Re: How to monitor resources on Linux.

John R Allgood <jallgood(at)the-allgoods(dot)net> writes:
> I have some questions on memory resources and linux. We are
> currently running Dell Poweredge 2950 with dual core opeterons and 8GB
> RAM. Postgres version is 7.4.17 on RHEL4. Could someone explain to me
> how to best monitor the memory resources on this platform. Top shows a
> high memory usage nearly all is being used.

That's meaningless: what you have to look at is the breakdown of *how*
it is being used. The normal state of affairs is that there is no
"free" memory to speak of, because the kernel will keep around cached
disk pages as long as it can, so as to save a read if they are
referenced again. You're only in memory trouble when the percentage
used for disk buffers gets real small.

> ipcs -m shows the following
> output. If I am looking at this correctly each of the postgres entries
> represents a postmaster with the number of connections. If I calculate
> the first entry it comes to around 3.4GB of RAM being used is this
> correct.

That's *completely* wrong. It's shared memory, so by definition there
is one copy, not one per process.

One thing you have to watch out for is that "top" tends to report some
or all shared memory as part of the address space of each attached
process; so adding up the process sizes shown by top gives a
ridiculously inflated estimate. However, it's tough to tell exactly how
much is being double-counted :-(. I tend to look at top's aggregate
numbers, which are pretty real, and ignore the per-process ones.

> We have started running into memory issues

How do you know that?

Another good tool is to watch "vmstat 1" output. If you see a lot of
swapin/swapout traffic, then maybe you do indeed need more RAM.

> We have a 2 node cluster running about 10 separate postmasters divided
> evenly on each node.

I was wondering why so many postgres-owned shmem segments. Is it
intentional that you've given them radically different amounts of
memory? Some of these guys are scraping along with just a minimal
number of buffers ...

> 0x0052ea91 163845 postgres 600 133947392 26
> 0x00530db9 196614 postgres 600 34529280 24
> 0x00530201 229383 postgres 600 34529280 21
> 0x005305e9 262152 postgres 600 4915200 3
> 0x005311a1 294921 postgres 600 34529280 28
> 0x0052fe19 327690 postgres 600 4915200 4

regards, tom lane