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

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Date:   Thu, 15 Oct 2020 11:33:53 -0700
From:   Andi Kleen <andi@...stfloor.org>
To:     Or Gerlitz <gerlitz.or@...il.com>
Cc:     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

On Thu, Oct 15, 2020 at 05:53:40PM +0300, Or Gerlitz wrote:
> Hi,
> 
> Earlier Intel processors (e.g E5-2650) support the more of classical
> two stall events (for backend and frontend [1]) and then perf shows
> the nice measure of stalled cycles per instruction - e.g here where we
> have IPC of 0.91 and CSPI (see [2]) of 0.68:

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.

-Andi