Re: UPDATEDs slowing SELECTs in a fully cached database

Thanks Craig.

Yep, I am not seeing the SELECTs slow down (measurably) during checkpoints
(i.e. when dirty pages are flushed to disk), but only during writing of the WAL
files. The buffers shared and OS are big enough to hold the entire database, so
evicting cached data should not be necessary. (The database including indexes
can fit into 16 or so GB, and I have 68GB on that machine).
Interestingly I initially thought there might be a correlation between
checkpointing and slower SELECTs, but it turns out that checkpointing just
slowed down IO to the WAL - until I move it to its own drive, and then increased
the effect I was seeing.

I'll do more research and try to provide more useful details.

Thanks for the pg_catalog link, I'll have a look at it.

-- Lars

----- Original Message ----
From: Craig Ringer <craig(at)postnewspapers(dot)com(dot)au>
To: pgsql-performance(at)postgresql(dot)org
Sent: Sun, July 10, 2011 4:11:39 PM
Subject: Re: [PERFORM] UPDATEDs slowing SELECTs in a fully cached database

On 11/07/2011 4:34 AM, lars wrote:
> I have since moved the WAL to its own EBS volume (ext4, data=writeback)
> to make it easier to monitor IO.
> The times where the SELECTs slow down coincide with heavy write traffic
> to the WAL volume.

In theory, UPDATEs shouldn't be blocking or slowing SELECTs. Whether that holds
up to the light of reality, real-world hardware, and software implementation
detail, I really don't know. I avoided responding to your first mail because I
generally work with smaller and less performance critical databases so I haven't
accumulated much experience with fine-tuning.

If your SELECTs were slower *after* your UPDATEs I'd be wondering if your
SELECTs are setting hint bits on the pages touched by the UPDATEs. See:
http://wiki.postgresql.org/wiki/Hint_Bits . It doesn't sound like that's the
case if the SELECTs are slowed down *during* a big UPDATE that hasn't yet
committed, though.

Could it just be cache pressure - either on shm, or operating system disk cache?
All the dirty buffers that have to be flushed to WAL and to the heap may be
evicting cached data your SELECTs were benefitting from. Unfortunately,
diagnostics in this area are ... limited ... though some of the pg_catalog views
(http://www.postgresql.org/docs/9.0/static/catalogs.html) may offer some
information.

--
Craig Ringer

POST Newspapers
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