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SubjectRe: [PATCH 1/6] x86/intel_rdt/mba_sc: Add documentation for MBA software controller


On Wed, 4 Apr 2018, Thomas Gleixner wrote:

> On Tue, 3 Apr 2018, Shivappa Vikas wrote:
> > On Tue, 3 Apr 2018, Thomas Gleixner wrote:
> > > On Thu, 29 Mar 2018, Vikas Shivappa wrote:
> > > The L2 external bandwidth is higher than the L3 external bandwidth.
> > >
> > > Is there any information available from CPUID or whatever source which
> > > allows us to retrieve the bandwidth ratio or the absolute maximum
> > > bandwidth per level?
> >
> > There is no information in cpuid on the bandwidth available. Also we have seen
> > from our experiments that the increase is not perfectly linear (delta
> > bandwidth increase from 30% to 40% may not be same as 70% to 80%). So we
> > currently dynamically caliberate this delta for the software controller.
>
> I assume you mean: calibrate
>
> Though I don't see anything which looks remotely like calibration.
> Calibration means that you determine the exact parameters by observation and
> then can use the calibrated values afterwards. But that's not what you are
> doing. So please don't claim its calibration.
>
> You observe behaviour which depends on the workload and other
> factors. That's not calibration. If you change the MSR by a granularity
> value then you calculate the bandwidth delta vs. the previous MSR
> value. That only makes sense and works when the application is having the
> same memory access patterns accross both observation periods.
>
> And of course, this won't be necessarily linear because if you throttle the
> application then it gets less work done per CPU time slice and the
> resulting stalls will also have side effects on the requested amount of
> memory and therefore distort the measurement. Ditto the other way
> around.
>
> There are too many factors influencing this, so claiming that it's
> calibration is window dressing at best. Even worse it suggests that it's
> something accurate, which subverts your goal of reducing confusion.
>
> Adaptive control might be an acceptable description, though given the
> amount of factors which play into that it's still an euphemism for
> 'heuristic'.

Agree we donot really caliberate and the only thing we guarentee is that
the actual bandwidth in bytes < user specified bandwidth bytes. This is
what the hardware guarenteed when we specified the values in percentage
as well but just that it was confusing.

>
> > > What's also missing from your explanation is how that feedback loop behaves
> > > under different workloads.
> > >
> > > Is this assuming that the involved threads/cpus actually try to utilize
> > > the bandwidth completely?
> >
> > No, the feedback loop only guarentees that the usage will not exceed what the
> > user specifies as max bandwidth. If it is using below the max value it does
> > not matter how much less it is using.
> > >
> > > What happens if the threads/cpus are only using a small set because they
> > > are idle or their computations are mostly cache local and do not need
> > > external bandwidth? Looking at the implementation I don't see how that is
> > > taken into account.
> >
> > The feedback only kicks into action if a rdtgroup uses more bandwidth than the
> > max specified by the user. I specified that it is always "ensure the "actual
> > b/w
> > 354 < user b/w" " and can add more explanation on these scenarios.
>
> Please finally stop to use this horrible 'b/w' thingy. It makes my eyes bleed
> everytime.

Will fix - this was a text from already existing documentation.

>
> > Also note that we are using the MBM counters for this feedback loop. Now that
> > the interface is much more useful because we have the same rdtgroup that is
> > being monitored and controlled. (vs. if we had the perf mbm the group of
> > threads in resctrl mba and in mbm could be different and would be hard to
> > measure what the threads/cpus in the resctrl are using).
>
> Why does that make me smile?

I know why :) Full credits to you as you had suggested to rewrite the
cqm/mbm in resctrl which is definitely very good in long term !

Thanks,
Vikas

>
> Thanks,
>
> tglx
>

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