for something that was written to make it easy to see the underlying mathematics rather than for speed and he duly obliged with several great examples. I took one of these examples and stripped it down even further to its very bare bones.  In doing so I may have made his example close to useless from a computational finance point of view but it gave me something nice and simple to play with.

What I ended up with was a simple piece of code that uses monte carlo techniques to find the distribution of two correlated assets: original_corr.m

```%ORIGINAL_CORR - The original, unoptimised code that simulates two correlated assets

%% Correlated asset information
CurrentPrice = [78 102];       %Initial Prices of the two stocks
Corr = [1 0.4; 0.4 1];         %Correlation Matrix
T = 500;                       %Number of days to simulate = 2years = 500days
n = 100000;                    %Number of simulations
dt = 1/250;                    %Time step (1year = 250days)
Div=[0.01 0.01];               %Dividend
Vol=[0.2 0.3];                 %Volatility

%%Market Information
r = 0.03;                      %Risk-free rate

%% Define storages
SimulPriceA=zeros(T,n);    %Simulated Price of Asset A
SimulPriceA(1,:)=CurrentPrice(1);
SimulPriceB=zeros(T,n);    %Simulated Price of Asset B
SimulPriceB(1,:)=CurrentPrice(2);

%% Generating the paths of stock prices by Geometric Brownian Motion
UpperTriangle=chol(Corr);    %UpperTriangle Matrix by Cholesky decomposition

for i=1:n
Wiener=randn(T-1,2);
CorrWiener=Wiener*UpperTriangle;
for j=2:T
SimulPriceA(j,i)=SimulPriceA(j-1,i)*exp((r-Div(1)-Vol(1)^2/2)*dt+Vol(1)*sqrt(dt)*CorrWiener(j-1,1));
SimulPriceB(j,i)=SimulPriceB(j-1,i)*exp((r-Div(2)-Vol(2)^2/2)*dt+Vol(2)*sqrt(dt)*CorrWiener(j-1,2));
end
end

%% Plot the distribution of final prices
% Comment this section out if doing timings
% subplot(1,2,1);hist(SimulPriceA(end,:),100);
% subplot(1,2,2);hist(SimulPriceB(end,:),100);```

On my laptop, this code takes 10.82 seconds to run on average.  If you comment out the final two lines then it’ll take a little longer and will produce a histogram of the distribution of final prices.

I’m going to take this code and modify it in various ways to see how different techniques and technologies can make it run more quickly. Here is a list of everything I have done so far.

Vectorisation – removing loops to make code go faster

Now, the most obvious optimisation that we can do with code like this is to use vectorisation to get rid of that double loop. The cumprod command is the key to doing this and the resulting code looks as follows: optimised_corr1.m

```%OPTIMISED_CORR1 - A pure-MATLAB optimised code that simulates two correlated assets

%% Correlated asset information
CurrentPrice = [78 102];       %Initial Prices of the two stocks
Corr = [1 0.4; 0.4 1];         %Correlation Matrix
T = 500;                       %Number of days to simulate = 2years = 500days
Div=[0.01 0.01];               %Dividend
Vol=[0.2 0.3];                 %Volatility

%% Market Information
r = 0.03;                      %Risk-free rate

%% Simulation parameters
n=100000;                       %Number of simulation
dt=1/250;                       %Time step (1year = 250days)

%% Define storages
SimulPrices=repmat(CurrentPrice,n,1);
CorrWiener = zeros(T-1,2,n);

%% Generating the paths of stock prices by Geometric Brownian Motion
UpperTriangle=chol(Corr);    %UpperTriangle Matrix by Cholesky decomposition

for i=1:n
CorrWiener(:,:,i)=randn(T-1,2)*UpperTriangle;
end
Volr = repmat(Vol,[T-1,1,n]);
Divr = repmat(Div,[T-1,1,n]);

%% do simulation
sim = cumprod(exp((r-Divr-Volr.^2./2).*dt+Volr.*sqrt(dt).*CorrWiener));
%get just the final prices
SimulPrices = SimulPrices.*reshape(sim(end,:,:),2,n)';

%% Plot the distribution of final prices
% Comment this section out if doing timings
%subplot(1,2,1);hist(SimulPrices(:,1),100);
%subplot(1,2,2);hist(SimulPrices(:,2),100);```

This code takes an average of 4.19 seconds to run on my laptop giving us a factor of 2.58 times speed up over the original.  This improvement in speed is not without its cost, however, and the price we have to pay is memory.  Let’s take a look at the amount of memory used by MATLAB after running these two versions. First, the original

```>> clear all
>> memory
Maximum possible array:              13344 MB (1.399e+010 bytes) *
Memory available for all arrays:     13344 MB (1.399e+010 bytes) *
Memory used by MATLAB:                 553 MB (5.800e+008 bytes)
Physical Memory (RAM):                8106 MB (8.500e+009 bytes)

*  Limited by System Memory (physical + swap file) available.
>> original_corr
>> memory
Maximum possible array:              12579 MB (1.319e+010 bytes) *
Memory available for all arrays:     12579 MB (1.319e+010 bytes) *
Memory used by MATLAB:                1315 MB (1.379e+009 bytes)
Physical Memory (RAM):                8106 MB (8.500e+009 bytes)

*  Limited by System Memory (physical + swap file) available.```

Now for the vectorised version

```>> %now I clear the workspace and check that all memory has been recovered%
>> clear all
>> memory
Maximum possible array:              13343 MB (1.399e+010 bytes) *
Memory available for all arrays:     13343 MB (1.399e+010 bytes) *
Memory used by MATLAB:                 555 MB (5.818e+008 bytes)
Physical Memory (RAM):                8106 MB (8.500e+009 bytes)

*  Limited by System Memory (physical + swap file) available.
>> optimised_corr1
>> memory
Maximum possible array:              10297 MB (1.080e+010 bytes) *
Memory available for all arrays:     10297 MB (1.080e+010 bytes) *
Memory used by MATLAB:                3596 MB (3.770e+009 bytes)
Physical Memory (RAM):                8106 MB (8.500e+009 bytes)

*  Limited by System Memory (physical + swap file) available.```

So, the original version used around 762Mb of RAM whereas the vectorised version used 3041Mb. If you don’t have enough memory then you may find that the vectorised version runs very slowly indeed!

Adding a loop to the vectorised code to make it go even faster!

Simple vectorisation improvements such as the one above are sometimes so effective that it can lead MATLAB programmers to have a pathological fear of loops. This fear is becoming increasingly unjustified thanks to the steady improvements in MATLAB’s Just In Time (JIT) compiler.  Discussing the details of MATLAB’s JIT is beyond the scope of these articles but the practical upshot is that you don’t need to be as frightened of loops as you used to.

In fact, it turns out that once you have finished vectorising your code, you may be able to make it go even faster by putting a loop back in (not necessarily thanks to the JIT though)! The following code takes an average of 3.42 seconds on my laptop bringing the total speed-up to a factor of 3.16 compared to the original.  The only difference is that I have added a loop over the variable ii to split up the cumprod calculation over groups of 125 at a time.

I have a confession: I have no real idea why this modification causes the code to go noticeably faster.  I do know that it uses a lot less memory; using the memory command, as I did above, I determined that it uses around 10Mb compared to 3041Mb of the original vectorised version.  You may be wondering why I set sub_size to be 125 since I could have chosen any divisor of 100000.  Well, I tried them all and it turned out that 125 was slightly faster than any other on my machine.  Maybe we are seeing some sort of CPU cache effect?  I just don’t know: optimised_corr2.m

```%OPTIMISED_CORR2 - A pure-MATLAB optimised code that simulates two correlated assets
%% Correlated asset information
CurrentPrice = [78 102];       %Initial Prices of the two stocks
Corr = [1 0.4; 0.4 1];         %Correlation Matrix
T = 500;                       %Number of days to simulate = 2years = 500days
Div=[0.01 0.01];               %Dividend
Vol=[0.2 0.3];                 %Volatility

%% Market Information
r = 0.03;                      %Risk-free rate

%% Simulation parameters
n=100000;                       %Number of simulation
sub_size = 125;
dt=1/250;                       %Time step (1year = 250days)

%% Define storages
SimulPrices=repmat(CurrentPrice,n,1);
CorrWiener = zeros(T-1,2,sub_size);

%% Generating the paths of stock prices by Geometric Brownian Motion
UpperTriangle=chol(Corr);    %UpperTriangle Matrix by Cholesky decomposition
Volr = repmat(Vol,[T-1,1,sub_size]);
Divr = repmat(Div,[T-1,1,sub_size]);

for ii = 1:sub_size:n
for i=1:sub_size
CorrWiener(:,:,i)=randn(T-1,2)*UpperTriangle;
end

%% do simulation
sim = cumprod(exp((r-Divr-Volr.^2./2).*dt+Volr.*sqrt(dt).*CorrWiener));
%get just the final prices
SimulPrices(ii:ii+sub_size-1,:) = SimulPrices(ii:ii+sub_size-1,:).*reshape(sim(end,:,:),2,sub_size)';
end
%% Plot the distribution of final prices
% Comment this section out if doing timings
%subplot(1,2,1);hist(SimulPrices(:,1),100);
%subplot(1,2,2);hist(SimulPrices(:,2),100);```

Some things that might have worked (but didn’t)

1. In general, functions are faster than scripts in MATLAB because MATLAB employs more aggressive optimisation techniques for functions.  In this case, however, it didn’t make any noticeable difference on my machine.  Try it out for yourself with optimised_corr3.m
2. When generating the correlated random numbers, the above code performs lots of small matrix multiplications:
```   for i=1:sub_size
CorrWiener(:,:,i)=randn(T-1,2)*UpperTriangle;
end```

It is often the case that you can get more flops out of a system by doing a single large matrix-matrix multiply than lots of little ones. So, we could do this instead:

```%Generate correlated random numbers
%using one big multiplication
randoms = randn(sub_size*(T-1),2);
CorrWiener = randoms*UpperTriangle;
CorrWiener = reshape(CorrWiener,(T-1),sub_size,2);
CorrWiener = permute(CorrWiener,[1 3 2]);```

Sadly, however, any gains that we might have made by doing a single matrix-matrix multiply are lost when the resulting matrix is reshaped and permuted into the form needed for the rest of the code (on my machine at least).  Feel free to try for yourself using optimised_corr4.m – the input argument of which determines the sub_size.

Can you do better using nothing but pure matlab (i.e. no mex, parallel computing toolbox, GPUs or other such trickery…they are for later articles)?  If so then feel free to contact me and let me know.

Acknowledgements

Thanks to Yong Woong Lee of the Manchester Business School as well as various employees at The Mathworks for useful discussions and advice.  Any mistakes that remain are all my own :)

The test laptop

• Laptop model: Dell XPS L702X
• CPU: Intel Core i7-2630QM @2Ghz software overclockable to 2.9Ghz. 4 physical cores but total 8 virtual cores due to Hyperthreading.
• GPU: GeForce GT 555M with 144 CUDA Cores.  Graphics clock: 590Mhz.  Processor Clock:1180 Mhz. 3072 Mb DDR3 Memeory
• RAM: 8 Gb
• OS: Windows 7 Home Premium 64 bit.
• MATLAB: 2011b

Next Time

In the second part of this series I look at how to run this code on the GPU using The Mathworks’ Parallel Computing Toolbox.

## Using Intel’s SPMD Compiler (ispc) with MATLAB on Linux

December 28th, 2011

Modern CPUs are capable of parallel processing at multiple levels with the most obvious being the fact that a typical CPU contains multiple processor cores.  My laptop, for example, contains a quad-core Intel Sandy Bridge i7 processor and so has 4 processor cores. You may be forgiven for thinking that, with 4 cores, my laptop can do up to 4 things simultaneously but life isn’t quite that simple.

The first complication is hyper-threading where each physical core appears to the operating system as two or more virtual cores.  For example, the processor in my laptop is capable of using hyper-threading and so I have access to up to 8 virtual cores!  I have heard stories where unscrupulous sales people have passed off a 4 core CPU with hyperthreading as being as good as an 8 core CPU…. after all, if you fire up the Windows Task Manager you can see 8 cores and so there you have it!  However, this is very far from the truth since what you really have is 4 real cores with 4 brain damaged cousins.  Sometimes the brain damaged cousins can do something useful but they are no substitute for physical cores.  There is a great explanation of this technology at makeuseof.com.

The second complication is the fact that each physical processor core contains a SIMD (Single Instruction Multiple Data) lane of a certain width. SIMD lanes, aka  SIMD units or vector units, can process several numbers simultaneously with a single instruction rather than only one a time.  The 256-bit wide SIMD lanes on my laptop’s processor, for example, can operate on up to 8 single (or 4 double) precision numbers per instruction.  Since each physical core has its own SIMD lane this means that a 4 core processor could theoretically operate on up to 32 single precision (or 16 double precision) numbers per clock cycle!

So, all we need now is a way of programming for these SIMD lanes!

Intel’s SPMD Program Compiler, ispc, is a free product that allows programmers to take direct advantage of the SIMD lanes in modern CPUS using a C-like syntax.  The speed-ups compared to single-threaded code can be impressive with Intel reporting up to 32 times speed-up (on an i7 quad-core) for a single precision Black-Scholes option pricing routine for example.

Using ispc on MATLAB

Since ispc routines are callable from C, it stands to reason that we’ll be able to call them from MATLAB using mex.  To demonstrate this, I thought that I’d write a sqrt function that works faster than MATLAB’s built-in version.  This is a tall order since the sqrt function is pretty fast and is already multi-threaded.  Taking the square root of 200 million random numbers doesn’t take very long in MATLAB:

```>> x=rand(1,200000000)*10;
>> tic;y=sqrt(x);toc
Elapsed time is 0.666847 seconds.```

This might not be the most useful example in the world but I wanted to focus on how to get ispc to work from within MATLAB rather than worrying about the details of a more interesting example.

Step 1 – A reference single-threaded mex file

Before getting all fancy, let’s write a nice, straightforward single-threaded mex file in C and see how fast that goes.

```#include <math.h>
#include "mex.h"

void mexFunction( int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[] )
{
double *in,*out;
int rows,cols,num_elements,i;

/*Get pointers to input matrix*/
in = mxGetPr(prhs[0]);
/*Get rows and columns of input matrix*/
rows = mxGetM(prhs[0]);
cols = mxGetN(prhs[0]);
num_elements = rows*cols;

/* Create output matrix */
plhs[0] = mxCreateDoubleMatrix(rows, cols, mxREAL);
/* Assign pointer to the output */
out = mxGetPr(plhs[0]);

for(i=0; i<num_elements; i++)
{
out[i] = sqrt(in[i]);
}
}```

Save the above to a text file called sqrt_mex.c and compile using the following command in MATLAB

` mex sqrt_mex.c`

Let’s check out its speed:

```>> x=rand(1,200000000)*10;
>> tic;y=sqrt_mex(x);toc
Elapsed time is 1.993684 seconds.```

Well, it works but it’s quite a but slower than the built-in MATLAB function so we still have some work to do.

Step 2 – Using the SIMD lane on one core via ispc

Using ispc is a two step process.  First of all you need the .ispc program

```export void ispc_sqrt(uniform double vin[], uniform double vout[],
uniform int count) {
foreach (index = 0 ... count) {
vout[index] = sqrt(vin[index]);
}
}```

Save this to a file called ispc_sqrt.ispc and compile it at the Bash prompt using

`ispc -O2 ispc_sqrt.ispc -o ispc_sqrt.o -h ispc_sqrt.h --pic`

This creates an object file, ispc_sqrt.o, and a header file, ispc_sqrt.h. Now create the mex file in MATLAB

```#include <math.h>
#include "mex.h"
#include "ispc_sqrt.h"

void mexFunction( int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[] )
{
double *in,*out;
int rows,cols,num_elements,i;

/*Get pointers to input matrix*/
in = mxGetPr(prhs[0]);
/*Get rows and columns of input matrix*/
rows = mxGetM(prhs[0]);
cols = mxGetN(prhs[0]);
num_elements = rows*cols;

/* Create output matrix */
plhs[0] = mxCreateDoubleMatrix(rows, cols, mxREAL);
/* Assign pointer to the output */
out = mxGetPr(plhs[0]);

ispc::ispc_sqrt(in,out,num_elements);
}```

Call this ispc_sqrt_mex.cpp and compile in MATLAB with the command

` mex ispc_sqrt_mex.cpp ispc_sqrt.o`

Let’s see how that does for speed:

```>> tic;y=ispc_sqrt_mex(x);toc
Elapsed time is 1.379214 seconds.```

So, we’ve improved on the single-threaded mex file a bit (1.37 instead of 2 seconds) but it’s still not enough to beat the MATLAB built-in.  To do that, we are going to have to use the SIMD lanes on all 4 cores simultaneously.

Step 3 – A reference multi-threaded mex file using OpenMP

Let’s step away from ispc for a while and see how we do with something we’ve seen before– a mex file using OpenMP (see here and here for previous articles on this topic).

```#include <math.h>
#include "mex.h"
#include <omp.h>

void do_calculation(double in[],double out[],int num_elements)
{
int i;

#pragma omp parallel for shared(in,out,num_elements)
for(i=0; i<num_elements; i++){
out[i] = sqrt(in[i]);
}
}

void mexFunction( int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[] )
{
double *in,*out;
int rows,cols,num_elements,i;

/*Get pointers to input matrix*/
in = mxGetPr(prhs[0]);
/*Get rows and columns of input matrix*/
rows = mxGetM(prhs[0]);
cols = mxGetN(prhs[0]);
num_elements = rows*cols;

/* Create output matrix */
plhs[0] = mxCreateDoubleMatrix(rows, cols, mxREAL);
/* Assign pointer to the output */
out = mxGetPr(plhs[0]);

do_calculation(in,out,num_elements);
}```

Save this to a text file called openmp_sqrt_mex.c and compile in MATLAB by doing

` mex openmp_sqrt_mex.c CFLAGS="\\$CFLAGS -fopenmp" LDFLAGS="\\$LDFLAGS -fopenmp"`

Let’s see how that does (OMP_NUM_THREADS has been set to 4):

```>> tic;y=openmp_sqrt_mex(x);toc
Elapsed time is 0.641203 seconds.```

That’s very similar to the MATLAB built-in and I suspect that The Mathworks have implemented their sqrt function in a very similar manner. Theirs will have error checking, complex number handling and what-not but it probably comes down to a for-loop that’s been parallelized using Open-MP.

Step 4 – Using the SIMD lanes on all cores via ispc

To get a ispc program to run on all of my processors cores simultaneously, I need to break the calculation down into a series of tasks. The .ispc file is as follows

```task void
ispc_sqrt_block(uniform double vin[], uniform double vout[],
uniform int block_size,uniform int num_elems){
uniform int index_start = taskIndex * block_size;
uniform int index_end = min((taskIndex+1) * block_size, (unsigned int)num_elems);

foreach (yi = index_start ... index_end) {
vout[yi] = sqrt(vin[yi]);
}
}

export void
ispc_sqrt_task(uniform double vin[], uniform double vout[],
uniform int block_size,uniform int num_elems,uniform int num_tasks)
{

launch[num_tasks] < ispc_sqrt_block(vin, vout, block_size, num_elems) >;
}
```

Compile this by doing the following at the Bash prompt

`ispc -O2 ispc_sqrt_task.ispc -o ispc_sqrt_task.o -h ispc_sqrt_task.h --pic`

We’ll need to make use of a task scheduling system. The ispc documentation suggests that you could use the scheduler in Intel’s Threading Building Blocks or Microsoft’s Concurrency Runtime but a basic scheduler is provided with ispc in the form of tasksys.cpp (I’ve also included it in the .tar.gz file in the downloads section at the end of this post), We’ll need to compile this too so do the following at the Bash prompt

`g++ tasksys.cpp -O3 -Wall -m64 -c -o tasksys.o -fPIC`

Finally, we write the mex file

```#include <math.h>
#include "mex.h"

void mexFunction( int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[] )
{
double *in,*out;
int rows,cols,i;

unsigned int num_elements;
unsigned int block_size;

/*Get pointers to input matrix*/
in = mxGetPr(prhs[0]);
/*Get rows and columns of input matrix*/
rows = mxGetM(prhs[0]);
cols = mxGetN(prhs[0]);
num_elements = rows*cols;

/* Create output matrix */
plhs[0] = mxCreateDoubleMatrix(rows, cols, mxREAL);
/* Assign pointer to the output */
out = mxGetPr(plhs[0]);

block_size = 1000000;

}```

In the above, the input array is divided into tasks where each task takes care of 1 million elements. Our 200 million element test array will, therefore, be split into 200 tasks– many more than I have processor cores. I’ll let the task scheduler worry about how to schedule these tasks efficiently across the cores in my machine. Compile this in MATLAB by doing

`mex ispc_sqrt_task_mex.cpp ispc_sqrt_task.o tasksys.o`

Now for crunch time:

```>> x=rand(1,200000000)*10;
>> tic;ys=sqrt(x);toc   %MATLAB's built-in
Elapsed time is 0.670766 seconds.
>> tic;y=ispc_sqrt_task_mex(x);toc  %my version using ispc
Elapsed time is 0.393870 seconds.```

There we have it! A version of the sqrt function that works faster than MATLAB’s own by virtue of the fact that I am now making full use of the SIMD lanes in my laptop’s Sandy Bridge i7 processor thanks to ispc.

Although this example isn’t very useful as it stands, I hope that it shows that using the ispc compiler from within MATLAB isn’t as hard as you might think and is yet another tool in the arsenal of weaponry that can be used to make MATLAB faster.

• Single threaded with ispc: 1.37 seconds
• MATLAB built-in: 0.67 seconds