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

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Date:	Sun, 15 Mar 2009 22:47:29 +0100
From:	"Alexander van Heukelum" <heukelum@...tmail.fm>
To:	"Ingo Molnar" <mingo@...e.hu>, "Sam Ravnborg" <sam@...nborg.org>
Cc:	"Geert Uytterhoeven" <geert@...ux-m68k.org>,
	"linux-kbuild" <linux-kbuild@...r.kernel.org>,
	"LKML" <linux-kernel@...r.kernel.org>,
	"Roman Zippel" <zippel@...ux-m68k.org>
Subject: Re: [PATCH 2/2] kconfig: improve seed in randconfig


On Sun, 15 Mar 2009 19:54:07 +0100, "Ingo Molnar" <mingo@...e.hu> said:
> 
> * Sam Ravnborg <sam@...nborg.org> wrote:
> 
> > 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.
> 
> doing:
> 
>          seed = (unsigned int)((now.tv_sec+1)*(now.tv_usec+1));
> 
> ought to settle any practical doubts.

Or maybe (and I think better...)

    seed = (unsigned int)(now.tv_sec ^ now.tv_usec);

Greetings,
    Alexander

> 	Ing

o :)

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