Re: Memory reporting on CentOS Linux

On Fri, 14 Aug 2009, Scott Carey wrote:

> The memory used by postgres for shared memory is the largest of all SHR
> columns for postgres columns.  Or, about 7.9GB.   So, postgres is using
> about 7.9GB for shared memory, and very little for anything else.

It's a good idea to check this result against the actual shared memory
block allocated.  If the server has been up long enough to go through all
of shared_buffers once, the results should be close.  You can look at the
block under Linux using "ipcs -m"; the one you want should look something
like this:

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x0052e2c1 21757972   gsmith     600        548610048  10

That represents a bit over 512MB worth of allocated memory for the server.
Alternately, you can use "pmap -d" on a PostgreSQL process to find the
block, something like this works:

$ pmap -d 13961 | egrep "^Address|shmid"
Address   Kbytes Mode  Offset           Device    Mapping
96c41000  535752 rw-s- 0000000000000000 000:00009   [ shmid=0x14c0014 ]

I have given up on presuming the summary values top shows are good for
anything on Linux.  I look at /proc/meminfo to see how much RAM is free,
and to figure out what's going on with the server processes I use:

ps -e -o pid,rss,vsz,size,cmd | grep postgres

And compute my own totals (one of these days I'm going to script that
process).  Useful reading on this topic:

http://virtualthreads.blogspot.com/2006/02/understanding-memory-usage-on-linux.html
http://mail.nl.linux.org/linux-mm/2003-03/msg00077.html
http://forums.gentoo.org/viewtopic.php?t=175419

Most confusion about what's going on here can be resolved by spending some
quality time with pmap.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD