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

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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

>
>
>

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Re: [PATCH 4/6] x86/mbm: Memory bandwidth monitoring event management Peter Zijlstra <peterz@infradead.org> - 2016-03-08 00:10 +0100
  RE: [PATCH 4/6] x86/mbm: Memory bandwidth monitoring event  management "Luck, Tony" <tony.luck@intel.com> - 2016-03-08 00:30 +0100
    Re: [PATCH 4/6] x86/mbm: Memory bandwidth monitoring event management Peter Zijlstra <peterz@infradead.org> - 2016-03-08 09:50 +0100
      Re: [PATCH 4/6] x86/mbm: Memory bandwidth monitoring event  management Vikas Shivappa <vikas.shivappa@intel.com> - 2016-03-10 23:50 +0100
  Re: [PATCH 4/6] x86/mbm: Memory bandwidth monitoring event  management Vikas Shivappa <vikas.shivappa@intel.com> - 2016-03-11 00:00 +0100

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