Re: adding wait_start column to pg_locks

On 2021-01-15 15:23, torikoshia wrote:
> Thanks for your reviewing and comments!
> 
> On 2021-01-14 12:39, Ian Lawrence Barwick wrote:
>> Looking at the code, this happens as the wait start time is being 
>> recorded in
>> the lock record itself, so always contains the value reported by the 
>> latest lock
>> acquisition attempt.
> 
> I think you are right and wait_start should not be recorded
> in the LOCK.
> 
> 
> On 2021-01-15 11:48, Ian Lawrence Barwick wrote:
>> 2021年1月15日(金) 3:45 Robert Haas <robertmhaas@gmail.com>:
>> 
>>> On Wed, Jan 13, 2021 at 10:40 PM Ian Lawrence Barwick
>>> <barwick@gmail.com> wrote:
>>>> It looks like the logical place to store the value is in the
>>> PROCLOCK
>>>> structure; ...
>>> 
>>> That seems surprising, because there's one PROCLOCK for every
>>> combination of a process and a lock. But, a process can't be waiting
>>> for more than one lock at the same time, because once it starts
>>> waiting to acquire the first one, it can't do anything else, and
>>> thus
>>> can't begin waiting for a second one. So I would have thought that
>>> this would be recorded in the PROC.
>> 
>> Umm, I think we're at cross-purposes here. The suggestion is to note
>> the time when the process started waiting for the lock in the
>> process's
>> PROCLOCK, rather than in the lock itself (which in the original
>> version
>> of the patch resulted in all processes with an interest in the lock
>> appearing
>> to have been waiting to acquire it since the time a lock acquisition
>> was most recently attempted).
> 
> AFAIU, it seems possible to record wait_start in the PROCLOCK but
> redundant since each process can wait at most one lock.
> 
> To confirm my understanding, I'm going to make another patch that
> records wait_start in the PGPROC.

Attached a patch.

I noticed previous patches left the wait_start untouched even after
it acquired lock.
The patch also fixed it.

Any thoughts?


Regards,

--
Atsushi Torikoshi