Re: LWlock:LockManager waits

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.


Frits Hoogland




> On 9 Apr 2024, at 09:54, James Pang <jamespang886@gmail.com> 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
> 
> Laurenz Albe <laurenz.albe@cybertec.at <mailto:laurenz.albe@cybertec.at>> 於 2024年4月9日週二 下午12:31寫道：
>> 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