Re: [PATCH 4/6] x86/mbm: Memory bandwidth monitoring event management

[Vikas Shivappa]

On Tue, 8 Mar 2016, Peter Zijlstra wrote:

> On Mon, Mar 07, 2016 at 11:27:26PM +0000, Luck, Tony wrote:
> > > > +		bytes = mbm_current->interval_bytes * MSEC_PER_SEC;
> > > > +		do_div(bytes, diff_time);
> > > > +		mbm_current->bandwidth = bytes;
> > > > +		mbm_current->interval_bytes = 0;
> > > > +		mbm_current->interval_start = cur_time;
> > > > +	}
> > > > > +
> > > > +	return mbm_current;
> > > > +}
> > >
> > > How does the above time tracking deal with the event not actually having
> > > been scheduled the whole time?
> >
> > That's been the topic of a few philosophical debates ... what exactly are
> > we trying to say when we report that a process has a "memory bandwidth"
> > of, say, 1523 MBytes/s?  We need to know both the amount of data moved
> > and to pick an interval to measure and divide by. Does it make a difference
> > whether the process voluntarily gave up the cpu for some part of the interval
> > (by blocking on I/O)? Or did the scheduler time-slice it out to run other jobs?
> >
> > The above code gives the average bandwidth across the last interval
> > (with a minimum interval size of 100ms to avoid craziness with rounding
> > errors on exceptionally tiny intervals). Some folks apparently want to get
> > a "rate" directly from perf.  I think many folks will find the "bytes" counters
> > more helpful (where they control the sample interval with '-I" flag to perf
> > utility).
>
> So why didn't any of that make it into the Changelog? This is very much
> different from any other perf driver, at the very least this debate
> should have been mentioned and the choice defended.
>
> Also, why are we doing the time tracking and divisions at all? Can't we
> simply report the number of bytes transferred and let userspace sort out
> the rest?
>
> Userspace is provided the time the event was enabled, the time the event
> was running and it can fairly trivially obtain walltime if it so desires
> and then it can compute whatever definition of bandwidth it wants to
> use.


We had discussions on removing the bw event. Discussed this with Tony and will 
update the patch by removing the bw events.. So this code will be removed.

thanks,
Vikas

>