Re: Bgwriter LRU cleaning: we've been going at this all wrong

Added to TODO:

* Consider wither increasing BM_MAX_USAGE_COUNT improves performance

http://archives.postgresql.org/pgsql-hackers/2007-06/msg01007.php

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Gregory Stark wrote:
>
> "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>
> > I don't really see why it's "overkill".
>
> Well I think it may be overkill in that we'll be writing out buffers that
> still have a decent chance of being hit again. Effectively what we'll be doing
> in the approximated LRU queue is writing out any buffer that reaches the 80%
> point down the list. Even if it later gets hit and pulled up to the head
> again.
>
> I suppose that's not wrong though, the whole idea of the clock sweep is that
> that's precisely the level of precision to which it makes sense to approximate
> the LRU. Ie, that any point in the top 20% is equivalent to any other and when
> we use a buffer we want to promote it to somewhere "near" the head but any
> point in the top 20% is good enough. Then any point in the last 20% should be
> effectively "good enough" too be considered a target buffer to clean as well.
>
> If we find it's overkill then what we should consider doing is raising
> BM_MAX_USAGE_COUNT. That's effectively tuning the percentage of the lru chain
> that we decide we try to keep clean.
>
> --
> Gregory Stark
> EnterpriseDB http://www.enterprisedb.com
>
>
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