netdev - Re: perf measure for stalled cycles per instruction on newer Intel processors

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Date:   Sun, 18 Oct 2020 18:00:27 -0700
From:   Andi Kleen <andi@...stfloor.org>
To:     Or Gerlitz <gerlitz.or@...il.com>
Cc:     Andi Kleen <andi@...stfloor.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Brendan Gregg <bgregg@...flix.com>,
        Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: perf measure for stalled cycles per instruction on newer Intel
 processors

> > Don't use it. It's misleading on a out-of-order CPU because you don't
> > know if it's actually limiting anything.
> >
> > If you want useful bottleneck data use --topdown.
> 
> So running again, this time with the below params, I got this output
> where all the right most column is colored red. I wonder what can be
> said on the amount/ratio of stalls for this app - if you can maybe recommend
> some posts of yours to better understand that, I saw some comment in the
> perf-stat man page and some lwn article but wasn't really able to figure it out.

TopDown determines what limits the execution the most.

The application is mostly backend bound (55-72%). This can be either memory
issues (more common), or sometimes also execution issues. Standard perf
doesn't support a further break down beyond these high level categories,
but there are alternative tools that do (e.g. mine is "toplev" in
https://github.com/andikleen/pmu-tools or VTune)

Some references on TopDown:
https://github.com/andikleen/pmu-tools/wiki/toplev-manual
http://bit.ly/tma-ispass14

The tools above would also allow you to sample where the stalls
are occuring.

-Andi