Re: [PATCH] PostgreSQL 9.4 mmap(2) performance regression on FreeBSD...

On Tue, Aug 12, 2014 at 12:59 PM, Andres Freund <andres@2ndquadrant.com> wrote:
> On 2014-08-12 09:42:30 -0700, Sean Chittenden wrote:
>> One of the patches that I've been sitting on and am derelict in punting
>> upstream is the attached mmap(2) flags patch for the BSDs. Is there any
>> chance this can be squeezed in to the PostreSQL 9.4 release?
>>
>> The patch is trivial in size and is used to add one flag to mmap(2) calls in
>> dsm_impl.c.  Alan Cox (FreeBSD alc, not Linux) and I went back and forth
>> regarding PostgreSQL's use of mmap(2) and determined that the following is
>> correct and will prevent a likely performance regression in PostgreSQL 9.4.
>> In PostgreSQL 9.3, all mmap(2) calls were called with the flags MAP_ANON |
>> MAP_SHARED, whereas in PostgreSQL 9.4 this is not the case.
>
> The performancewise important call to mmap will still use that set of
> flags, no? That's the one backing shared_buffers.
>
> The mmap backend for *dynamic* shared memory (aka dsm) is *NOT* supposed
> to be used on common platforms. Both posix and sysv shared memory will
> be used before falling back to the mmap() backend.

Hmm, yeah.  This might still be a good thing to do (because what do we
lose?) but it shouldn't really be an issue in practice.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company