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Re: Problem with 8.4 stats collector high load

From: Jakub Ouhrabka <kuba(at)comgate(dot)cz>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: Problem with 8.4 stats collector high load
Date: 2010-02-16 19:16:46
Message-ID: 4B7AEF1E.1060206@comgate.cz (view raw or flat)
Thread:
Lists: pgsql-hackers
 > Ideally, autovacuum would only request a new copy of the file if the
 > one it got was considerably out of date.  Obviously a tenth of a
 > second is not old enough.

I've tried to look at it and found that's already implemented - see 
autovac_refresh_stats(). STATS_READ_DELAY which is set to 1s. Am I 
reading the code correctly? If so then 1s is not enough for big clusters.

I guess it would be feasible to crank STATS_READ_DELAY up a little bit, 
say to 10s. What do you think?

Kuba

Dne 16.2.2010 19:59, Alvaro Herrera napsal(a):
> Jakub Ouhrabka wrote:
>>> Maybe you should decrease naptime a bit.
>>
>> That did the trick, thanks!
>>
>>> Yes.  There were some changes that needed to be done to autovacuum so
>>> that it didn't read the stats file too often, but I don't recall if I
>>> got around to it.
>>
>> I looked at the strace output and there are *writes* to the file not
>> reads. Why? Is it a consequence of this optimization?
>>
>> Release notes 8.4:
>>
>> Reduce I/O load of writing the statistics collection file by writing
>> the file only when requested (Martin Pihlak)
>>
>> Was autovacuum requesting to write this 20MB file 650x per minute?
>
> Yes, exactly.
>
> Ideally, autovacuum would only request a new copy of the file if the one
> it got was considerably out of date.  Obviously a tenth of a second is
> not old enough.
>

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