Horton

Re: [HACKERS] make async slave to wait for lsn to be replayed

Alvaro Herrera <alvherre@alvh.no-ip.org>

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From: Alvaro Herrera <alvherre@alvh.no-ip.org>
To: Alexander Korotkov <aekorotkov@gmail.com>
Cc: Bharath Rupireddy <bharath.rupireddyforpostgres@gmail.com>, Kartyshov Ivan <i.kartyshov@postgrespro.ru>, Euler Taveira <euler@eulerto.com>, Thomas Munro <thomas.munro@gmail.com>, Peter Eisentraut <peter@eisentraut.org>, Amit Kapila <amit.kapila16@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Peter Smith <smithpb2250@gmail.com>, pgsql-hackers@lists.postgresql.org
Date: 2024-04-03T06:58:41Z
Lists: pgsql-hackers 
Hello, I noticed that commit 06c418e163e9 uses waitLSN->mutex (a
spinlock) to protect the contents of waitLSN -- and it's used to walk an
arbitrary long list of processes waiting ... and also, an arbitrary
number of processes could be calling this code.  I think using a
spinlock for this is unwise, as it'll cause busy-waiting whenever
there's contention.  Wouldn't it be better to use an LWLock for this?
Then the processes would sleep until the lock is freed.

While nosing about the code, other things struck me:

I think there should be more comments about WaitLSNProcInfo and
WaitLSNState in waitlsn.h.

In addLSNWaiter it'd be better to assign 'cur' before acquiring the
lock.

Is a plan array really the most efficient data structure for this,
considering that you have to reorder each time you add an element?
Maybe it is, but wouldn't it make sense to use memmove() when adding one
element rather iterating all the remaining elements to the end of the
queue?

I think the include list in waitlsn.c could be tightened a bit:

@@ -18,28 +18,18 @@
 #include <math.h>
 
 #include "pgstat.h"
-#include "fmgr.h"
-#include "access/transam.h"
-#include "access/xact.h"
 #include "access/xlog.h"
-#include "access/xlogdefs.h"
 #include "access/xlogrecovery.h"
-#include "catalog/pg_type.h"
 #include "commands/waitlsn.h"
-#include "executor/spi.h"
 #include "funcapi.h"
 #include "miscadmin.h"
-#include "storage/ipc.h"
 #include "storage/latch.h"
-#include "storage/pmsignal.h"
 #include "storage/proc.h"
 #include "storage/shmem.h"
-#include "storage/sinvaladt.h"
-#include "utils/builtins.h"
 #include "utils/pg_lsn.h"
 #include "utils/snapmgr.h"
-#include "utils/timestamp.h"
 #include "utils/fmgrprotos.h"
+#include "utils/wait_event_types.h"

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"Las cosas son buenas o malas segun las hace nuestra opinión" (Lisias)

Commits

  1. Ensure standby promotion point in 043_wal_replay_wait.pl

  2. Minor cleanup related to pg_wal_replay_wait() procedure

  3. Adjust pg_wal_replay_wait() procedure behavior on promoted standby

  4. pg_wal_replay_wait(): Fix typo in the doc

  5. Implement pg_wal_replay_wait() stored procedure

  6. Revert: Implement pg_wal_replay_wait() stored procedure

  7. Call WaitLSNCleanup() in AbortTransaction()

  8. Clarify what is protected by WaitLSNLock

  9. Use an LWLock instead of a spinlock in waitlsn.c

  10. Use the pairing heap instead of a flat array for LSN replay waiters

  11. Minor improvements for waitlsn.c

  12. Make the order of the header file includes consistent