On Tue, Jul 23, 2024 at 10:35 AM Amit Kapila <amit.kapil...@gmail.com> wrote: > > On Tue, Jul 9, 2024 at 12:39 AM John H <johnh...@gmail.com> wrote: > > > > > Out of curiosity, did you compare with standby_slot_names_from_syncrep > > > set to off > > > and standby_slot_names not empty? > > > > I didn't think 'standby_slot_names' would impact TPS as much since > > it's not grabbing the SyncRepLock but here's a quick test. > > Writer with 5 synchronous replicas, 10 pg_recvlogical clients and > > pgbench all running from the same server. > > > > Command: pgbench -c 4 -j 4 -T 600 -U "ec2-user" -d postgres -r -P 5 > > > > Result with: standby_slot_names = > > 'replica_1,replica_2,replica_3,replica_4,replica_5' > > > > latency average = 5.600 ms > > latency stddev = 2.854 ms > > initial connection time = 5.503 ms > > tps = 714.148263 (without initial connection time) > > > > Result with: standby_slot_names_from_syncrep = 'true', > > synchronous_standby_names = 'ANY 3 (A,B,C,D,E)' > > > > latency average = 5.740 ms > > latency stddev = 2.543 ms > > initial connection time = 4.093 ms > > tps = 696.776249 (without initial connection time) > > > > Result with nothing set: > > > > latency average = 5.090 ms > > latency stddev = 3.467 ms > > initial connection time = 4.989 ms > > tps = 785.665963 (without initial connection time) > > > > Again I think it's possible to improve the synchronous numbers if we > > cache but I'll try that out in a bit. > > > > Okay, so the tests done till now conclude that we won't get the > benefit by using 'standby_slot_names_from_syncrep'. Now, if we > increase the number of standby's in both lists and still keep ANY 3 in > synchronous_standby_names then the results may vary. We should try to > find out if there is a performance benefit with the use of > synchronous_standby_names in the normal configurations like the one > you used in the above tests to prove the value of this patch. >
I didn't fully understand the parameters mentioned above, specifically what 'latency stddev' and 'latency average' represent.. But shouldn't we see the benefit/value of this patch by having a setup where a particular standby is slow in sending the response back to primary (could be due to network lag or other reasons) and then measuring the latency in receiving changes on failover-enabled logical subscribers? We can perform this test with both of the below settings and say make D and E slow in sending responses: 1) synchronous_standby_names = 'ANY 3 (A,B,C,D,E)' 2) standby_slot_names = A_slot, B_slot, C_slot, D_slot, E_slot. thanks Shveta
