PostgresSQL vs super pages

[Konstantin Belousov]

On Thu, Oct 11, 2018 at 02:01:20PM +1300, Thomas Munro wrote:
> On Thu, 11 Oct 2018 at 13:20, Konstantin Belousov <kostikbel at 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