Message-ID: <507573A4.1090605@optionshouse.com>
Date: Wed, 10 Oct 2012 08:09:56 -0500
From: Shaun Thomas <sthomas@optionshouse.com>
Reply-To: <sthomas@optionshouse.com>
Organization: Peak6
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CC: <pgsql-performance@postgresql.org>
Subject: Re: shared_buffers/effective_cache_size on 96GB server
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On 10/10/2012 02:12 AM, Strahinja Kustudi=C4=87 wrote:

>           total    used   free  shared   buffers     cached
> Mem:      96730   96418    311       0        71      93120

Wow, look at all that RAM. Something nobody has mentioned yet, you'll 
want to set some additional kernel parameters for this, to avoid getting 
occasional IO storms caused by dirty memory flushes.

vm.dirty_background_ratio =3D 1
vm.dirty_ratio =3D 5

Again, these would go in sysctl.conf, or /etc/sysctl.d/10-dbserver.conf 
or something. If you have a newer kernel, look into 
vm.dirty_background_bytes, and vm.dirty_bytes.

The why of this is brought up occasionally here, but it comes down to 
your vast amount of memory. The defaults for even late Linux kernels is 
5% for dirty_background_ratio, and 10% for dirty_ratio. So if you 
multiply it out, the kernel will allow about 4.8GB of dirty memory 
before attempting to flush it to disk. If that number reaches 9.6, the 
system goes synchronous, and no other disk writes can take place until 
*all 9.6GB* is flushed. Even with a fast disk subsystem, that's a pretty 
big gulp.

The idea here is to keep it writing in the background by setting a low 
limit, so it never reaches a critical mass that causes it to snowball 
into the more dangerous upper limit. If you have a newer kernel, the 
ability to set "bytes" is a much more granular knob that can be used to 
match RAID buffer sizes. You'll probably want to experiment with this a 
bit before committing to a setting.

> So it did a little swapping, but only minor, still I should probably
> decrease shared_buffers so there is no swapping at all.

Don't worry about that amount of swapping. As others have said here, you 
can reduce that to 0, and even then, the OS will still swap something 
occasionally. It's really just a hint to the kernel how much swapping 
you want to go on, and it's free to ignore it in cases where it knows 
some data won't be accessed after initialization or something, so it 
swaps it out anyway.


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

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