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Re: [Cluster-devel] GFS2: glock statistics gathering (RFC)



On Fri, Nov 04, 2011 at 04:57:31PM +0000, Steven Whitehouse wrote:
> Hi,
> 
> On Fri, 2011-11-04 at 12:31 -0400, David Teigland wrote:
> > On Fri, Nov 04, 2011 at 03:19:49PM +0000, Steven Whitehouse wrote:
> > > The three pairs of mean/variance measure the following
> > > things:
> > > 
> > >  1. DLM lock time (non-blocking requests)
> > 
> > You don't need to track and save this value, because all results will be
> > one of three values which can gather once:
> > 
> > short: the dir node and master node are local: 0 network round trip
> > medium: one is local, one is remote: 1 network round trip
> > long: both are remote: 2 network round trips
> > 
> > Once you've measured values for short/med/long, then you're done.
> > The distribution will depend on the usage pattern.
> > 
> The reason for tracking this is to be able to compare it with the
> blocking request value to (I hope) get a rough idea of the difference
> between the two, which may indicate contention on the lock. So this
> is really a "baseline" measurement.
> 
> Plus we do need to measure it, since it will vary according to a
> number of things, such as what hardware is in use.

Right, but the baseline shouldn't change once you have it.

> > > 2. To spot performance issues more easily
> > 
> > Apart from contention, I'm not sure there are many perf issues that dlm
> > measurements would help with.
> > 
> That is the #1 cause of reported performance issues, so top of our list
> to work on. The goal is to make it easier to track down the source of
> these kinds of problems.

I still think that time averages and computations sounds like a difficult
and indirect way of measuring contention... but see how it works.

Dave


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