Re: SYSV shared memory vs mmap performance

On Thu, Sep 13, 2012 at 08:30:03AM +0200, Francois Tigeot wrote:
> Hi,
>
> Given the recent decision to switch from SYSV shared memory to mmap and
> the concerns which were made with regard to performance on *BSD kernels,
> I've run a few Pgbench tests on a spare Xeon box.
>
> I tested PostgreSQL-9.3 from June 28th, as of commits:
> - c5b3451a8e72cb7825933d4f4827f467cb38b498 (mmap)
> - 5d594b73d988b1ac78c49d8a84deae6bae876d01 (sysv shared memory)
>
> I also used both Scientific Linux-6.2 and DragonFly BSD-3.1; the results
> are in the attached PDF document.
>
> To cut a long story short, Linux doesn't show any difference and DragonFly
> sees some heavy degradation under load. After a while, it starts swapping
> and performance goes to hell.
>
> The only *BSD system tested was DragonFly but I know from previous pgbench
> tests FreeBSD and NetBSD follow a similar performance curve
>
> The famous kern.ipc.shm_use_phys sysctl was set to 1, which is the default
> setting.

Just a reminder we might have *BSD performance issues with our use of
Posix shared memory in Postgres 9.3.  I am attaching the PDF the user
posted.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +