From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from out1.smtp.messagingengine.com ([66.111.4.25]:41785 "EHLO out1.smtp.messagingengine.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752373AbZCOVre convert rfc822-to-8bit (ORCPT ); Sun, 15 Mar 2009 17:47:34 -0400 Message-Id: <1237153649.32747.1305532855@webmail.messagingengine.com> From: "Alexander van Heukelum" Content-Disposition: inline Content-Transfer-Encoding: 8BIT Content-Type: text/plain; charset="UTF-8" MIME-Version: 1.0 References: <20090315102341.GA6051@uranus.ravnborg.org> <20090315102856.GB6292@uranus.ravnborg.org> <10f740e80903150353i233d0da5rabde6e97607e3435@mail.gmail.com> <20090315130930.GA7146@uranus.ravnborg.org> <20090315185406.GA19737@elte.hu> Subject: Re: [PATCH 2/2] kconfig: improve seed in randconfig In-Reply-To: <20090315185406.GA19737@elte.hu> Date: Sun, 15 Mar 2009 22:47:29 +0100 Sender: linux-kbuild-owner@vger.kernel.org List-ID: To: Ingo Molnar , Sam Ravnborg Cc: Geert Uytterhoeven , linux-kbuild , LKML , Roman Zippel On Sun, 15 Mar 2009 19:54:07 +0100, "Ingo Molnar" said: > > * Sam Ravnborg wrote: > > > On Sun, Mar 15, 2009 at 11:53:03AM +0100, Geert Uytterhoeven wrote: > > > On Sun, Mar 15, 2009 at 11:28, Sam Ravnborg 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 :) > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" > in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > >