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Re: [PATCH] randomize benchtests
- From: Torvald Riegel <triegel at redhat dot com>
- To: OndÅej BÃlka <neleai at seznam dot cz>
- Cc: Siddhesh Poyarekar <siddhesh dot poyarekar at gmail dot com>, c at domone dot kolej dot mff dot cuni dot cz, GNU C Library <libc-alpha at sourceware dot org>
- Date: Fri, 17 May 2013 13:16:01 +0200
- Subject: Re: [PATCH] randomize benchtests
- References: <20130422120018 dot GA30323 at domone dot kolej dot mff dot cuni dot cz> <CAAHN_R1pHJLoS3iP7KrQMmA4gPLawvuRWoK8Xy13VPMBbyPk+Q at mail dot gmail dot com> <20130422125625 dot GA30639 at domone dot kolej dot mff dot cuni dot cz> <1368786270 dot 3054 dot 3119 dot camel at triegel dot csb> <20130517104445 dot GA14264 at domone dot kolej dot mff dot cuni dot cz>
On Fri, 2013-05-17 at 12:44 +0200, OndÅej BÃlka wrote:
> On Fri, May 17, 2013 at 12:24:30PM +0200, Torvald Riegel wrote:
> > On Mon, 2013-04-22 at 14:56 +0200, OndÅej BÃlka wrote:
> > > On Mon, Apr 22, 2013 at 05:44:14PM +0530, Siddhesh Poyarekar wrote:
> > > > On 22 April 2013 17:30, OndÅej BÃlka <neleai@seznam.cz> wrote:
> > > > > + clock_gettime (CLOCK_PROCESS_CPUTIME_ID, &start);
> > > > > + for (k = 0; k < iters; k++)
> > > > > + {
> > > > > + i = rand_r (&seed)%NUM_SAMPLES;
> > > > > + BENCH_FUNC(i);
> > > > > + }
> > > > > + clock_gettime (CLOCK_PROCESS_CPUTIME_ID, &end);
> > > >
> > > > This is wrong. The interval also has the time taken to call rand_r.
> > > >
> > > This is not wrong. You are interested only on differences between
> > > implementations and adding same time from rand_r calls does not change
> > > that.
> >
> > But if we should be changing the rand_r implementation in the future
> > (e.g., if we might be getting HW support for it on a certain
> > architecture), then this will lead to a difference in all our
>
> From stdlib/rand_r.c:
>
> /* This algorithm is mentioned in the ISO C standard, here extended
> for 32 bits. */
>
> Given that we can break applications that depend on rand_r
> reproducibility it will not change.
> If you want fully specified generator use drand48.
My comment was about the performance of one call to rand_r. This isn't
about where or how it's specified, it's just about the performance, and
trying to keep it reproducible.