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Re: Background LRU Writer/free list

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Greg Smith <gsmith(at)gregsmith(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Background LRU Writer/free list
Date: 2007-09-26 08:26:30
Message-ID: 200709260826.l8Q8QUF06377@momjian.us (view raw or flat)
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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

-- 
  Bruce Momjian  <bruce(at)momjian(dot)us>          http://momjian.us
  EnterpriseDB                               http://www.enterprisedb.com

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

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