On Wed, 23 May 2007, Peter Zijlstra wrote:
> On Wed, 2007-05-23 at 12:11 -0400, Jason Baron wrote:
> > On Wed, 23 May 2007, Peter Zijlstra wrote:
> >
> > > On Wed, 2007-05-23 at 10:40 -0400, Jason Baron wrote:
> > > > On Wed, 23 May 2007, Peter Zijlstra wrote:
> > > >
> > > > > Count lock contention events per lock class. Additionally track the first four
> > > > > callsites that resulted in the contention.
> > > > >
> > > >
> > > > I think that we need the total number of locking calls, not just the total
> > > > number of *contended* locking calls, in order to make the data
> > > > significant. Same for waittime. Yes, this pollutes the fastpath. In the
> > > > contention case, its one extra addition, and for the waittime, its a call
> > > > the sched_clock(). I don't think that's too much to pay for much more
> > > > meaningful data.
> > >
> > > The holdtime statistics add these numbers.
> > >
> >
> > ok, i see what you are saying...however, the 'waittime' statistic as
> > implemented, is only recorded when we don't get the lock immediately.
> > Thus, it's really measuring the waittime when there is lock contention. I
> > understand that in the non-contended case we are only talking a a few
> > cycles, but the fact that the non-contended case is not counted as another
> > waittime of even zero length (so no measurement is required), might skew
> > the stats a bit. For examples, if there was a lock that was almost never
> > contended but one time happened to be contended for a long time, its
> > average wait time would look really high.
>
> I'm not seeing how or why this is a problem. The number of contentions
> is reported on the same line, so it should be obvious.
>
> Your definition of wait-time is also obtainable from the numbers, one
> would just divide waittime-total by acquisitions, instead of
> contentions.
>
agreed, I just want to point out that under my definition of waitime, I
would have to make the assumption that the waittime is 0 for a lock that
is acquired without fallback to the slowpath. For a spinlock acquisition,
this is likely a fair assumption, however the trylock path for semaphores
can be longer.
Acked-by: Jason Baron <[email protected]>
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