Re: [PATCH v3 0/7] perf report: Add latency and parallelism profiling

[Namhyung Kim <namhyung@kernel.org>]

On Mon, Jan 27, 2025 at 10:58:47AM +0100, Dmitry Vyukov wrote:
> There are two notions of time: wall-clock time and CPU time.
> For a single-threaded program, or a program running on a single-core
> machine, these notions are the same. However, for a multi-threaded/
> multi-process program running on a multi-core machine, these notions are
> significantly different. Each second of wall-clock time we have
> number-of-cores seconds of CPU time.
> 
> Currently perf only allows to profile CPU time. Perf (and all other
> existing profilers to the be best of my knowledge) does not allow to
> profile wall-clock time.
> 
> Optimizing CPU overhead is useful to improve 'throughput', while
> optimizing wall-clock overhead is useful to improve 'latency'.
> These profiles are complementary and are not interchangeable.
> Examples of where latency profile is needed:
>  - optimzing build latency
>  - optimizing server request latency
>  - optimizing ML training/inference latency
>  - optimizing running time of any command line program
> 
> CPU profile is useless for these use cases at best (if a user understands
> the difference), or misleading at worst (if a user tries to use a wrong
> profile for a job).
> 
> This series add latency and parallelization profiling.
> See the added documentation and flags descriptions for details.
> 
> Brief outline of the implementation:
>  - add context switch collection during record
>  - calculate number of threads running on CPUs (parallelism level)
>    during report
>  - divide each sample weight by the parallelism level
> This effectively models that we were taking 1 sample per unit of
> wall-clock time.
> 
> We still default to the CPU profile, so it's up to users to learn
> about the second profiling mode and use it when appropriate.
> 
> Changes in v3:
>  - rebase and split into patches
>  - rename 'wallclock' to 'latency' everywhere
>  - don't enable latency profiling by default,
>    instead add record/report --latency flag

Thanks for doing this, much better now.  I've added some comments in
the thread.

Thanks,
Namhyung

> 
> Dmitry Vyukov (7):
>   perf report: Add machine parallelism
>   perf report: Add parallelism sort key
>   perf report: Switch filtered from u8 to u16
>   perf report: Add parallelism filter
>   perf report: Add latency output field
>   perf report: Add --latency flag
>   perf report: Add latency and parallelism profiling documentation
> 
> Cc: Namhyung Kim <namhyung@kernel.org>
> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> Cc: Ian Rogers <irogers@google.com>
> Cc: linux-perf-users@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> 
>  .../callchain-overhead-calculation.txt        |  5 +-
>  .../cpu-and-latency-overheads.txt             | 85 ++++++++++++++++++
>  tools/perf/Documentation/perf-report.txt      | 49 ++++++----
>  tools/perf/Documentation/tips.txt             |  3 +
>  tools/perf/builtin-record.c                   | 20 +++++
>  tools/perf/builtin-report.c                   | 39 ++++++++
>  tools/perf/ui/browsers/hists.c                | 27 +++---
>  tools/perf/ui/hist.c                          | 64 +++++++++----
>  tools/perf/util/addr_location.c               |  1 +
>  tools/perf/util/addr_location.h               |  7 +-
>  tools/perf/util/event.c                       | 11 +++
>  tools/perf/util/events_stats.h                |  2 +
>  tools/perf/util/hist.c                        | 90 +++++++++++++++----
>  tools/perf/util/hist.h                        | 26 +++++-
>  tools/perf/util/machine.c                     |  7 ++
>  tools/perf/util/machine.h                     |  6 ++
>  tools/perf/util/session.c                     | 12 +++
>  tools/perf/util/session.h                     |  1 +
>  tools/perf/util/sort.c                        | 69 ++++++++++++--
>  tools/perf/util/sort.h                        |  3 +-
>  tools/perf/util/symbol.c                      | 34 +++++++
>  tools/perf/util/symbol_conf.h                 |  8 +-
>  22 files changed, 498 insertions(+), 71 deletions(-)
>  create mode 100644 tools/perf/Documentation/cpu-and-latency-overheads.txt
> 
> 
> base-commit: 91b7747dc70d64b5ec56ffe493310f207e7ffc99
> -- 
> 2.48.1.262.g85cc9f2d1e-goog
>