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

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Message-ID: <3908561D78D1C84285E8C5FCA982C28F3A000C1D@ORSMSX114.amr.corp.intel.com>
Date:	Mon, 7 Mar 2016 23:27:26 +0000
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Vikas Shivappa <vikas.shivappa@...ux.intel.com>
CC:	"Shivappa, Vikas" <vikas.shivappa@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"Shankar, Ravi V" <ravi.v.shankar@...el.com>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	"Anvin, H Peter" <h.peter.anvin@...el.com>
Subject: RE: [PATCH 4/6] x86/mbm: Memory bandwidth monitoring event
 management

>> +		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