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Re: Caching (was Re: choosing the right platform)

From: Jean-Luc Lachance <jllachan(at)nsd(dot)ca>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: jim(at)nasby(dot)net, "scott(dot)marlowe" <scott(dot)marlowe(at)ihs(dot)com>,Matthew Nuzum <cobalt(at)bearfruit(dot)org>,"'Josh Berkus'" <josh(at)agliodbs(dot)com>,"'Pgsql-Performance'" <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Caching (was Re: choosing the right platform)
Date: 2003-04-10 14:27:16
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance

What appends when PG scans a table that is is too big to fit in the
Won't the whole cache get trashed and swapped off to disk?
Shouldn't there be a way to lock some tables in PG cache?
Who about caracterizing some of the RAM like: scan, index, small
frequently used tables.


Tom Lane wrote:
> [...]
> PG is *not* any smarter about the usage patterns of its disk buffers
> than the kernel is; it uses a simple LRU algorithm that is surely no
> brighter than what the kernel uses.  (We have looked at smarter buffer
> recycling rules, but failed to see any performance improvement.)  So the
> notion that PG can do a better job of cache management than the kernel
> is really illusory.  About the only advantage you gain from having data
> directly in PG buffers rather than kernel buffers is saving the CPU
> effort needed to move data across the userspace boundary --- which is
> not zero, but it's sure a lot less than the time spent for actual I/O.
> So my take on it is that you want shared_buffers fairly small, and let
> the kernel do the bulk of the heavy lifting for disk cache.  That's what
> it does for a living, so let it do what it does best.  You only want
> shared_buffers big enough so you don't spend too many CPU cycles shoving
> data back and forth between PG buffers and kernel disk cache.  The
> default shared_buffers setting of 64 is surely too small :-(, but my
> feeling is that values in the low thousands are enough to get past the
> knee of that curve in most cases.

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