Re: Slow standby snapshot

Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2022-11-15 23:14:42 +0000, Simon Riggs wrote:
>> Hence more frequent compression is effective at reducing the overhead.
>> But too frequent compression slows down the startup process, which
>> can't then keep up.
>> So we're just looking for an optimal frequency of compression for any
>> given workload.

> What about making the behaviour adaptive based on the amount of wasted effort
> during those two operations, rather than just a hardcoded "emptiness" factor?

Not quite sure how we could do that, given that those things aren't even
happening in the same process. But yeah, it does feel like the proposed
approach is only going to be optimal over a small range of conditions.

> I don't think the xids % KAX_COMPRESS_FREQUENCY == 0 filter is a good idea -
> if you have a workload with plenty subxids you might end up never compressing
> because xids divisible by KAX_COMPRESS_FREQUENCY will end up as a subxid
> most/all of the time.

Yeah, I didn't think that was too safe either. It'd be more reliable
to use a static counter to skip all but every N'th compress attempt
(something we could do inside KnownAssignedXidsCompress itself, instead
of adding warts at the call sites).

regards, tom lane