Re: Shared memory usage

First off, posting to two lists like you did (-general and -performance)
is frowned on here. Pick whichever is more appropriate for the topic and
post to just that one; in your case, the performance list would be more
appropriate, and I'm only replying to there.

On Sun, 26 Aug 2007, Max Zorloff wrote:

> shared_buffers is set to 60000, yet they use a minimal part of that.
> PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
> 11492 postgres 16 0 530m 72m 60m S 14 1.2 0:50.91 postmaster

Looks to me like PostgreSQL is grabbing 530MB worth of memory on your
system. run the ipcs command to see how big the block that's dedicated to
the main server is; I suspect you'll find it's at 400MB just like you
expect it to be. Here's an example from my server which has a 256MB
shared_buffers:

-bash-3.00$ ipcs
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x0052e2c1 1114114 postgres 600 277856256 3

Also: when you've got top running, hit the "c" key and the postmaster
processes will give you more information about what they're doing you may
find helpful.

> All the indexes and half of the database should be in the shared memory,
> is it not? Or am I completely missing what are the shared_buffers for?
> If so, then how do I put my indexes and at least a part of the data into
> memory?

You can find out what's inside the shared_buffers cache by using the
installing the contrib/pg_buffercache module against your database. The
README.pg_buffercache file in there gives instructions on how to install
it, and the sample query provided there should tell you what you're
looking for here.

> Where do I find my OS disk cache settings? I'm using Linux.

You can get a summary of how much memory Linux is using to cache data by
running the free command, and more in-depth information is available if
you look at the /proc/meminfo information. I have a paper you may find
helpful here, it has more detail in it than you need but it provides some
pointers to resources to help you better understand how memory management
in Linux works: http://www.westnet.com/~gsmith/content/linux-pdflush.htm

--
* Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD