Re: Tracking wait event for latches

On Thu, Sep 22, 2016 at 1:52 AM, Michael Paquier
<michael(dot)paquier(at)gmail(dot)com> wrote:
> Then comes the interesting bits: I don't think that it is a good idea
> to associate only one event for a waiting point in the system views.
> Say that at a waiting point WaitLatch is called with
> WL_POSTMASTER_DEATH and WL_TIMEOUT, I'd rather let the user know that
> both of them have been set up and not choose just one of them using
> some hierarchy. But I rather think that we should list all of them
> depending on the WL_* flags set up by the caller. Here comes a second
> patch: let's use the last byte of the wait events and add this new
> text[] column in pg_stat_activity, say wait_details. So for a waiting
> point it would be possible to tell to the user that it is waiting on
> '{socket,timeout,postmaster_death}' for example.

I think this is focusing on the wrong thing. What we need here is not
infinite detail on exactly what is going on under the hood but useful
classification rules. For example, as I said in my email to Thomas,
being able to exclude all cases of waiting for client I/O is useful
because those aren't signs of a problem in the way that LWLock or Lock
waits are. It's better for us to provide that classification using
the existing system than for users to have to work out exactly which
things ought to be excluded on the basis of specific event names.

On the other hand, I submit that knowing which waits will be
interrupted by the death of the postmaster and which will not doesn't
add much. In fact, I think that's almost an anti-feature, because it
will encourage users to care about details that are very rarely
relevant.

> Then comes a third patch: addition of a system view to list all the
> activity happening for processes that are not dependent on
> max_connections. pg_stat_activity has the advantage, as Tom mentioned
> a couple of days ago, that one can just run count(*) on it to estimate
> the number of connections left. I think this is quite helpful. Or we
> could just add a column in pg_stat_activity to mark a process type: is
> it a system process or not? This deserves its own discussion for sure.

Sure.

--
Robert Haas
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