Re: High SYS CPU - need advise

On 11/20/2012 04:08 PM, Jeff Janes wrote:

> Shaun Thomas reports one that is (I assume) not read intensive, but
> his diagnosis is that this is a kernel bug where a larger
> shared_buffers for no good reason causes the kernel to kill off its
> page cache.

We're actually very read intensive. According to pg_stat_statements, we
regularly top out at 42k queries per second, and pg_stat_database says
we're pushing 7k TPS.

But I'm still sure this is a kernel bug. Moving from 4GB to 6GB or 8GB
causes the kernel to cut the active page cache in half, in addition to
freeing 1/4 of RAM to just sit around doing nothing. That in turn causes
kswapd to work constantly, while our IO drivers work to undo the damage.
It's a positive feedback loop that I can reliably drive the load up to
800+ on an 800-client pgbench with two threads, all while having 0% CPU
free.

Make that 4GB, and not only does the problem completely disappear, but
the load settles down to around 9, and the machine becomes about 60%
idle. Something in there is fantastically broken, but I can't point a
finger at what.

I was just piping in because, in absence of an obvious PG-related
culprit, the problem could be the OS itself. It certainly was in our case.

That, or PG has a memory leak that only appears at > 4GB of shared buffers.

--
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 500 | Chicago IL, 60604
312-444-8534
sthomas@optionshouse.com

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