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Re: wal_buffers, redux

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Cc: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: wal_buffers, redux
Date: 2012-03-14 02:21:54
Message-ID: CA+Tgmob3=c2t=cmGQd_yhBM7AmtXhOmbG+=p4WDKcFPmhKDycg@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
On Tue, Mar 13, 2012 at 10:02 PM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:
> On Wed, Mar 14, 2012 at 7:20 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> On Tue, Mar 13, 2012 at 3:48 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>> On Mon, Mar 12, 2012 at 4:45 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
>>>> Rerunning all 4 benchmarks (both 16MB and 32MB wal_buffers on both
>>>> machines) with fsync=off (as well as synchronous_commit=off still)
>>>> might help clarify things.
>>>
>>> I reran the 32-client benchmark on the IBM machine with fsync=off and got this:
>>>
>>> 32MB: tps = 26809.442903 (including connections establishing)
>>> 16MB: tps = 26651.320145 (including connections establishing)
>>>
>>> That's a speedup of nearly a factor of two, so clearly fsync-related
>>> stalls are a big problem here, even with wal_buffers cranked up
>>> through the ceiling.
>>
>> And here's a tps plot with wal_buffers = 16MB, fsync = off.  The
>> performance still bounces up and down, so there's obviously some other
>> factor contributing to latency spikes
>
> Initialization of WAL file? Do the latency spikes disappear if you start
> benchmark after you prepare lots of the recycled WAL files?

The latency spikes seem to correspond to checkpoints, so I don't think
that's it.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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