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Re: UPDATEDs slowing SELECTs in a fully cached database

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: "lars" <lhofhansl(at)yahoo(dot)com>
Cc: "Ivan Voras" <ivoras(at)freebsd(dot)org>, <pgsql-performance(at)postgresql(dot)org>
Subject: Re: UPDATEDs slowing SELECTs in a fully cached database
Date: 2011-07-13 14:46:36
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
[combining responses to two posts on this thread by lars]
lars <lhofhansl(at)yahoo(dot)com> wrote:
> On the face of it, though, this looks like Postgres would not be
> that useful as database that resides (mostly) in the cache.
I've mentioned this in a hand-wavy general sense, but I should have
mentioned specifics ages ago: for a database where the active
portion of the database is fully cached, it is best to set
seq_page_cost and random_page_cost to the same value, somewhere in
the 0.1 to 0.05 range.  (In your case I would use 0.05.)  In highly
cached databases I have sometimes also found it necessary to
increase cpu_tuple_cost.  (In your case I might try 0.02.)
This won't directly address the specific issue you've been
investigating in this thread, but it will tend to give you faster
plans for your actual environment without needing to fuss with
things on a query-by-query basis.  It may indirectly mitigate the
problem at hand through heavier use of indexes which would reduce
pruning and hint-bit setting by readers.
> Interesting. When you did you test, did you also find WAL write
> activity when running x the first time after y?
I wasn't able to check that in this quick, ad hoc run.
> Oh, it's just switched off for testing, so that I can control
> when vacuum runs and make sure that it's not skewing the results
> while I am measuring something.
Again, that's an impulse I can certainly understand, but the problem
is that turning autovacuum off skews results in other ways, such as
forcing readers to do maintenance work which might otherwise be done
in a cost-limited background process.  Or if that didn't happen you
would be constantly chasing through lots of dead line pointers which
would hurt performance in another way.  It's probably best to
consider autovacuum an integral part of normal database operations
and run benchmarks like this with it operational.  This will also
give you an opportunity to tune thresholds and costing factors to
evaluate the impact that such adjustments have on potential
> For a fully cached database I would probably want to switch off
> HOT pruning and compaction (which from what we see is done
> synchronously with the select) and leave it up to the asynchronous
> auto vacuum to do that. But maybe I am still not quite
> understanding the performance implications.
Code comments indicate that they expect the pruning to be a pretty
clear win on multiple reads, although I don't know how much that was
benchmarked.  Jeff does raise a good point, though -- it seems odd
that WAL-logging of this pruning would need to be synchronous.  We
support asynchronous commits -- why not use that feature
automatically for transactions where the only writes are this sort
of thing.  Which raises an interesting question -- what happens to
the timings if your SELECTs are done with synchronous_commit = off?
I wonder if it would make any sense to implicitly use async commit
for a transaction which is declared READ ONLY or which never
acquires and XID?

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Next:From: Merlin MoncureDate: 2011-07-13 14:52:53
Subject: Re: UPDATEDs slowing SELECTs in a fully cached database
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