Re: missing madvise functionality

Jakub Jelinek
2007-04-03T20:29:37Z

On Tue, Apr 03, 2007 at 01:17:09PM -0700, Ulrich Drepper wrote:
> Andrew Morton wrote:
> > Ulrich, could you suggest a little test app which would demonstrate this
> > behaviour?
>
> It's not really reliably possible to demonstrate this with a small
> program using malloc. You'd need something like this mysql test case
> which Rik said is not hard to run by yourself.
>
> If somebody adds a kernel interface I can easily produce a glibc patch
> so that the test can be run in the new environment.
>
> But it's of course easy enough to simulate the specific problem in a
> micro benchmark. If you want that let me know.

I think something like following testcase which simulates what free
and malloc do when trimming/growing a non-main arena.

My guess is that all the page zeroing is pretty expensive as well and
takes significant time, but I haven't profiled it.

#include <pthread.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <unistd.h>

void *
tf (void *arg)
{
(void) arg;
size_t ps = sysconf (_SC_PAGE_SIZE);
void *p = mmap (NULL, 128 * ps, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
if (p == MAP_FAILED)
exit (1);
int i;
for (i = 0; i < 100000; i++)
{
/* Pretend to use the buffer. */
char *q, *r = (char *) p + 128 * ps;
size_t s;
for (q = (char *) p; q < r; q += ps)
*q = 1;
for (s = 0, q = (char *) p; q < r; q += ps)
s += *q;
/* Free it. Replace this mmap with
madvise (p, 128 * ps, MADV_THROWAWAY) when implemented. */
if (mmap (p, 128 * ps, PROT_NONE,
MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0) != p)
exit (2);
/* And immediately malloc again. This would then be deleted. */
if (mprotect (p, 128 * ps, PROT_READ | PROT_WRITE))
exit (3);
}
return NULL;
}

int
main (void)
{
pthread_t th[32];
int i;
for (i = 0; i < 32; i++)
if (pthread_create (&th[i], NULL, tf, NULL))
exit (4);
for (i = 0; i < 32; i++)
pthread_join (th[i], NULL);
return 0;
}

Jakub