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Re: LWLock Queue Jumping

From: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
To: pgsql-hackers(at)postgresql(dot)org, stefan(at)kaltenbrunner(dot)cc
Subject: Re: LWLock Queue Jumping
Date: 2009-08-30 17:44:53
Message-ID: f67928030908301044w77311615h9462869814509bfd@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-hackers
>
> ---------- Forwarded message ----------
> From: Stefan Kaltenbrunner <stefan(at)kaltenbrunner(dot)cc>
> To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
> Date: Sun, 30 Aug 2009 11:48:47 +0200
> Subject: Re: LWLock Queue Jumping
> Heikki Linnakangas wrote:
>
>> Greg Stark wrote:
>>
>>> On Fri, Aug 28, 2009 at 8:07 PM, Simon Riggs<simon(at)2ndquadrant(dot)com>
>>> wrote:
>>>
>>>> WALInsertLock is heavily contended and likely always will be even if we
>>>> apply some of the planned fixes.
>>>>
>>> I've lost any earlier messages, could you resend the raw data on which
>>> this is based?
>>>
>>
>> I don't have any pointers right now, but WALInsertLock does often show
>> up as a bottleneck in write-intensive benchmarks.
>>
>
> yeah I recently ran accross that issue with testing concurrent COPY
> performance:
>
>
> http://www.kaltenbrunner.cc/blog/index.php?/archives/27-Benchmarking-8.4-Chapter-2bulk-loading.html
> discussed here:
>
> http://archives.postgresql.org/pgsql-hackers/2009-06/msg01019.php



It looks like this is the bulk loading of data into unindexed tables.  How
good is that as a target for optimization?  I can see several (quite
difficult to code and maintain) ways to make bulk loading into unindexed
tables faster, but they would not speed up the more general cases.


> and (iirc) also here:
>
> http://archives.postgresql.org/pgsql-hackers/2009-06/msg01133.php



I played around a little with this, parallel bulk loads into a unindexed,
very skinny table.  If I hacked XLogInsert so that it did nothing but take
the WALInsertLock, release it, then return a fake RecPtr, it scaled better
but still not very well.  So giant leaps in throughput would need to involve
calling XLogInsert less often (or at least taking the WALInsertLock less
often).  You could nibble around the edges by tweaking what happens under
the WALInsertLock, but I don't think that that will get you big wins by
doing that for this case.  But again, how important is this case?  Are bulk
loads into skinny unindexed tables the best test-bed for improving
XLogInsert?

(Sorry, I think I forgot to change the subject on previous message.  Digests
are great if you only read, but for contributing I guess I have to change to
receiving each message)

Jeff

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