Re: [Xen-devel] [PATCH 07/19] xen: credit2: prevent load balancing to go mad if time goes backwards

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

On Thu, 2016-07-07 at 03:18 -0600, Jan Beulich wrote:
> > > > On 07.07.16 at 11:09, <george.dunlap@xxxxxxxxxx> wrote:
> > > Oh, okay - I agree on those two parts then. But the question on
> > > the
> > > usefulness of absolute vs relative times remains.
> > What is the usefulness of printing the relative time?  If you have
> > the
> > absolute time, you have some chance of catching mistakes like one
> > of the
> > times being 0 or something like that; or of being able to correlate
> > it
> > with another time printed somewhere else (for instance, a timestamp
> > from
> > a trace record).
> Well, having had to deal with time going backwards elsewhere
> (both in the past and recently) I have always found it cumbersome
> to work out the delta from huge (far into the billions) absolute
> numbers, and therefore consider logging the delta more useful -
>
And, in general, I agree with you. In this case, however...

> apart from seeing at the first glance whether a particular delta is
> positive or negative, this also allows at almost the first glance to
> at least recognize the magnitude of the difference. But anyway ...
> 
true, for the magnitude part, but in this case we are only logging
anything if delta is negative. So the fact itself that there is
something being logged, tells about the sign of the delta. And...

> > In any case, I think it's really a bike shed.  Dario is the one who
> > has
> > used this error message to find an actual bug recently, so I'll let
> > him
> > decide what he thinks the most useful thing to print here is.
> ... fine with me; it was just a question after all.
> 
...I caught two bugs thanks to this. The (more recent) one about lack
of monotonicity, having two absolute values or a delta would have been
the same. For the other one (the fact that we were using stale time
samples because we were calling NOW() and then [potentially] waiting
for acquiring a lock) seeing that there were repeating occurrences of
the same absolute value of now, did indeed helped both identifying and
debugging the issue, while having just a delta wouldn't have been as
effective.

And I've also done what George refers to, i.e., look for the exact
value printed in the trace record, to have an idea of what was
happening, and again having absolutes revealed useful for this.

So, yes, I think it's actually fine to leave the message as it is.

Thanks and Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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