Re: suboverflowed subtransactions concurrency performance optimize

On Mon, 30 Aug 2021 at 11:25, Andrey Borodin <x4mmm@yandex-team.ru> wrote:
>
> Hi Pengcheng!
>
> You are solving important problem, thank you!
>
> > 30 авг. 2021 г., в 13:43, Pengchengliu <pengchengliu@tju.edu.cn> написал(а):
> >
> > To resolve this performance problem, we think about a solution which cache
> > SubtransSLRU to local cache.
> > First we can query parent transaction id from SubtransSLRU, and copy the
> > SLRU page to local cache page.
> > After that if we need query parent transaction id again, we can query it
> > from local cache directly.
>
> A copy of SLRU in each backend's cache can consume a lot of memory.

Yes, copying the whole SLRU into local cache seems overkill.

> Why create a copy if we can optimise shared representation of SLRU?

transam.c uses a single item cache to prevent thrashing from repeated
lookups, which reduces problems with shared access to SLRUs.
multitrans.c also has similar.

I notice that subtrans. doesn't have this, but could easily do so.
Patch attached, which seems separate to other attempts at tuning.

On review, I think it is also possible that we update subtrans ONLY if
someone uses >PGPROC_MAX_CACHED_SUBXIDS.
This would make subtrans much smaller and avoid one-entry-per-page
which is a major source of cacheing.
This would means some light changes in GetSnapshotData().
Let me know if that seems interesting also?

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