PostgresSQL vs super pages

[Konstantin Belousov]

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.

> mmap(0x0,4194304,PROT_READ|PROT_WRITE,MAP_SHARED|MAP_HASSEMAPHORE|MAP_NOSYNC|MAP_ALIGNED_SUPER,46,0x0)
> = 35081158656 (0x82b000000)
> close(46)                                        = 0 (0x0)