RE: [PATCH 00/12] Cqm2: Intel Cache quality monitoring fixes

[Shivappa Vikas]

On Mon, 6 Feb 2017, Luck, Tony wrote:

> > 12) Whatever fs or syscall is provided instead of perf syscalls, it
> > should provide total_time_enabled in the way perf does, otherwise is
> > hard to interpret MBM values.
>
> It seems that it is hard to define what we even mean by memory bandwidth.
>
> If you are measuring just one task and you find that the total number of bytes
> read is 1GB at some point, and one second later the total bytes is 2GB, then
> it is clear that the average bandwidth for this process is 1GB/s. If you know
> that the task was only running for 50% of the cycles during that 1s interval,
> you could say that it is doing 2GB/s ... which is I believe what you were
> thinking when you wrote #12 above.  But whether that is right depends a
> bit on *why* it only ran 50% of the time. If it was time-sliced out by the
> scheduler ... then it may have been trying to be a 2GB/s app. But if it
> was waiting for packets from the network, then it really is using 1 GB/s.

Is the requirement is to have both enabled and run time or just enabled time 
(enabled time must be easy to report - just the wall time from start trace to 
end trace)?

This is not reported correctly in the upstream perf cqm and for
cgroup -C we dont report it either (since we report the package).

Thanks,
Vikas

> All bets are off if you are measuring a service that consists of several
> tasks running concurrently. All you can really talk about is the aggregate
> average bandwidth (total bytes / wall-clock time). It makes no sense to
> try and factor in how much cpu time each of the individual tasks got.
>
> -Tony