Re: [PATCH v5 0/8] perf report: Add latency and parallelism profiling

Dmitry Vyukov
2025-02-10T07:17:19Z

On Fri, 7 Feb 2025 at 19:30, Andi Kleen <ak@linux.intel.com> wrote:
>
> > > I assume it just works, but might be worth checking.
> >
> > Yes, it seems to just work as one would assume. Things just combine as intended.
>
> Great.
>
> >
> > > It was intended to address some of these issues too.
> >
> > What issue? Latency profiling? I wonder what approach you had in mind?
>
> The problem with gaps in parallelism is usually how things change
> over time. If you have e.g. idle periods they tend to look different
> in the profile. with the full aggregation you don't see it, but with
> a time series it tends to stand out.
>
> But yes that approach usually only works for large gaps. I guess
> yours is better for more fine grained issues.
>
> And I guess it might not be the most intutive for less experienced
> users.
>
> This is BTW actually a case for using a perf data GUI like hotspot or
> vtune which can visualize this better and you can zoom arbitrarily.
> Standard perf has only timechart for it, but it's a bit clunky to use
> and only shows the reschedules.
>
> > Also (1) user still needs to understand the default profile is wrong,
> > (2) be proficient with perf features, (3) manually aggregate lots of
> > data (time slicing increases amount of data in the profile X times),
> > (4) deal with inaccuracy caused by edge effects (e.g. slice is 1s, but
> > program phase changed mid-second).
>
> If you're lucky and the problem is not long tail you can use a high
> percentage cut off (--percent-limit) to eliminate most of the data.
>
> Then you just have "topN functions over time" which tends to be quite
> readable. One drawback of that approach is that it doesn't show
> the "other", but perhaps we'll fix that one day.
>
> But yes that perf has too many options and is not intuitive and most
> people miss most of the features is an inherent problem. I don't have
> a good solution for that unfortunately, other than perhaps better
> documentation.

I don't think this is a solution :(

I provided lots of rationale for making this latency profiling enabled
by default in this patch series for this reason. If we just capture
context switches, then we can show both overhead and latency, even if
we sort by overhead by default, people would still see the latency
column and may start thinking/asking questions.
But this is not happening, so mostly people on this thread will know about it :)

> > But it does open some interesting capabilities in combination with a
> > latency profile, e.g. the following shows how parallelism was changing
> > over time.
> >
> > for perf make profile:
>
> Very nice! Looks useful.
>
> Perhaps add that variant to tips.txt too.

That's a good idea.
I am waiting for other feedback to not resend the series just because of this.

> > perf report -F time,latency,parallelism --time-quantum=1s
> >
> > # Time Latency Parallelism
> > # ............ ........ ...........
> > #
> > 1795957.0000 1.42% 1
> > 1795957.0000 0.07% 2
> > 1795957.0000 0.01% 3
> > 1795957.0000 0.00% 4
> >
> > 1795958.0000 4.82% 1
> > 1795958.0000 0.11% 2
> > 1795958.0000 0.00% 3
> > ...
> > 1795964.0000 1.76% 2
> > 1795964.0000 0.58% 4
> > 1795964.0000 0.45% 1
> > 1795964.0000 0.23% 10
> > 1795964.0000 0.21% 6
> >
> > /\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\
> >
> > Here it finally started running on more than 1 CPU.
>
>
> -Andi