Horton

LogwrtResult contended spinlock

Alvaro Herrera <alvherre@2ndquadrant.com>

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From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Pg Hackers <pgsql-hackers@lists.postgresql.org>
Cc: Jaime Casanova <jaime.casanova@2ndQuadrant.com>
Date: 2020-08-31T18:21:56Z
Lists: pgsql-hackers

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Jaime Casanova recently reported a situation where pglogical replicating
from 64 POS sites to a single central (64-core) node, each with two
replication sets, causes XLog's info_lck to become highly contended
because of frequently reading LogwrtResult.  We tested the simple fix of
adding a new LWLock that protects LogwrtResult and LogwrtRqst; that
seems to solve the problem easily enough.

At first I wanted to make the new LWLock cover only LogwrtResult proper,
and leave LogwrtRqst alone.  However on doing it, it seemed that that
might change the locking protocol in a nontrivial way.  So I decided to
make it cover both and call it a day.  We did verify that the patch
solves the reported problem, at any rate.

-- 
Álvaro Herrera                PostgreSQL Expert, https://www.2ndQuadrant.com/

Commits

  1. Remove bogus assertion in pg_atomic_monotonic_advance_u64

  2. Add XLogCtl->logInsertResult

  3. Operate XLogCtl->log{Write,Flush}Result with atomics

  4. Split XLogCtl->LogwrtResult into separate struct members

  5. Introduce atomic read/write functions with full barrier semantics.

  6. Reduce the number of GetFlushRecPtr() calls done by walsenders.

  7. Remove most volatile qualifiers from xlog.c