On Tue, 26 Jun 2007, Heikki Linnakangas wrote:

How much of the buffer cache do you think we should try to keep clean? And how large a percentage of the buffer cache do you think have usage_count=0 at any given point in time?


What I discovered is that most of the really bad checkpoint pause cases I ran into involved most of the buffer cache being dirty while also having a non-zero usage count, which left the background writer hard-pressed to work usefully (the LRU writer couldn't do anything, and the all-scan was writing wastefully). I was seeing >90% dirty+usage_count>0 in the really ugly spots.

What I like about Tom's idea is that it will keep the LRU writer in the best possible zone for that case (writing out madly right behind the LRU sweeper as counts get to zero) while still being fine on the more normal ones like you describe. In particular, it should cut down on how much client backends write buffers in an overloaded case considerably.

That will vary widely depending on your workload, of course, but keeping 1/4 of the buffer cache clean seems like overkill to me.

What may need to happen here is to add Tom's approach, but perhaps restrain it using the current auto-tuning LRU patch's method of estimating how many clean buffers are needed in the near future. Particularly on large buffer caches, the idea of getting so far ahead of the sweep that you're looping all the away around and following right behind the clock sweep point may be overkill, but I think it will help enormously on smaller caches that are often very dirty.

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* Greg Smith [EMAIL PROTECTED] http://www.gregsmith.com Baltimore, MD

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