> >
> > 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);
Maybe better - but the one suggested by Ingo is more intuitive,
and is what I implmented already.
I have pushed out the following patch.
[And I know it breaks the 80 char limit].
Sam
commit b0fe551000179c868d46266278a890eab878baca
Author: Ingo Molnar <mingo@xxxxxxx>
Date: Thu Mar 12 15:15:31 2009 +0100
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@xxxxxxx>
Cc: Roman Zippel <zippel@xxxxxxxxxxxxxx>
[sam: fix for systems where usec is zero - noticed by Geert Uytterhoeven]
Signed-off-by: Sam Ravnborg <sam@xxxxxxxxxxxx>
diff --git a/scripts/kconfig/conf.c b/scripts/kconfig/conf.c
index 3e1057f..d190092 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 +465,22 @@ int main(int ac, char **av)
input_mode = set_yes;
break;
case 'r':
+ {
+ struct timeval now;
+ unsigned int seed;
+
+ /*
+ * Use microseconds derived seed,
+ * compensate for systems where it may be zero
+ */
+ gettimeofday(&now, NULL);
+
+ seed = (unsigned int)((now.tv_sec + 1) * (now.tv_usec + 1));
+ srand(seed);
+
input_mode = set_random;
- srand(time(NULL));
break;
+ }
case 'h':
printf(_("See README for usage info\n"));
exit(0);
--
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