Autovacuum launcher process launches worker process at high frequency

Hi all,

I found the kind of strange behaviour of the autovacuum launcher
process when XID anti-wraparound vacuum.

Suppose that a database (say test_db) whose age of frozenxid is about
to reach max_autovacuum_max_age has three tables T1 and T2.
T1 is very large and is frequently updated, so vacuum takes long time
for vacuum.
T2 is static and already frozen table, thus vacuum can skip to vacuum
whole table.
And anti-wraparound vacuum was already executed on other databases.

Once the age of datfrozenxid of test_db exceeded
max_autovacuum_max_age, autovacuum launcher launches worker process in
order to do anti-wraparound vacuum on testdb.
A worker process assigned to test_db begins to vacuum T1, it takes long time.
Meanwhile another worker process is assigned to test_db and completes
to vacuum on T2 and exits.

After for while, the autovacuum launcher launches new worker again and
worker is assigned to test_db again.
But that worker exits quickly because there is no table we need to
vacuum. (T1 is being vacuumed by another worker process).
When new worker process starts, worker process sends SIGUSR2 signal to
launcher process to wake up him.
Although the launcher process executes WaitLatch() after launched new
worker, it is woken up and launches another new worker process soon
again.
As a result, launcher process launches new worker process at extremely
high frequency regardless of autovacuum_naptime, which increase cpu
use rate.

Why does auto vacuum worker need to wake up launcher process after started?

autovacuum.c:L1604
/* wake up the launcher */
if (AutoVacuumShmem->av_launcherpid != 0)
kill(AutoVacuumShmem->av_launcherpid, SIGUSR2);

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center