Reduce SyncRepLock contention on the commit path

Hi hackers,

On a primary with synchronous replication, every commit that wrote WAL
goes through SyncRepWaitForLSN(), and every standby reply goes through
SyncRepReleaseWaiters(). Both take SyncRepLock exclusively, although
much of their work does not need to happen under the lock.

The attached series moves that work out of the critical section. The
four patches are independent; only 0003 changes observable behaviour.

0001 -- Wake released waiters after dropping the queue lock.

SyncRepWakeQueue() sets each released backend's latch while holding
SyncRepLock. Setting a latch may call kill(), so at high commit rates
the walsender makes one syscall per released commit while committers
queue on the same lock.

The patch collects released procs in a list and wakes them after dropping
the lock, as ProcArrayGroupClearXid() already does with ProcArrayLock.
The unlink, write barrier, and state update remain under the lock: a
waiter reads syncRepState without it and must not see itself as completed
while still on the queue.

0002 -- Compute synced positions before taking the queue lock.

SyncRepReleaseWaiters() takes SyncRepLock before scanning the walsender
slots for synced write, flush, and apply positions. That scan takes a
spinlock per slot, allocates memory, and sorts quorum results.

The patch moves the scan before the lock. The consumers only move lsn[]
forward, so a result that becomes stale while waiting for the lock simply
does not advance it. The patch also avoids the lock when the walsender
is not a synchronous standby.

0003 -- Release waiters once per drained batch of standby replies.

ProcessStandbyReplyMessage() calls SyncRepReleaseWaiters() for every
reply, even when several replies are already waiting in the socket.
Except for the last one, each pass then uses positions that the next
message immediately replaces.

The patch marks a release as pending and runs it once after draining the
socket. The main risk is losing a pending release on an early exit: no
other process will wake the committers acknowledged by that reply.

The goodbye, EOF, invalid and unexpected message paths all run the
pending release before returning. A normal standby shutdown sends its
final reply and goodbye together, so that path matters in practice.
Errors while parsing a later message, including a torn message, go
through WalSndErrorCleanup(), which releases waiters after dropping the
locks.

0004 -- Skip the lock when the acknowledgement has already arrived.

This patch mirrors lsn[] in an atomic watermark and checks it before
SyncRepWaitForLSN() takes the lock. The watermark is updated under the
lock immediately after lsn[]. Both only move forward, so a stale read
may take the slow path unnecessarily but cannot skip a required wait.

I am least sure that 0004 is worth the extra shared state; see below.

Prior work
----------

0001 was proposed in Thomas Munro's "Latches vs lwlock contention"
thread as part of a general SetLatches() facility:

https://www.postgresql.org/message-id/CA%2BhUKGKmO7ze0Z6WXKdrLxmvYa%3DzVGGXOO30MMktufofVwEm1A%40mail.gmail.com

The heavyweight-lock part of that work was committed in November 2024,
but SetLatches() and its sync-rep use were not. As far as I can tell,
the sync-rep patch did not get a separate review. I used a list local to
syncrep.c to keep the change contained and avoid the open questions
around buffers and allocation in the general facility.

There is also precedent for 0004. Michael Paquier's 2e57790836c ("Fix
race with synchronous_standby_names at startup", April 2025) reads
WalSndCtl->sync_standbys_status without the lock. The same monotonicity
argument applies here; the LSN needs the atomics API because it is 64
bits wide.

Measurements
------------

I compared devel master with the same master plus all four patches on
two hosts connected by a dedicated 100 GbE link (RTT 0.11 ms):

primary 4-socket Xeon Platinum 8580, 240 threads, 2 TB RAM, NVMe
standby 2-socket Xeon Gold 5320, 104 threads, 1 TB RAM, NVMe

The test used pgbench scale 2000, fillfactor 70, 750 clients,
-M prepared, the built-in TPC-B script, and 10-minute runs with
synchronous_commit = on, fsync and full_page_writes on. postgres used
two primary sockets and pgbench a third.

Each point started from the same prepared cluster, and the standby was
rebuilt from a fresh base backup. I ran three interleaved pairs,
alternating base and patched:

tps runs 122250/122591/123091 -> 134849/134447/132740
mean tps 122644 -> 134012 +9.3%
mean latency 6.085 ms -> 5.558 ms -8.7%
failed transactions: none

The spread was 0.7% for base and 1.6% for patched.

pg_stat_activity samples taken every 5 seconds show the same effect.
The queue lock and standby acknowledgement wait are both named SyncRep;
wait_event_type separates them:

LWLock/SyncRep samples 12222 -> 6912 -43%
IPC/SyncRep samples 20699 -> 14694 -29%

Per committed transaction, lock-wait samples fell from 0.0997 to 0.0516.
The queue-lock wait was roughly halved while throughput rose by 9%. The
standby acknowledgement wait also fell by 35% per transaction, as
committers stopped queueing for SyncRepLock before waiting for the
standby.

To see which patches contributed, I also ran -DLWLOCK_STATS builds on a
small single-host setup: 16 threads, scale 20, 128 clients, 30 seconds.
Across two runs, base -> patched:

walsender acquisitions/commit 0.900-1.106 -> 0.538-0.610
backend acquisitions/commit 0.9983-0.9995 -> 0.977-0.984
commits that blocked 9.34-10.20% -> 0.24-0.27%

At 32 clients, blocked acquisitions fell from 37809 to 362.

0003 makes most of the difference: the walsender takes the lock roughly
half as often. 0004 rarely takes its fast path, saving only 1.6-2.3% of
backend acquisitions at 128 clients. With synchronous_commit = on, it
can only catch a commit if an acknowledgement for a later transaction
happens to cover its LSN.

I also ran four interleaved, 60-second single-client pairs. Mean
latency was 0.760 ms on base and 0.762 ms patched, so deferring the
release to the end of a one-reply drain showed no measurable delay.

Testing
-------

0003 adds src/test/recovery/t/056_syncrep_release.pl for two cases where
a deferred release could be lost:

* A stopped walsender resumes after the standby has applied the commit
and shut down, then drains the final reply and goodbye together.

* An injection point after a drained reply stands in for a torn message.
The walreceiver is stopped while replay advances from WAL already on
disk, so the first reply after resume carries the apply position the
committer needs.

Notes
-------

The SyncRepReleaseWaiters() call on configuration reload is outside the
reply drain and is unchanged by 0003.

0001 allocates a MaxBackends-sized wake list once per process in
TopMemoryContext. I am open to changing that if there is a better fit.

The patches are against 7e6e294e4e4.

-------

Review would be especially helpful on two points:

* Did I miss any exit path from the reply drain that must run the
deferred release?

* Is 0004 worth its new atomic in WalSndCtlData for a 1.6-2.3% reduction
in backend lock acquisitions in these tests?

I plan to register 0001-0003 for the September CommitFest and drop 0004,
unless there is a workload where its fast path is more useful.

Regards,
Vadim Ponomarev