Re: Autotuning Group Commit
От | Simon Riggs |
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Тема | Re: Autotuning Group Commit |
Дата | |
Msg-id | 1106383657.31592.76.camel@localhost.localdomain обсуждение исходный текст |
Ответ на | Re: Autotuning Group Commit ("Jim C. Nasby" <decibel@decibel.org>) |
Ответы |
Re: Autotuning Group Commit
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Список | pgsql-hackers |
On Sat, 2005-01-22 at 00:18 -0600, Jim C. Nasby wrote: > 1) I'm in favor of autotuning anything possible. > 2) In addition to turning group_commit on and off, what about also > adjusting the commit delay, based on statistics of recent commits? It > might require a slightly larger sample set (maybe the last 100 commits), > but it seems it would provide more flexibility (hence more usefulness) > to the autotuning. > > I belive you'd want to first calculate the elapsed time between each > commit in the sample set, then look for groupings of elapsed time. If > you have a set that looks like this: > > Time (ms) Number > 2 * > 4 * > 6 > 8 ** > 10 * > 12 ****** > 14 **** > 16 ** > 18 > 20 * > > then you'd want a delay of 16ms. I think this calculation could be done > fairly quickly by grouping the commits into buckets of different elapsed > times, then look for the largest elapsed time that has a number of > commits greater than the mean number of commits for all the buckets. But > I'm not a statistician, hopefully someone here is. :) Yes, I considered that, but since we're talking about a frequently executed piece of code, I was hoping to keep it short and sweet. What do others think? The other issue is the likely time granularity for many OS will be 10ms anyway, so you have a choice of 0, 10, 20, 30ms... Overall, group commit isn't much use until the log disk is getting very busy. Delays don't really need to be more than a disk rotation, so even a laptop can manage 11ms between sequential writes. I'd suggest hardcoding commit_delay at 10ms, but we still need an on/off switch, so it seems sensible to keep it. We may be in a better position to use fine grained settings in the future. -- Best Regards, Simon Riggs
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