linux-kernel - Re: [patch 10/10] *Tests* Scheduler profiling

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Message-ID: <20070711050238.GC4025@Krystal>
Date:	Wed, 11 Jul 2007 01:02:38 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"Li, Tong N" <tong.n.li@...el.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 10/10] *Tests* Scheduler profiling - Use immediate values

Hi,

* Li, Tong N (tong.n.li@...el.com) wrote:
> Mathieu,
> 
> > 	cycles_per_iter = 0.0;
> > 	for (i=0; i<NR_TESTS; i++) {
> > 		time1 = get_cycles();
> > 		for (j = 0; j < NR_ITER; j++) {
> > 			testval = &array[random() % ARRAY_SIZE];
> > 		}
> > 		time2 = get_cycles();
> > 		cycles_per_iter += (time2 - time1)/(double)NR_ITER;
> > 	}
> > 	cycles_per_iter /= (double)NR_TESTS;
> > 	printf("Just getting the pointer, doing noting with it, cycles
> per
> > iteration (mean) : %g\n", cycles_per_iter);
> > 
> 
> Some comments on the code:
> 
> 1. random() is counted in cycle_per_iter, which can skew the results.
> You could pre-compute the random addresses and store them in an array.
> Then, during the actual timing, walk the array:
> 
> index = 0;
> for (i = 0; i < ARRAY_SIZE; i++)
>       index = *(int *)(array + index * CACHE_LINE_SIZE);
> 
> 2. You may want to flush the cache before the timing starts.
> 
> 3. You want to access memory at the cache-line granularity to avoid
> addresses falling into the same line (and thus unwanted hits).
> 

This is true, my test code was not perfect. Thanks for the hints.
The improvements you propose will clearly accelerate my test program
quite a bit, but I doubt that it will cause even higher memory
latencies. Although using a random() at each memory access is slow, it
should give a good enough dispersion. And since do 3 cache trashing
passes in my code, I make sure that each and every cache lines are
trashed. In fact, since I do multiple accesses to each cache line (as
you noted in point 3), it takes more time, but makes it more certain
that I hit all of them at least once.

> If you do these, I expect you'll get a higher memory latency.
> 

I will use these comments in my next tests, thanks. :) However, I still
feel confident that the numbers I got from my run still hold.

Mathieu

>   tong
> 

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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