Re: Optimize LISTEN/NOTIFY

On Thu, Nov 6, 2025, at 09:33, Joel Jacobson wrote:
> On Thu, Nov 6, 2025, at 00:21, Chao Li wrote:
>> That’s what we don’t know. We now lack a performance test for
>> evaluating how “direct advancement” efficiently helps if it only
>> handles sleeping listeners. So what I was suggesting is that we should
>> first create some tests, maybe also add a few more statistics, so that
>> we can evaluate different solutions. If a simple implementation that
>> only handles sleeping listeners would have performed good enough, of
>> course we can take it; otherwise we may need to either pursue a better
>> solution.

Changes since v23:

* The advancingPos flag has been split into two fields:
    bool isAdvancing: indicates if a backend is currently advancing
    QueuePosition advancingPos: the target position the backend will advance to

* The logic in SignalBackends has been reworked and simplified,
  thanks to the new isAdvancing and advancingPos fields.
  I now think it's finally easy to reason about why each branch
  in SignalBackends must be correct.

I've also attached 0003-optimize_listen_notify-v24.txt that adds
instrumentation that can then be used together with the
benchmark/correctness tool pg_async_notify_test-v24.c.
This has been very helpful to me, to develop an intuition for its
concurrency behavior. I hope it can help others as well.
The 0003 patch is only for testing and not part of the patchset,
hence the .txt.

% gcc -Wall -Wextra -O2 -pthread \
  -I$(pg_config --includedir-server) \
  -I$(pg_config --includedir) \
  -L$(pg_config --libdir) \
  -o pg_async_notify_test-v24 pg_async_notify_test-v24.c \
  -lpq -pthread -lm

% ./pg_async_notify_test-v24 --listeners 10 --notifiers 10 --channels 10 --duration 10
10 s: 301622 sent (29409/s), 3015940 received (294185/s)
Notification Latency Distribution:
 0.00-0.01ms                0 (0.0%) avg: 0.000ms
 0.01-0.10ms     #          25 (0.0%) avg: 0.074ms
 0.10-1.00ms     #          289 (0.0%) avg: 0.461ms
 1.00-10.00ms    #########  2824257 (93.6%) avg: 4.193ms
 10.00-100.00ms  #          189923 (6.3%) avg: 24.893ms
>100.00ms       #          1453 (0.0%) avg: 109.662ms

SignalBackends Statistics:
signaled_targeted  #          1251569 (9.4%)
advancing_behind   #          108505 (0.8%)
advancing_ahead    #          207494 (1.6%)
idle_behind        #          408641 (3.1%)
avoided_wakeups    #######    10589740 (79.6%)
already_ahead      #          744087 (5.6%)

asyncQueueReadAllNotifications Statistics:
necessary_wakeups    ########   1525695 (86.3%)
unnecessary_wakeups  #          242106 (13.7%)

/Joel