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Re: [HACKERS] Automatically setting work_mem

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Luke Lonergan <llonergan(at)greenplum(dot)com>, pgsql-patches(at)postgresql(dot)org, Martijn van Oosterhout <kleptog(at)svana(dot)org>, Qingqing Zhou <zhouqq(at)cs(dot)toronto(dot)edu>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: [HACKERS] Automatically setting work_mem
Date: 2006-04-22 03:07:07
Message-ID: 200604220307.k3M377W05944@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
Where are we on this patch?

---------------------------------------------------------------------------

Simon Riggs wrote:
> On Tue, 2006-03-21 at 17:47 -0500, Tom Lane wrote:
> 
> > I'm fairly unconvinced about Simon's underlying premise --- that we
> > can't make good use of work_mem in sorting after the run building phase
> > --- anyway.  
> 
> We can make good use of memory, but there does come a point in final
> merging where too much is of no further benefit. That point seems to be
> at about 256 blocks per tape; patch enclosed for testing. (256 blocks
> per tape roughly doubles performance over 32 blocks at that stage).
> 
> That is never the case during run building - more is always better.
> 
> > If we cut back our memory usage 
> Simon inserts the words: "too far"
> > then we'll be forcing a
> > significantly more-random access pattern to the temp file(s) during
> > merging, because we won't be able to pre-read as much at a time.
> 
> Yes, thats right.
> 
> If we have 512MB of memory that gives us enough for 2000 tapes, yet the
> initial runs might only build a few runs. There's just no way that all
> 512MB of memory is needed to optimise the performance of reading in a
> few tapes at time of final merge.
> 
> I'm suggesting we always keep 2MB per active tape, or the full
> allocation, whichever is lower. In the above example that could release
> over 500MB of memory, which more importantly can be reused by subsequent
> sorts if/when they occur.
> 
> 
> Enclose two patches:
> 1. mergebuffers.patch allows measurement of the effects of different
> merge buffer sizes, current default=32
> 
> 2. reassign2.patch which implements the two kinds of resource
> deallocation/reassignment proposed.
> 
> Best Regards, Simon Riggs
> 

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-- 
  Bruce Momjian   http://candle.pha.pa.us
  EnterpriseDB    http://www.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +

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