Message-ID: <492D4FF0.2000606@enterprisedb.com>
Date: Wed, 26 Nov 2008 15:32:32 +0200
Organization: EnterpriseDB
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To: Tom Lane <tgl@sss.pgh.pa.us>
CC: PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: Visibility map, partial vacuums
References: <4905AE17.7090305@enterprisedb.com>
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	<4925664C.3090605@enterprisedb.com>
	<26361.1227467112@sss.pgh.pa.us>
	<492A6032.6080000@enterprisedb.com>
	<18086.1227537479@sss.pgh.pa.us>
	<492D4460.1000809@enterprisedb.com> <5856.1227705135@sss.pgh.pa.us>
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From: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>

Tom Lane wrote:
> Heikki Linnakangas <heikki.linnakangas@enterprisedb.com> writes:
>> There is another problem, though, if the map is frequently probed for 
>> pages that don't exist in the map, or the map doesn't exist at all. 
>> Currently, the size of the map file is kept in relcache, in the 
>> rd_vm_nblocks_cache variable. Whenever a page is accessed that's > 
>> rd_vm_nblocks_cache, smgrnblocks is called to see if the page exists, 
>> and rd_vm_nblocks_cache is updated. That means that every probe to a 
>> non-existing page causes an lseek(), which isn't free.
> 
> Well, considering how seldom new pages will be added to the visibility
> map, it seems to me we could afford to send out a relcache inval event
> when that happens.  Then rd_vm_nblocks_cache could be treated as
> trustworthy.
> 
> Maybe it'd be worth doing that for the FSM too.  The frequency of
> invals would be higher, but then again the reference frequency is
> probably higher too?

A relcache invalidation sounds awfully heavy-weight. Perhaps a 
light-weight invalidation event that doesn't flush the entry altogether, 
but just resets the cached sizes?

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com