From help-octave-request at bevo dot che dot wisc dot edu Sat Jan 17 21:58:44 2004
Subject: Re: Octave and threaded ATLAS and FFTW
From: "Dmitri A. Sergatskov" <dmitri at unm dot edu>
To: "Henry F. Mollet" <mollet at pacbell dot net>
Cc: "John W. Eaton" <jwe at bevo dot che dot wisc dot edu>,    Octave_post <help-octave@bevo.che.wisc.edu>
Date: Sat, 17 Jan 2004 20:58:06 -0700

Henry F. Mollet wrote:
> With a 400 Mhz Power PC G3 the two items tested take considerably longer. I
> can understand a factor of 2000/400 = 5. Not that I need the speed, but does
> it imply that vecLib (with BLAS and LAPACk) in Mac OS X Dev Tools is about
> 4-10 times slower compared to  ATLAS 2.6.0?
> Henry
> 
> octave:1> tic; t1=cputime ; a=rand(2000); t2=cputime ; toc
> ans = 5.0177
> octave:2> t2-t1
> ans = 3.1200 % ca. 20 times longer
> octave:3> tic; t1=cputime ; b=inv(a); t2=cputime ; toc
> ans = 87.499
> octave:4> t2-t1
> ans = 72.130 % ca. 39 times longer

I was trying to make a different point. On a lightly loaded system
the tic/toc and cputime normally give very close to each other results
withing 0.1 second or even better. E.g. (on AthlonXP 1700MHz single cpu):

octave:7> tic; t1=cputime ; a=rand(2000); t2=cputime ; toc
ans = 0.17543
octave:8> t2-t1
ans = 0.17000
octave:9> tic; t1=cputime ; b=inv(a); t2=cputime ; toc
ans = 9.1615
octave:10> t2-t1
ans = 9.1500
octave:11> tic; t1=cputime ; c=b*a; t2=cputime ; toc
ans = 5.9397
octave:12> t2-t1
ans = 5.9200

If I load a CPU (run say setiathome at nice 1) then they start to differe more:

octave:13> tic; t1=cputime ; a=rand(2000); t2=cputime ; toc
ans = 0.18272
octave:14> t2-t1
ans = 0.17000
octave:15> tic; t1=cputime ; b=inv(a); t2=cputime ; toc
ans = 17.289
octave:16> t2-t1
ans = 9.1900
octave:17> tic; t1=cputime ; c=b*a; t2=cputime ; toc
ans = 11.818
octave:18> t2-t1
ans = 5.9700

That is well understood. What is odd is that when I recompile octave against
thread-enabled ATLAS (and consequently had to link-in pthread library)
the cputime numbers stopped making sense.

As far as your (Henry Mollet) data, I have two suspicions:
Since you do not have octave-forge installed you are using slow rand()
which came with octave proper.
The fact that you get significant discrepancy between tic/toc and cputime()
data suggests that you might be running some CPU intensive job on the
background.

The number still puzzling and I am wondering if it might be the case of
G3 vs G4 optimization and using wrong library for the CPU.
I admit knowing next to nothing about Macs so don't shoot me if it sounds
stupid.

Regards,

Dmitri.

p.s. I did verify with a stopwatch that tic/toc times on threaded Octave
are correct, so for large problems one can get perhaps up to 50% speedup
for some tasks (matrix multiplication is one of them). I will post
some numbers soon.



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