Re: SubtransControlLock and performance problems

On Mon, 2020-02-17 at 15:03 +0000, Lars Aksel Opsahl wrote:
> I have tested in branch ( https://github.com/larsop/resolve-overlap-and-gap/tree/add_postgis_topology_using_func)
> where I use only have functions and no procedures and I still have the same problem with subtransaction locks.
>
> Can I based on this assume that the problem is only related to exceptions ?

No, it is related to BEGIN ... EXCEPTION ... END blocks, no matter if
an exception is thrown or not.

As soon as execution enters such a block, a subtransaction is started.

> Does this mean that if have 32 threads running in parallel and I get 2 exceptions in each thread I have reached a state where I will get contention ?

No, it means that if you enter a block with an EXCEPTION clause more
than 64 times in a single transaction, performance will drop.

> Is it any way increase from 64 to a much higher level, when compiling the code ?

Yes, you can increase PGPROC_MAX_CACHED_SUBXIDS in src/include/storage/proc.h

> Basically what I do here is that I catch exceptions when get them and tries to solve the problem in a alternative way.

Either use shorter transactions, or start fewer subtransactions.

Yours,
Laurenz Albe
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