RE: Wake up autovacuum launcher from postmaster when a worker exits - Mailing list pgsql-hackers

> On Thu, Jan 8, 2026 at 11:57 AM Heikki Linnakangas <hlinnaka@iki.fi> wrote:
> >
> > When an autovacuum worker exits, ProcKill() sends SIGUSR2 to the
> > launcher. I propose moving that responsibility to the postmaster, because:
> >
> > * It's simpler IMHO
> >
> > * The postmaster is already responsible for sending the signal if
> > fork() fails
> >
> > * It makes it consistent with background workers. When a background
> > worker exits, the postmaster sends the signal to the launching process
> > (if requested).
> >
> > * Postmaster doesn't need to worry about sending the signal to the
> > wrong process if the launcher's PID is reused, because it always has
> > up-to-date PID information, because the launcher is postmaster's child
> > process. That risk was negligible to begin with, but this eliminates
> > completely, so we don't need the comment excusing it it anymore.
> 
> It sounds reasonable to me too. +1.
> 
> Regards,
> 
> --
> Masahiko Sawada
> Amazon Web Services: https://aws.amazon.com
> 
Hi all,

I have completed the testing for this patch with a focus on the signaling logic from postmaster to launcher.

Test Environment:
- autovacuum_max_workers = 1
- autovacuum_naptime = 5s
- Multiple databases (4 total)

Observations:
I performed cross-database vacuum tests. With the 1-worker limit, I observed that the launcher starts the next worker
fora pending database immediately (within the ~1.25s scheduled stagger) after the previous worker exits.
 
The logs show a seamless handover between worker processes (e.g., Worker A exits at 16:06:20.209, and the next
scheduledWorker B starts at 16:06:21.447). 
 

2026-01-09 16:06:20.209 CST [1918017] LOG:  automatic vacuum of table "db_test2.public.t2": index scans: 0
    pages: 222 removed, 0 remain, 222 scanned (100.00% of total), 0 eagerly scanned
    tuples: 50000 removed, 0 remain, 0 are dead but not yet removable
    removable cutoff: 813, which was 1 XIDs old when operation ended
    new relfrozenxid: 813, which is 3 XIDs ahead of previous value
    frozen: 0 pages from table (0.00% of total) had 0 tuples frozen
    visibility map: 222 pages set all-visible, 222 pages set all-frozen (0 were all-visible)
    index scan not needed: 0 pages from table (0.00% of total) had 0 dead item identifiers removed
    avg read rate: 0.000 MB/s, avg write rate: 2.794 MB/s
    buffer usage: 697 hits, 0 reads, 4 dirtied
    WAL usage: 450 records, 4 full page images, 158352 bytes, 32768 full page image bytes, 0 buffers full
    memory usage: dead item storage 0.02 MB accumulated across 0 resets (limit 64.00 MB each)
    system usage: CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.01 s
2026-01-09 16:06:21.447 CST [1918022] DEBUG:  autovacuum: processing database "template1"
2026-01-09 16:06:22.699 CST [1918027] DEBUG:  autovacuum: processing database "postgres"
2026-01-09 16:06:23.946 CST [1918044] DEBUG:  autovacuum: processing database "db_test1"
2026-01-09 16:06:23.958 CST [1918044] LOG:  automatic vacuum of table "db_test1.public.t1": index scans: 0
    pages: 222 removed, 0 remain, 222 scanned (100.00% of total), 0 eagerly scanned
    tuples: 50000 removed, 0 remain, 0 are dead but not yet removable
    removable cutoff: 814, which was 1 XIDs old when operation ended
    new relfrozenxid: 814, which is 7 XIDs ahead of previous value
    frozen: 0 pages from table (0.00% of total) had 0 tuples frozen
    visibility map: 222 pages set all-visible, 222 pages set all-frozen (0 were all-visible)
    index scan not needed: 0 pages from table (0.00% of total) had 0 dead item identifiers removed
    avg read rate: 0.000 MB/s, avg write rate: 2.793 MB/s
    buffer usage: 697 hits, 0 reads, 4 dirtied
    WAL usage: 450 records, 4 full page images, 158352 bytes, 32768 full page image bytes, 0 buffers full
    memory usage: dead item storage 0.02 MB accumulated across 0 resets (limit 64.00 MB each)
    system usage: CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.01 s

This confirms that the postmaster successfully notifies the launcher and the worker slot is freed appropriately before
thenotification.
 

The patch looks correct and robust. +1 from my side.

Best Regards,
Yuan Li(carol)