Re: was there a change in FreeBSD SHM implementation from

On Wed, 10 Jul 2002, Vivek Khera wrote:

> I have a dual cpu box with 2GB RAM dedicated to running Postgres.
> Last week, I upgraded FreeBSD from 4.4-STABLE to 4.6-RELEASE-p1. When
> I went to restart postgres, it complained that it could not allocate
> the shared memory segment. I'm running Postgres 7.2.1.

This came up on the list a few days ago. FreeBSD 4.6 apparently has
some sort of system limit on how much shared memory it will allocate,
probably to keep those using shared memory from eating up too much
of the available VM. This is not a problem because you want to allocate
a fairly small amount of shared memory.

> 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...

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

See previous discussions on this (postgresql-general) list over
the past couple of weeks for details. (Is this in the FAQ yet?)

cjs
--
Curt Sampson <cjs(at)cynic(dot)net> +81 90 7737 2974 http://www.netbsd.org
Don't you know, in this new Dark Age, we're all light. --XTC