Re: Background LRU Writer/free list

This has been saved for the 8.4 release:

http://momjian.postgresql.org/cgi-bin/pgpatches_hold

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Greg Smith wrote:
> I'm mostly done with my review of the "Automatic adjustment of
> bgwriter_lru_maxpages" patch. In addition to issues already brought up
> with that code, there are some small things that need to be done to merge
> it with the recent pg_stat_bgwriter patch, and I have some concerns about
> its unbounded scanning of the buffer pool; I'll write that up in more
> detail or just submit an improved patch as I get time this week.
>
> But there's a fundamental question that has been bugging me, and I think
> it impacts the direction that code should take. Unless I'm missing
> something in my reading, buffers written out by the LRU writer aren't ever
> put onto the free list. I assume this is to stop from prematurely
> removing buffers that contain useful data. In cases where a substantial
> percentage of the buffer cache is dirty, the LRU writer has to scan a
> significant portion of the pool looking for one of the rare clean buffers,
> then write it out. When a client goes to grab a free buffer afterward, it
> has to scan the same section of the pool to find the now clean buffer,
> which seems redundant.
>
> With the new patch, the LRU writer is fairly well bounded in that it
> doesn't write out more than it thinks it will need; you shouldn't get into
> a situation where many more pages are written than will be used in the
> near future. Given that mindset, shouldn't pages the LRU scan writes just
> get moved onto the free list?
>
> --
> * Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD
>
> ---------------------------(end of broadcast)---------------------------
> TIP 5: don't forget to increase your free space map settings

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