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

[Luck, Tony]

>> +		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).

-Tony