Re: Loop unrolling: black magic or stochastic process?

[dgorur <dgorur at gmail dot com>]

In my original code, I have valid data that I read into the x array before
calling the doLoop(...) function. In an effort to isolate the problem, I
created this simple test code to help diagnose the problem. While I
understand your concern, I was not convinced that this was the problem: if
it were, why should the removal of the print statement consistently reduce
the time taken? To test this, I added the following code to main, before the
call to doLoop(...)

	srand(time(NULL));
	for (i=0;i<DATA_COUNT;i++){
		for (j=0;j<INPUT_DIM;j++)
			x[i][j] = (double) rand()/RAND_MAX;
	}

Sure enough, the results show that this makes no difference:

[dgorur@mary008-0304-dhcp-217 Desktop]$ ./a.out
Beginning loop...
Rows processed = 100000.
t1 - t0 = 49 seconds.
Done. Time elapsed = 49 seconds.

To see if there was something else wrong with my coding or with compiler
flags, or whether it was a compiler issue, I tested the original code on
another computer, running a different OS (Linux), and using a different
version of gcc (4.1.2). This machine also happened to have an Intel compiler
installed, so I thought I'd see what happened using that. The results have
made my problem irrelevant. Thank you all for the rapid responses. By the
way, this *is* on a Linux machine, I found the shell script for the funky
bash prompt on xkcd.

-Dev

Using the Intel compiler (icc)

C:\home\dgorur\sparseGP>icc -O3 -funroll-loops sparseGP.c -o sparsegp_icc
C:\home\dgorur\sparseGP>./sparsegp_icc 
This is sparseGP.

Reading data from databaseLarge.dat...
Done. 84000 data points read.

Beginning correlation loop...
Rows processed = 84000,
Time elapsed = 0 seconds.

Using gcc
C:\home\dgorur\sparseGP>gcc -lm -O3 -funroll-loops sparseGP.c -o
sparsegp_gcc
C:\home\dgorur\sparseGP>./sparsegp_gcc
This is sparseGP.

Reading data from databaseLarge.dat...
Done. 84000 data points read.

Beginning correlation loop...
Rows processed = 84000,
Time elapsed = 278 seconds.


Tim Prince-3 wrote:
> 
> dgorur wrote:
>> Hi,
>> 
>> I've been getting unpredictable results with gcc -funroll-loops. 
> As the other reply suggested, you can't expect consistent results when
> working on uninitialized data, and no amount of code tuning will
> compensate for it.
> I wonder about your selection of time(); as you're not threading, and
> appear to be interested in comparing code execution times, why not clock()
> ?
> An obvious question, which you could answer better for yourself, as you
> have chosen such a quirky version of gcc, is whether -funroll-loops is
> producing the apparently desired result of unrolling the inner loop fully.
>  If not, why not use
>  --param max-unroll-times=6   (3,2)
> to set a more suitable amount of unrolling?
> 
> 
> 

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