Weird case: Loop runs faster with fprintf?
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Hi,
I have a huge code with many functions. In that, the function that takes the most time has 4 nested loops, and I was trying to estimate the time it would take in total by printing the current counter in the loop. In doing so, I realized that the loop runs much faster when I print! I do printing via fprintf. Below is the nested loop section of the function:
if(efkFlag(3))
for i1=1:an.movf
rID = ID.movf(i1);
rplace = (i1-1)*sim.dim+(1:sim.dim);
for i2=1:an.movf
rnnID = ID.movf(i2);
rnnplace = (i2-1)*sim.dim+(1:sim.dim);
for i3=1:an.fcalc
t1 = ID.fcalc(i3);
t2 = RN(i3);
t3 = R(1:t2,1,i3);
t4 = W(rID,t1);
t5 = W(rID,t3);
t6 = bsxfun(@minus,t4,t5');
t7 = W(rnnID,t1);
t8 = W(rnnID,t3);
t9 = bsxfun(@minus,t7,t8');
t10 = bsxfun(@times,t6,t9);
t11 = find(SMP(:,1)==t1,1);
if(~isempty(t11))
t10 = SMP(t11,3).*t10;
end
for i4=1:t2
t12 = (i4-1)*sim.dim+(1:sim.dim);
t13 = KR(t12,1:sim.dim,i3);
t14 = t10(i4).*t13;
K(rplace,rplace) = K(rplace,rplace)+t14;
K(rplace,rnnplace) = K(rplace,rnnplace)-t14;
end
end
end
fprintf('stiffness: %3.0f %3.0f %6.2f\n',i1,an.movf,toc/60);
end
end
Does anyone have an idea about why the loop would run faster?
Thanks,
2 个评论
Seyhan Emre Gorucu
2012-8-7
Run the same case 1000 times. The first time you run it gives a large value. 1000 times takes an average. Did you do profiling or tic toc?
Emre
2012-8-9
回答（3 个）
per isakson
2012-8-8
编辑：per isakson
2012-8-9
I cannot reproduce your result with the function, cssm (see below), on R2012a 64bit, Windows7. Here are two results recorded after several runs.
...
>> cssm(10);
Elapsed time is 10.712166 seconds.
...
>> cssm(10);
1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17, ...
Elapsed time is 10.700414 seconds.
.
- The time recorded with tic/toc is hardly affected by fprintf.
- the fprintf-output displays while looping (handled be a parallel process?)
- profile shows "zero" time for fprintf
- Is "Free Memory" (Windows Task Manager) close to zero anytime during the execution of your program?
function M11 = cssm( N )
M01 = randn( N );
M02 = randn( N );
tic
for ii = 1 : 100
for jj = 1 : 100
for kk = 1 : 100
M11 = bsxfun( @times, M01, M02 );
end
end
% fprintf( '%d,', ii )
end
fprintf( '\n' )
toc
end
2 个评论
Emre
2012-8-9
per isakson
2012-8-9
Hi Embre, I assume that
- you can reproduce my results with my function, cssm
- you have profiled your code with and without fprintf. (The Matlab function, PROFILE.) What does it tell?
Why not send the problem to the tech support together with a clear instruction on how to run it.
Sean de Wolski
2012-8-6
0 个投票
It might be even faster if you have just a drawnow where the fprintf(etc) is.
Daniel Shub
2012-8-7
0 个投票
Are you sure it is running faster. I am not sure that fprintf actually prints to the screen in the middle of your loop. It may wait until the nested loops are finished before flushing all the answers to the screen. This would make it look like it is going fast.
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