Re: Synchronizing slots from primary to standby

On Fri, Mar 8, 2024 at 9:56 AM Ajin Cherian <itsajin(at)gmail(dot)com> wrote:
>
>> Pushed with minor modifications. I'll keep an eye on BF.
>>
>> BTW, one thing that we should try to evaluate a bit more is the
>> traversal of slots in StandbySlotsHaveCaughtup() where we verify if
>> all the slots mentioned in standby_slot_names have received the
>> required WAL. Even if the standby_slot_names list is short the total
>> number of slots can be much larger which can lead to an increase in
>> CPU usage during traversal. There is an optimization that allows to
>> cache ss_oldest_flush_lsn and ensures that we don't need to traverse
>> the slots each time so it may not hit frequently but still there is a
>> chance. I see it is possible to further optimize this area by caching
>> the position of each slot mentioned in standby_slot_names in
>> replication_slots array but not sure whether it is worth.
>>
>>
>
> I tried to test this by configuring a large number of logical slots while making sure the standby slots are at the end of the array and checking if there was any performance hit in logical replication from these searches.
>

Thanks Ajin and Nisha.

We also plan:
1) Redoing XLogSendLogical time-log related test with
'sync_replication_slots' enabled.
2) pg_recvlogical test to monitor lag in StandbySlotsHaveCaughtup()
for a large number of slots.
3) Profiling to see if StandbySlotsHaveCaughtup() is noticeable in the
report when there are a large number of slots to traverse.

thanks
Shveta