linux-kernel - Re: [PATCH 2/2] kconfig: improve seed in randconfig

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Message-ID: <20090315130930.GA7146@uranus.ravnborg.org>
Date:	Sun, 15 Mar 2009 14:09:30 +0100
From:	Sam Ravnborg <sam@...nborg.org>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:	linux-kbuild <linux-kbuild@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Roman Zippel <zippel@...ux-m68k.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 2/2] kconfig: improve seed in randconfig

On Sun, Mar 15, 2009 at 11:53:03AM +0100, Geert Uytterhoeven wrote:
> On Sun, Mar 15, 2009 at 11:28, Sam Ravnborg <sam@...nborg.org> wrote:
> > 'make randconfig' uses glibc's rand function, and the seed of
> > that PRNG is set via:
> >
> >                        srand(time(NULL));
> >
> > But 'time()' only increases once every second - freezing the
> > randconfig result within a single second.
> >
> > My Nehalem testbox does randconfig much faster than 1 second
> >  and i have a few scripts that do 'randconfig until condition X'
> > loops.
> >
> > Those scripts currently waste a lot of CPU time due to randconfig
> > changing its seed only once per second currently.
> >
> > Change the seed to be micrseconds based. (I checked the statistical
> > spread of the seed - the now.tv_sec*now.tv_usec multiplication
> > there further improves it.)
> 
> > +                       gettimeofday(&now, NULL);
> > +
> > +                       seed = (unsigned int)(now.tv_sec*now.tv_usec);
> 
> Just wondering: may there be some platforms that don't offer microsecond
> resolution, and tv_usec is always zero?
That would indeed be bad for the seed.
Googling did not turn up anything.

	Sam
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