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

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Message-ID: <10f740e80903150353i233d0da5rabde6e97607e3435@mail.gmail.com>
Date:	Sun, 15 Mar 2009 11:53:03 +0100
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Sam Ravnborg <sam@...nborg.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 11:28, 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?

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds
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