Tom Lane wrote:
Heikki Linnakangas <[EMAIL PROTECTED]> writes:
On further thought, there is one workload where removing the non-LRU part would be counterproductive:


If you have a system with a very bursty transaction rate, it's possible that when it's time for a checkpoint, there hasn't been any WAL logged activity since last checkpoint, so we skip it. When that happens, the buffer cache might still be full of dirty pages, because of hint bit updates. That still isn't a problem on it's own, but if you then do a huge batch update, you have to flush those dirty pages at that point. It would be better to trickle flush those dirty pages during the idle period.

But wouldn't the LRU-based scan accomplish that?

It only scans bgwriter_lru_percent buffers ahead of the clock hand. If the hand isn't moving, it keeps scanning the same buffers over and over again. You can crank it all the way up to 100%, though, in which case it would work, but that starts to get expensive CPU-wise.

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

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