Re: RFC: replace pg_stat_activity.waiting with something more descriptive

On Mon, Feb 22, 2016 at 10:05 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>> I wouldn't bother tinkering with it at this point. The value isn't
>> going to be recorded on disk anywhere, so it will be easy to change
>> the way it's computed in the future if we ever need to do that.
>>
>
> Okay. Find the rebased patch attached with this mail. I have moved
> this patch to upcoming CF.

I would call the functions pgstat_report_wait_start() and
pgstat_report_wait_end() instead of pgstat_report_start_waiting() and
pgstat_report_end_waiting().

I think pgstat_get_wait_event_type should not return HWLock, a term
that appears nowhere in our source tree at present. How about just
"Lock"?

I think the wait event types should be documented - and the wait
events too, perhaps.

Maybe it's worth having separate wait event type names for lwlocks and
lwlock tranches. We could report LWLockNamed and LWLockTranche and
document the difference: "LWLockNamed indicates that the backend is
waiting for a specific, named LWLock. The event name is the name of
that lock. LWLockTranche indicates that the backend is waiting for
any one of a group of locks with similar function. The event name
identifies the general purpose of locks in that group."

There's no requirement that every session have every tranche
registered. I think we should consider displaying "extension" for any
tranche that's not built-in, or at least for tranches that are not
registered (rather than "unknown wait event").

+ if (lock->tranche == 0 && lockId < NUM_INDIVIDUAL_LWLOCKS)

Isn't the second part automatically true at this point?

The changes to LockBufferForCleanup() don't look right to me. Waiting
for a buffer pin might be a reasonable thing to define as a wait
event, but it shouldn't reported as if we were waiting on the LWLock
itself.

What happens if an error is thrown while we're in a wait?

Does this patch hurt performance?

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company