Re: measuring lwlock-related latency spikes

On Sun, Apr 1, 2012 at 10:27 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> So lock starvation on the control lock would cause a long wait after
> each I/O, making it look like an I/O problem.

Except that both of the locks involved in his smoking gun occur
*after* the control lock has already been acquired. The one that's
actually being blocked for a long time is in fact acquiring a shared
lock which the queue jumping couldn't be hurting.

We know you're convinced about the queue jumping being a problem, and
it's definitely a plausible problem, but I think you need exactly the
kind of instrumentation Robert is doing here to test that theory.
Without it even if everyone agreed it was a real problem we would have
no idea whether a proposed change fixed it.


Fwiw this instrumentation is *amazing*. As a user this kind of rare
random stall is precisely the kind of thing that totally kills me. I
would so much rather run a web site on a database where each query
took twice as long but it guaranteed that no query would take over a
second than one that was twice as fast on average but occasionally
gets stuck for 12s.


-- 
greg