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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
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