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

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Message-ID: <20090315102856.GB6292@uranus.ravnborg.org>
Date:	Sun, 15 Mar 2009 11:28:56 +0100
From:	Sam Ravnborg <sam@...nborg.org>
To:	linux-kbuild <linux-kbuild@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Cc:	Roman Zippel <zippel@...ux-m68k.org>, Ingo Molnar <mingo@...e.hu>
Subject: [PATCH 2/2] kconfig: improve seed in randconfig

'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.)

Signed-off-by: Ingo Molnar <mingo@...e.hu>
Signed-off-by: Sam Ravnborg <sam@...nborg.org>
---
 scripts/kconfig/conf.c |   15 ++++++++++++++-
 1 files changed, 14 insertions(+), 1 deletions(-)

diff --git a/scripts/kconfig/conf.c b/scripts/kconfig/conf.c
index 3e1057f..4f1b488 100644
--- a/scripts/kconfig/conf.c
+++ b/scripts/kconfig/conf.c
@@ -11,6 +11,7 @@
 #include <time.h>
 #include <unistd.h>
 #include <sys/stat.h>
+#include <sys/time.h>
 
 #define LKC_DIRECT_LINK
 #include "lkc.h"
@@ -464,9 +465,21 @@ int main(int ac, char **av)
 			input_mode = set_yes;
 			break;
 		case 'r':
+		{
+			struct timeval now;
+			unsigned int seed;
+
+			/*
+			 * Use microseconds derived seed:
+			 */
+			gettimeofday(&now, NULL);
+
+			seed = (unsigned int)(now.tv_sec*now.tv_usec);
+			srand(seed);
+
 			input_mode = set_random;
-			srand(time(NULL));
 			break;
+		}
 		case 'h':
 			printf(_("See README for usage info\n"));
 			exit(0);
-- 
1.6.0.2.GIT

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