I need to rectify myself: LWLock is not a spinlock (anymore). The documentation in lwlock.c makes it clear that it used to be spinlock, but now is a counter modified by atomic instructions.
Oh, I forgot to answer:
you mean too many concurrent sessions trying to acquire lock on same relation , then waiting on "LockManager" LWlock,right?
This is the point: no, it’s not about the same relation.
The LWLock:LockManager is a wait event that is raised when competing for the LWLock that protects the shared Lock structure, which holds all of the locks of the database.
On 9 Apr 2024, at 09:54, James Pang <jamespang886@xxxxxxxxx> wrote:
you mean too many concurrent sessions trying to acquire lock on same relation , then waiting on "LockManager" LWlock,right? this contention occurred on parsing ,planning, or execute step ?
Thanks,
James On Tue, 2024-04-09 at 11:07 +0800, James Pang wrote:
> we found sometimes , with many sessions running same query "select ..." at the same time, saw many sessions waiting on "LockManager". for example, pg_stat_activity show. It's a production server, so no enable trace_lwlocks flag. could you direct me what's the possible reason and how to reduce this "lockmanager" lock? all the sql statement are "select " ,no DML.
>
> time wait_event count(pid)
> 2024-04-08 09:00:06.043996+00 | DataFileRead | 42
> 2024-04-08 09:00:06.043996+00 | | 15
> 2024-04-08 09:00:06.043996+00 | LockManager | 31
> 2024-04-08 09:00:06.043996+00 | BufferMapping | 46
> 2024-04-08 09:00:07.114015+00 | LockManager | 43
> 2024-04-08 09:00:07.114015+00 | DataFileRead | 28
> 2024-04-08 09:00:07.114015+00 | ClientRead | 11
> 2024-04-08 09:00:07.114015+00 | | 11
That's quite obvious: too many connections cause internal contention in the database.
Reduce the number of connections by using a reasonably sized connection pool.
Yours,
Laurenz Albe
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