Re: Assertion failure in SnapBuildInitialSnapshot()

On Thu, Dec 8, 2022 at 8:17 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Mon, Nov 21, 2022 at 4:31 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> One idea to fix this issue is that in
> ReplicationSlotsComputeRequiredXmin(), we compute the minimum xmin
> while holding both ProcArrayLock and ReplicationSlotControlLock, and
> release only ReplicationSlotsControlLock before updating the
> replication_slot_xmin. I'm concerned it will increase the contention
> on ProcArrayLock but I've attached the patch for discussion.
>

But what kind of workload are you worried about? This will be called
while processing XLOG_RUNNING_XACTS to update
procArray->replication_slot_xmin/procArray->replication_slot_catalog_xmin
only when required. So, if we want we can test some concurrent
workloads along with walsenders doing the decoding to check if it
impacts performance.

What other way we can fix this? Do you think we can try to avoid
retreating xmin values in ProcArraySetReplicationSlotXmin() to avoid
this problem? Personally, I think taking the lock as proposed by your
patch is a better idea. BTW, this problem seems to be only logical
replication specific, so if we are too worried then we can change this
locking only for logical replication.

--
With Regards,
Amit Kapila.