Re: was there a change in FreeBSD SHM implementation from

>>>>> "CS" == Curt Sampson <cjs@cynic.net> writes:

>> For those familiar with postgres, I was using shared_buffers=100000
>> with 4.4, but had to back that down to 32000 for 4.6.  This is
>> obviously impacting performance...

CS> Yes, I'm sure your performance improved. By going from 780 MB to
CS> 25 MB of shared memory, you just increased the amount of data the
CS> OS can buffer from about 1.1 GB to 1.8 GB. You've just increased
CS> your cache size by about 60%.

Actually performance went down.  Way down.  I disagree with your
argument that increasing the cache will help, since the cache is not
needed if you don't pushd out your SHM pages in the first place and
need to reload them quickly.

What it turned out to be was that SHMMAX was larger than my SHMALL.
Why the kernel let me build it that way without warnings is a hole in
the config system, but not fatal.  Bumping up SHMALL fixed my issue,
and now my performance is back to normal.

Perhaps Postgres could identify the smaller of SHMMAX and SHMALL when
reporting the failure to get the memory, and indicate which one is too
small.

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