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gdgc
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Why GPU computation does not scale linearly with the number of loops ?

Asked by gdgc
on 19 Dec 2017
Latest activity Commented on by Joss Knight
on 20 Dec 2017
Good evening everyone,
I'm currently trying to speed up my code by converting it to GPU computation. But I'm facing a problem I didn't have with CPU : The time doesn't scale linearly with the number of loops.
Here is the code :
Nloops = 500;
dt=1e-1; % time step
n=3e3; % number of vortices
z1=complex(gpuArray.randn(1,n),gpuArray.randn(1,n)); % vortices position
C1=repmat(1e-2*gpuArray.randn(n,1),1,n); % circulation
D=gpuArray.eye(n);
nD=~D;
tic
for ii=1:Nloops
Z1 = repmat(z1,n,1);
z1 = z1 + (dt*0.5i/pi) * (sum((C1.*nD)./(Z1-Z1.'+D),1));
end
toc
The result is quite surprising
Nloops = 100 : Elapsed time is 0.014950 seconds.
Nloops = 500 : Elapsed time is 17.072178 seconds.
While on CPU it scales well ( 3 and 15 seconds respectively ), do you have an idea why it scales so badly on GPU ?

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1 Answer

Answer by Matt J
on 19 Dec 2017
 Accepted Answer

tic and toc are not accurate measures of GPU execution time. Use gputimeit() instead.

  11 Comments

I'm using the Titan X.
I am using a 650M card (2012 time frame) with R2017b.
I tried adding in a gather() to force the computation to finish, but it did not seem to make any difference. However, either way (with or without gather) if I run the tests close together then it takes additional time, but if I wait the timer is faster. This suggests the GPU might not have finished (even with the gather.)
Note: in the source below, I use 1e3 nodes not 3e3, so as to avoid filling my GPU memory.
function testtime
dt=1e-1; % time step
n=1e3; % number of vortices
z1 = complex(gpuArray.randn(1,n),gpuArray.randn(1,n)); % vortices position
C1=repmat(1e-2*gpuArray.randn(n,1),1,n); % circulation
D=gpuArray.eye(n);
nD=~D;
gputimeit(@()gather(LoopPart(z1)))
function z1 = LoopPart(z1)
Nloops = 500;
for ii=1:Nloops
Z1 = repmat(z1,n,1);
z1 = z1 + (dt*0.5i/pi) * (sum((C1.*nD)./(Z1-Z1.'+D),1));
end
end
end
Try removing the repmat from your loop, with automatic scalar expansion you don't need it: Z1-Z1.' == z1-z1.'

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