Re: Slow standby snapshot

On Fri, 16 Sept 2022 at 17:08, Michail Nikolaev
<michail(dot)nikolaev(at)gmail(dot)com> wrote:
>
> Hello everyone.
>
> To find the best frequency for calling KnownAssignedXidsCompress in
> Simon's patch, I made a set of benchmarks. It looks like each 8th xid
> is a little bit often.
>
> Setup and method is the same as previous (1). 16-core machines,
> max_connections = 5000. Tests were running for about a day, 220 runs
> in total (each version for 20 times, evenly distributed throughout the
> day).
>
> Staring from 60th second, 30 seconds-long transaction was started on primary.
>
> Graphs in attachment. So, looks like 64 is the best value here. It
> gives even a little bit more TPS than smaller values.
>
> Yes, such benchmark does not cover all possible cases, but it is
> better to measure at least something when selecting constants :)

This is very good and clear, thank you.

> If someone has an idea of different benchmark scenarios - please share them.

> So, updated version (with 64 and some commit message) in attachment too.
>
> [1]: https://www.postgresql.org/message-id/flat/CANtu0ohzBFTYwdLtcanWo4%2B794WWUi7LY2rnbHyorJdE8_ZnGg%40mail.gmail.com#379c1be7b8134ada5a574078d51b64c6

I've cleaned up the comments and used a #define for the constant.

IMHO this is ready for commit. I will add it to the next CF.

--
Simon Riggs http://www.EnterpriseDB.com/