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From: Josh Berkus <josh@agliodbs.com>
To: pgsql-performance@postgresql.org
Subject: Re: Some performance testing?
Date: Tue, 14 Apr 2015 12:58:32 -0700
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Scott,

> Can confirm that for pg purposes, 3.2 is basically broken in some not
> to great ways. We've had servers that were overloaded at load factors
> of 20 or 30 drop down to 5 or less with just a change from 3.2 to
> 3.11/3.13 on ubuntu 12.04

That's correct, and 3.5 shares the same problems.  The underlying issue
was that 3.X was tweaked to be MUCH more aggressive about
cache-clearing, to the point where it would be evicting data from the FS
cache which had just been read in and hadn't even been used yet.  For
some reason, this aggressive eviction got worse the more processes on
the system which were using the FS cache, so where you really see it is
when you have more processes with cache than you have cores.

It's pretty easy to demonstrate just using pgbench, with a database
larger than RAM, and 2X as many clients as cores.  You'll see that
kernels 3.2 and 3.5 will do 3X to 5X as much IO for the same workload as
3.10 and later will do.

Greame,

On 04/09/2015 04:01 AM, Graeme B. Bell wrote:> performance with 2.6:
(pgbench, size 100, 32 clients)
>
> 48 651 transactions per second (read only)
> 6 504 transactions per second (read-write)
>
>
> performance with 3.18 (pgbench, size 100, 32 clients)
>
> 129 303 transactions per second  (read only)
> 16 895 transactions (read-write)

Thanks for that data!  I'm glad to see that 3.18 has improved so much.

-- 
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com


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