Re: walsender performance regression due to logical decoding on standby changes

Hi,

On 2023-05-17 12:53:15 -0700, Andres Freund wrote:
> I'll try to come up with a benchmark without the issues I pointed out in
> https://postgr.es/m/20230517194331.ficfy5brpfq5lrmz%40awork3.anarazel.de

Here we go:

setup:

create primary
SELECT pg_create_physical_replication_slot('reserve', true);
create standby using pg_basebackup

create WAL:
psql -c CREATE TABLE testtable_logged(other_data int default 1);' && \
  c=16; PGOPTIONS='-c synchronous_commit=off' /path-to-pgbench --random-seed=0 -n -c$c -j$c -t1000 -P1 -f <( echo
"INSERTINTO testtable_logged SELECT generate_series(1, 1000)" ) && \
 
  psql -c "SELECT pg_create_restore_point('end');"

benchmark:
rm -rf /tmp/test && \
  cp -ar /srv/dev/pgdev-dev-standby /tmp/test && \
  cp -ar /srv/dev/pgdev-dev/pg_wal/* /tmp/test/pg_wal/ && \
  sync && \
  /usr/bin/time -f '%es' /path-to-postgres -D /tmp/test -c recovery_target_action=shutdown -c recovery_target_name=end
-cshared_buffers=1GB -c fsync=off
 

That way I can measure how long it takes to replay exactly the same WAL, and
also take profiles of exactly the same work, without influencing time results.

I copy the WAL files to the primary to ensure that walreceiver (standby) /
walsender (primary) performance doesn't make the result variability higher.


                      max_walsenders=10     max_walsenders=100
e101dfac3a5 reverted  7.01s                 7.02s
093e5c57d50 / HEAD    8.25s                 19.91s
bharat-v3             7.14s            7.13s

So indeed, bharat-v3 largely fixes the issue.

The regression of v3 compared to e101dfac3a5 reverted seems pretty constant at
~0.982x, independent of the concrete max_walsenders value. Which makes sense,
the work is constant.


To make it more extreme, I also tested a workload that is basically free to replay:

c=16; /srv/dev/build/m-opt/src/bin/pgbench/pgbench --random-seed=0 -n -c$c -j$c -t5000 -P1 -f <( echo "SELECT
pg_logical_emit_message(false,'c', 'a') FROM generate_series(1, 1000)" ) && psql -c "SELECT
pg_create_restore_point('end');"


                      max_walsenders=10     max_walsenders=100
e101dfac3a5 reverted  1.70s                 1.70s
093e5c57d50 / HEAD    3.00s                 14.56s
bharat-v3             1.88s                 1.88s

In this extreme workload we still regress by ~0.904x.

I'm not sure how much it's worth worrying about that - this is a quite
unrealistic testcase.

FWIW, if I just make WalSndWakeup() do nothing, I still see a very small, but
reproducible, overhead: 1.72s - that's just the cost of the additional
external function call.

If I add a no-waiters fastpath using proclist_is_empty() to
ConditionVariableBroadcast(), I get 1.77s. So the majority of the remaining
slowdown indeed comes from the spinlock acquisition in
ConditionVariableBroadcast().

Greetings,

Andres Freund