pgsql: Optimize WAL insertion lock acquisition and release with some at

Optimize WAL insertion lock acquisition and release with some atomics

The WAL insertion lock variable insertingAt is currently being read
and written with the help of the LWLock wait list lock to avoid any read
of torn values. This wait list lock can become a point of contention on
a highly concurrent write workloads.

This commit switches insertingAt to a 64b atomic variable that provides
torn-free reads/writes. On platforms without 64b atomic support, the
fallback implementation uses spinlocks to provide the same guarantees
for the values read. LWLockWaitForVar(), through
LWLockConflictsWithVar(), reads the new value to check if it still needs
to wait with a u64 atomic operation. LWLockUpdateVar() updates the
variable before waking up the waiters with an exchange_u64 (full memory
barrier). LWLockReleaseClearVar() now uses also an exchange_u64 to
reset the variable. Before this commit, all these steps relied on
LWLockWaitListLock() and LWLockWaitListUnlock().

This reduces contention on LWLock wait list lock and improves
performance of highly-concurrent write workloads. Here are some
numbers using pg_logical_emit_message() (HEAD at d6677b93) with various
arbitrary record lengths and clients up to 1k on a rather-large machine
(64 vCPUs, 512GB of RAM, 16 cores per sockets, 2 sockets), in terms of
TPS numbers coming from pgbench:
message_size_b | 16 | 64 | 256 | 1024
--------------------+--------+--------+--------+-------
patch_4_clients | 83830 | 82929 | 80478 | 73131
patch_16_clients | 267655 | 264973 | 250566 | 213985
patch_64_clients | 380423 | 378318 | 356907 | 294248
patch_256_clients | 360915 | 354436 | 326209 | 263664
patch_512_clients | 332654 | 321199 | 287521 | 240128
patch_1024_clients | 288263 | 276614 | 258220 | 217063
patch_2048_clients | 252280 | 243558 | 230062 | 192429
patch_4096_clients | 212566 | 213654 | 205951 | 166955
head_4_clients | 83686 | 83766 | 81233 | 73749
head_16_clients | 266503 | 265546 | 249261 | 213645
head_64_clients | 366122 | 363462 | 341078 | 261707
head_256_clients | 132600 | 132573 | 134392 | 165799
head_512_clients | 118937 | 114332 | 116860 | 150672
head_1024_clients | 133546 | 115256 | 125236 | 151390
head_2048_clients | 137877 | 117802 | 120909 | 138165
head_4096_clients | 113440 | 115611 | 120635 | 114361

Bharath has been measuring similar improvements, where the limit of the
WAL insertion lock begins to be felt when more than 256 concurrent
clients are involved in this specific workload.

An extra patch has been discussed to introduce a fast-exit path in
LWLockUpdateVar() when there are no waiters, still this does not
influence the write-heavy workload cases discussed as there are always
waiters. This will be considered separately.

Author: Bharath Rupireddy
Reviewed-by: Nathan Bossart, Andres Freund, Michael Paquier
Discussion: https://postgr.es/m/CALj2ACVF+6jLvqKe6xhDzCCkr=rfd6upaGc3477Pji1Ke9G7Bg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/71e4cc6b8ec6a08f81973bd387fe575134cd0bdf

Modified Files
--------------
src/backend/access/transam/xlog.c | 4 ++--
src/backend/storage/lmgr/lwlock.c | 46 ++++++++++++++++++---------------------
src/include/storage/lwlock.h | 6 ++---
3 files changed, 26 insertions(+), 30 deletions(-)