On 07/15, Paul E. McKenney wrote:
>
> On Wed, Jul 15, 2015 at 08:15:11PM +0200, Peter Zijlstra wrote:
> >
> > No, it makes the read-side primitive contain an unconditional memory
> > barrier, that forgoes the entire point.
> >
> > The writers are stupidly expensive already for they need global
> > serialization, optimizing them in any way doesn't make sense.
>
> That could well be the case, but it would be good to see the numbers.
Please see the discussion in another "change sb_writers to use
percpu_rw_semaphore".
The simple test-case from Dave
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <assert.h>
#define BUFLEN 1
#define FILESIZE (1 * 1024 * 1024)
char *testcase_description = "Separate file write";
void testcase(unsigned long long *iterations)
{
char buf[BUFLEN];
char tmpfile[] = "/run/user/1000/willitscale.XXXXXX";
int fd = mkstemp(tmpfile);
unsigned long size = 0;
memset(buf, 0, sizeof(buf));
assert(fd >= 0);
unlink(tmpfile);
while (1) {
int ret = write(fd, buf, BUFLEN);
assert(ret >= 0);
size += ret;
if (size >= FILESIZE) {
size = 0;
lseek(fd, 0, SEEK_SET);
}
(*iterations)++;
}
}
runs 12% faster if we "simply" remove mb's from sb_start/end_write().
percpu_rw_semaphore does this too and has the approximately same
performance, and we can (hopefully) remove this nontrivial, currently
not 100% correct, and very "special" code in fs/super.c.
Oleg.
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