The fact is, glibc is just total crap.
I tried to send uli a patch to just add caching. No go. I sent
*another* patch to at least make glibc use a sane interface (and the
cache if it needs to fall back on /proc/stat for some legacy reason).
We'll see what happens.
FWIW a rerun with this modified LD_PRELOAD that does caching seems
to have the same performance as the version that does sched_getaffinity.
So you're right. Caching indeed helps and my assumption that the child
would only do it once was incorrect.
The only problem I see with it is that it doesn't handle CPU hotplug,
but Paul's suggestion would fix that too.
Paul Eggbert suggested "caching for one second" - by just calling
"gettimtofday()" to see how old the cache is. That would work too.
Maybe we need a "standard LD_PRELOAD library to improve glibc" @)
-Andi
// gcc -fPIC -shared sysconf-caching.c -ldl -o sysconf-caching.so
#define _GNU_SOURCE 1
#include <dlfcn.h>
#include <sched.h>
#include <unistd.h>
static long int (*real_sysconf)(int name);
long int sysconf(int name)
{
if (!real_sysconf)
real_sysconf = dlsym(RTLD_NEXT, "sysconf");
if (name == _SC_NPROCESSORS_ONLN) {
static int cache = -1;
if (cache == -1)
cache = real_sysconf( _SC_NPROCESSORS_ONLN);
return cache;
}
return real_sysconf(name);
}