Re: AW: CommitDelay performance improvement

At 10:56 27/02/01 +0100, Zeugswetter Andreas SB wrote:
>
>> > I agree that 30k looks like the magic delay, and probably 30/5 would be a
>> > good conservative choice. But now I think about the choice of number, I
>> > think it must vary with the speed of the machine and length of the
>> > transactions; at 20tps, each TX is completing in around 50ms.
>
>I think disk speed should probably be the main factor.
>After the first run 30k/5 also seemed the best here, but running the test
>again shows, that the results are only reproducible after a new initdb.
>Anybody else see reproducible results without previous initdb ?


I think we want something that reflects the chance of a time-saving as a
result of a wait, which is why I suggested having each backend monitor
commits/sec, then basing the delay on some % of that number. eg. if
commits/sec = 1, then it's either low-load, or long tx's, in either case
CommitDelay won't help. Similarly, if we have 1000 commits/sec, then we
have a very fast system and/or disk, and CommitDelay of 10ms is clearly
glacial. 

AFAICS, dynamically monitoring commits/sec (or a similar statistic) is
TOWTG, but in all cases we need to set a max on CommitDelay to prevent
individual TXs getting too long (although I am unsure if the latter is
*really* necessary, it is far better to be safe).

Note: commits/sec need to be kept for each backend so we can remove the
contribution of the backend that is considering waiting.


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