On Wed, Feb 28, 2024 at 12:01:55PM -0800, Namhyung Kim wrote:
> On Wed, Feb 28, 2024 at 4:22 AM Arnaldo Carvalho de Melo
> <acme@xxxxxxxxxx> wrote:
> >
> > On Tue, Feb 27, 2024 at 09:33:35PM -0800, Namhyung Kim wrote:
> > > Currently it accounts the contention using delta between timestamps in
> > > lock:contention_begin and lock:contention_end tracepoints. But it means
> > > the lock should see the both events during the monitoring period.
> > >
> > > Actually there are 4 cases that happen with the monitoring:
> > >
> > > monitoring period
> > > / \
> > > | |
> > > 1: B------+-----------------------+--------E
> > > 2: B----+-------------E |
> > > 3: | B-----------+----E
> > > 4: | B-------------E |
> > > | |
> > > t0 t1
> > >
> > > where B and E mean contention BEGIN and END, respectively. So it only
> > > accounts the case 4 for now. It seems there's no way to handle the case
> > > 1. The case 2 might be handled if it saved the timestamp (t0), but it
> > > lacks the information from the B notably the flags which shows the lock
> > > types. Also it could be a nested lock which it currently ignores. So
> > > I think we should ignore the case 2.
> >
> > Perhaps have a separate output listing locks that were found to be with
> > at least tE - t0 time, with perhaps a backtrace at that END time?
>
> Do you mean long contentions in case 3? I'm not sure what do
> you mean by tE, but they started after t0 so cannot be greater
case 2
monitoring period
/ \
| |
2: B----+-------------E |
| | |
t0 tE t1
We get a notification for event E, right? We don´t have one for B,
because it happened before we were monitoring.
> than or equal to the monitoring period. Maybe we can try with
> say, 90% of period but we can still miss something.
>
> And collecting backtrace of other task would be racy as the it
> may not contend anymore.
>
> > With that we wouldn't miss that info, however incomplete it is and the
> > user would try running again, perhaps for a longer time, or start
> > monitoring before the observed workload starts, etc.
>
> Yeah, it can be useful. Let me think about it more.
>
> >
> > Anyway:
> >
> > Reviwed-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
>
> Thanks for your review!
> Namhyung
>
