Re: RFC: replace pg_stat_activity.waiting with something more descriptive - Mailing list pgsql-hackers

On Sat, Sep 12, 2015 at 3:24 PM, Amit Kapila <amit.kapila16@gmail.com> wrote:

On Fri, Aug 14, 2015 at 7:23 PM, Alexander Korotkov <aekorotkov@gmail.com> wrote:
>
> On Thu, Aug 6, 2015 at 1:01 PM, Ildus Kurbangaliev <i.kurbangaliev@postgrespro.ru> wrote:
>>
>>
>> I've looked deeper and I found PgBackendStatus to be not a suitable
>> place for keeping information about low level waits. Really, PgBackendStatus
>> is used to track high level information about backend. This is why auxiliary
>> processes don't have PgBackendStatus, because they don't have such information
>> to expose. But when we come to the low level wait events then auxiliary
>> processes are as useful for monitoring as backends are. WAL writer,
>> checkpointer, bgwriter etc are using LWLocks as well. This is certainly unclear
>> why they can't be monitored.
>>
>> This is why I think we shoudn't place wait event into PgBackendStatus. It
>> could be placed into PGPROC or even separate data structure with different
>> concurrency model which would be most suitable for monitoring.
>
>
> +1 for tracking wait events not only for backends
>
> Ildus, could you do following?
> 1) Extract LWLocks refactoring into separate patch.
> 2) Make a patch with storing current wait event information in PGPROC.
>

Now as Robert has committed standardization of lwlock names in

commit - aa65de04, let us try to summarize and work on remaining parts

of the patch. So I think now the next set of work is as follows:

1. Modify the tranche mechanism so that information about LWLocks

can be tracked easily.  For this already there is some discussion, ideas

and initial patch is floated in this thread and there doesn't seem to be much

conflicts, so we can write the patch for it.  I am planning to write or modify

the existing patch unless you, IIdus or anyone has objections or want to

write it, please let me know to avoid duplication of work. 

2. Track wait_event in pg_stat_activity.  Now here the main point where

we doesn't seem to be in complete agreement is that shall we keep it

as one byte in PgBackendStatus or use two or more bytes to store

wait_event information and still there is another point made by you to

store it in PGProc rather than in PgBackendStatus so that we can display

information of background processes as well.

Now as a matter of consensus, I think Tom has raised a fair point [1] against

storing this information in PGProc and I feel that it is relatively less

important to have such information about background processes and the

reason for same is explained upthread [2].  About having more than one-byte

to store information about various wait_events, I think now we will not have

more than 100 or so events to track, do you really think that anytime in forseeable

future we will have more than 256 important events which we would like to track?

So I think about this lets first try to build the consensus and then attempt to

write or modify the patch.

[1] - http://www.postgresql.org/message-id/4067.1439561494@sss.pgh.pa.us

[2] - http://www.postgresql.org/message-id/CAA4eK1JRQGTgRMRao6H65rA=fhifUCNQhX7ONgY58UM6rN1rxQ@mail.gmail.com

In order to build the consensus we need the roadmap for waits monitoring.

Would single byte in PgBackendStatus be the only way for tracking wait events? Could we have pluggable infrastructure in waits monitoring: for instance, hooks for wait event begin and end?

Limit of 256 wait events is probably OK. But we have no room for any additional parameters of wait event. For instance, if you notice high contention for buffer content lock then it would be nice to figure out: how many blocks are involved?, which blocks? We need to track additional parameters in order to answer this question.