Date: Sun, 14 Oct 2018 02:50:21 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Thomas Munro <munro@ip9.org> Cc: freebsd-hackers@freebsd.org, alc@freebsd.org, mjg@freebsd.org Subject: Re: PostgresSQL vs super pages Message-ID: <20181013235021.GX5335@kib.kiev.ua> In-Reply-To: <CADLWmXWS6qjt02bxUkd7BewfhYw69at8OYe%2Bh15%2B1OCpnpi6ng@mail.gmail.com> References: <CADLWmXU=7QM-oHmY=TMAQanQE-dnXY4v74Zm1kkEz3Gc=ip21A@mail.gmail.com> <20181011001954.GV5335@kib.kiev.ua> <CADLWmXWS6qjt02bxUkd7BewfhYw69at8OYe%2Bh15%2B1OCpnpi6ng@mail.gmail.com>
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On Thu, Oct 11, 2018 at 02:01:20PM +1300, Thomas Munro wrote:
> On Thu, 11 Oct 2018 at 13:20, Konstantin Belousov <kostikbel@gmail.com> wrote:
> > On Thu, Oct 11, 2018 at 12:59:41PM +1300, Thomas Munro wrote:
> > > shm_open("/PostgreSQL.1721888107",O_RDWR|O_CREAT|O_EXCL,0600) = 46 (0x2e)
> > > ftruncate(46,0x400000) = 0 (0x0)
> > Try to write zeroes instead of truncating.
> > This should activate the fast path in the fault handler, and if the
> > pages allocated for backing store of the shm object were from reservation,
> > you should get superpage mapping on the first fault without promotion.
>
> If you just write() to a newly shm_open()'d fd you get a return code
> of 0 so I assume that doesn't work. If you ftruncate() to the desired
> size first, then loop writing 8192 bytes of zeroes at a time, it
> works. But still no super pages. I tried also with a write buffer of
> 2MB of zeroes, but still no super pages. I tried abandoning
> shm_open() and instead using a mapped file, and still no super pages.
I did not quite scientific experiment, but you would need to try to find
the differences between what I did and what you observe. Below is the
naive test program that directly implements my suggestion, and the
output from the procstat -v for it after all things were set up.
/* $Id: shm_super.c,v 1.1 2018/10/13 23:49:37 kostik Exp kostik $ */
#include <sys/param.h>
#include <sys/mman.h>
#include <err.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#define M(x) ((x) * 1024 * 1024)
#define SZ M(4)
int
main(void)
{
char buf[128];
void *ptr;
off_t cnt;
int error, shmfd;
shmfd = shm_open(SHM_ANON, O_CREAT | O_RDWR, 0600);
if (shmfd == -1)
err(1, "shm_open");
error = ftruncate(shmfd, SZ);
if (error == -1)
err(1, "truncate");
memset(buf, 0, sizeof(buf));
for (cnt = 0; cnt < SZ; cnt += sizeof(buf)) {
error = write(shmfd, buf, sizeof(buf));
if (error == -1)
err(1, "write");
else if (error != sizeof(buf))
errx(1, "short write %d", (int)error);
}
ptr = mmap(NULL, SZ, PROT_READ | PROT_WRITE, MAP_SHARED |
MAP_ALIGNED_SUPER, shmfd, 0);
if (ptr == MAP_FAILED)
err(1, "mmap");
for (cnt = 0; cnt < SZ; cnt += PAGE_SIZE)
*((char *)ptr + cnt) = 0;
printf("ptr %p\n", ptr);
snprintf(buf, sizeof(buf), "procstat -v %d", getpid());
system(buf);
}
$ ./shm_super
ptr 0x800e00000
PID START END PRT RES PRES REF SHD FLAG TP PATH
98579 0x400000 0x401000 r-x 1 3 1 0 CN-- vn /usr/home/kostik/work/build/bsd/DEV/stuff/tests/shm_super
98579 0x600000 0x601000 rw- 1 1 1 0 ---- df
98579 0x800600000 0x800620000 r-x 32 34 146 72 CN-- vn /libexec/ld-elf.so.1
98579 0x800620000 0x800644000 rw- 24 24 1 0 ---- df
98579 0x80081f000 0x800820000 rw- 1 0 1 0 C--- vn /libexec/ld-elf.so.1
98579 0x800820000 0x800821000 rw- 1 1 1 0 ---- df
98579 0x800821000 0x8009b3000 r-x 402 440 146 72 CN-- vn /lib/libc.so.7
98579 0x8009b3000 0x800bb3000 --- 0 0 0 0 CN-- --
98579 0x800bb3000 0x800bbf000 rw- 12 0 1 0 C--- vn /lib/libc.so.7
98579 0x800bbf000 0x800bd9000 rw- 5 14 2 0 ---- df
98579 0x800c00000 0x800e00000 rw- 9 14 2 0 ---- df
98579 0x800e00000 0x801200000 rw- 1024 1030 3 0 --S- df
98579 0x801200000 0x801400000 rw- 6 1030 3 0 ---- df
98579 0x7fffdffff000 0x7ffffffdf000 --- 0 0 0 0 ---- --
98579 0x7ffffffdf000 0x7ffffffff000 rw- 4 4 1 0 ---D df
98579 0x7ffffffff000 0x800000000000 r-x 1 1 81 0 ---- ph
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