Re: [PATCH v3 7/8] perf report: Add latency and parallelism profiling documentation

From: Namhyung Kim
Date: Mon Feb 03 2025 - 23:28:01 EST

On Mon, Feb 03, 2025 at 03:30:42PM +0100, Dmitry Vyukov wrote:
> Describe latency and parallelism profiling, related flags, and differences
> with the currently only supported CPU-consumption-centric profiling.

It doesn't seem to have descriptions for the --latency option (for perf
record and report). Probably better to put them in the previous patch.

>
> Signed-off-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Ian Rogers <irogers@xxxxxxxxxx>
> Cc: linux-perf-users@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
> .../callchain-overhead-calculation.txt | 5 +-
> .../cpu-and-latency-overheads.txt | 85 +++++++++++++++++++
> tools/perf/Documentation/perf-report.txt | 49 +++++++----
> tools/perf/Documentation/tips.txt | 3 +
> 4 files changed, 123 insertions(+), 19 deletions(-)
>
> diff --git a/tools/perf/Documentation/callchain-overhead-calculation.txt b/tools/perf/Documentation/callchain-overhead-calculation.txt
> index 1a757927195ed..e0202bf5bd1a0 100644
> --- a/tools/perf/Documentation/callchain-overhead-calculation.txt
> +++ b/tools/perf/Documentation/callchain-overhead-calculation.txt
> @@ -1,7 +1,8 @@
> Overhead calculation
> --------------------
> -The overhead can be shown in two columns as 'Children' and 'Self' when
> -perf collects callchains. The 'self' overhead is simply calculated by
> +The CPU overhead can be shown in two columns as 'Children' and 'Self'
> +when perf collects callchains (and corresponding 'Wall' columns for
> +wall-clock overhead). The 'self' overhead is simply calculated by
> adding all period values of the entry - usually a function (symbol).
> This is the value that perf shows traditionally and sum of all the
> 'self' overhead values should be 100%.
> diff --git a/tools/perf/Documentation/cpu-and-latency-overheads.txt b/tools/perf/Documentation/cpu-and-latency-overheads.txt
> new file mode 100644
> index 0000000000000..3b6d637054651
> --- /dev/null
> +++ b/tools/perf/Documentation/cpu-and-latency-overheads.txt
> @@ -0,0 +1,85 @@
> +CPU and latency overheads
> +-------------------------
> +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.
> +Perf can measure overhead for both of these times (shown in 'overhead' and
> +'latency' columns for CPU and wall-clock time correspondingly).
> +
> +Optimizing CPU overhead is useful to improve 'throughput', while optimizing
> +latency overhead is useful to improve 'latency'. It's important to understand
> +which one is useful in a concrete situation at hand. For example, the former
> +may be useful to improve max throughput of a CI build server that runs on 100%
> +CPU utilization, while the latter may be useful to improve user-perceived
> +latency of a single interactive program build.
> +These overheads may be significantly different in some cases. For example,
> +consider a program that executes function 'foo' for 9 seconds with 1 thread,
> +and then executes function 'bar' for 1 second with 128 threads (consumes
> +128 seconds of CPU time). The CPU overhead is: 'foo' - 6.6%, 'bar' - 93.4%.
> +While the latency overhead is: 'foo' - 90%, 'bar' - 10%. If we try to optimize
> +running time of the program looking at the (wrong in this case) CPU overhead,
> +we would concentrate on the function 'bar', but it can yield only 10% running
> +time improvement at best.
> +
> +By default, perf shows only CPU overhead. To show latency overhead, use
> +'perf record --latency' and 'perf report':
> +
> +-----------------------------------
> +Overhead Latency Command
> + 93.88% 25.79% cc1
> + 1.90% 39.87% gzip
> + 0.99% 10.16% dpkg-deb
> + 0.57% 1.00% as
> + 0.40% 0.46% sh
> +-----------------------------------
> +
> +To sort by latency overhead, use 'perf report --latency':
> +
> +-----------------------------------
> +Latency Overhead Command
> + 39.87% 1.90% gzip
> + 25.79% 93.88% cc1
> + 10.16% 0.99% dpkg-deb
> + 4.17% 0.29% git
> + 2.81% 0.11% objtool
> +-----------------------------------
> +
> +To get insight into the difference between the overheads, you may check
> +parallelization histogram with '--sort=latency,parallelism,comm,symbol --hierarchy'

I think you need to omit latency in the sort key and recommend users to
use --latency option instead.

'perf report --hierarchy --latency --sort=parallelism,comm,symbol'


> +flags. It shows fraction of (wall-clock) time the workload utilizes different
> +numbers of cores ('Parallelism' column). For example, in the following case
> +the workload utilizes only 1 core most of the time, but also has some
> +highly-parallel phases, which explains significant difference between
> +CPU and wall-clock overheads:
> +
> +-----------------------------------
> + Latency Overhead Parallelism / Command / Symbol
> ++ 56.98% 2.29% 1
> ++ 16.94% 1.36% 2
> ++ 4.00% 20.13% 125
> ++ 3.66% 18.25% 124
> ++ 3.48% 17.66% 126
> ++ 3.26% 0.39% 3
> ++ 2.61% 12.93% 123
> +-----------------------------------
> +
> +By expanding corresponding lines, you may see what commands/functions run
> +at the given parallelism level:
> +
> +-----------------------------------
> + Latency Overhead Parallelism / Command / Symbol
> +- 56.98% 2.29% 1
> + 32.80% 1.32% gzip
> + 4.46% 0.18% cc1
> + 2.81% 0.11% objtool
> + 2.43% 0.10% dpkg-source
> + 2.22% 0.09% ld
> + 2.10% 0.08% dpkg-genchanges
> +-----------------------------------
> +
> +To see the normal function-level profile for particular parallelism levels
> +(number of threads actively running on CPUs), you may use '--parallelism'
> +filter. For example, to see the profile only for low parallelism phases
> +of a workload use '--latency --parallelism=1-2' flags.
> diff --git a/tools/perf/Documentation/perf-report.txt b/tools/perf/Documentation/perf-report.txt
> index 87f8645194062..7e0ba990d71e8 100644
> --- a/tools/perf/Documentation/perf-report.txt
> +++ b/tools/perf/Documentation/perf-report.txt
> @@ -44,7 +44,7 @@ OPTIONS
> --comms=::
> Only consider symbols in these comms. CSV that understands
> file://filename entries. This option will affect the percentage of
> - the overhead column. See --percentage for more info.
> + the overhead and latency columns. See --percentage for more info.
> --pid=::
> Only show events for given process ID (comma separated list).
>
> @@ -54,12 +54,12 @@ OPTIONS
> --dsos=::
> Only consider symbols in these dsos. CSV that understands
> file://filename entries. This option will affect the percentage of
> - the overhead column. See --percentage for more info.
> + the overhead and latency columns. See --percentage for more info.
> -S::
> --symbols=::
> Only consider these symbols. CSV that understands
> file://filename entries. This option will affect the percentage of
> - the overhead column. See --percentage for more info.
> + the overhead and latency columns. See --percentage for more info.
>
> --symbol-filter=::
> Only show symbols that match (partially) with this filter.
> @@ -68,6 +68,16 @@ OPTIONS
> --hide-unresolved::
> Only display entries resolved to a symbol.
>
> +--parallelism::
> + Only consider these parallelism levels. Parallelism level is the number
> + of threads that actively run on CPUs at the time of sample. The flag
> + accepts single number, comma-separated list, and ranges (for example:
> + "1", "7,8", "1,64-128"). This is useful in understanding what a program
> + is doing during sequential/low-parallelism phases as compared to
> + high-parallelism phases. This option will affect the percentage of
> + the overhead and latency columns. See --percentage for more info.
> + Also see the `CPU and latency overheads' section for more details.
> +
> -s::
> --sort=::
> Sort histogram entries by given key(s) - multiple keys can be specified
> @@ -87,6 +97,7 @@ OPTIONS
> entries are displayed as "[other]".
> - cpu: cpu number the task ran at the time of sample
> - socket: processor socket number the task ran at the time of sample
> + - parallelism: number of running threads at the time of sample
> - srcline: filename and line number executed at the time of sample. The
> DWARF debugging info must be provided.
> - srcfile: file name of the source file of the samples. Requires dwarf
> @@ -97,12 +108,14 @@ OPTIONS
> - cgroup_id: ID derived from cgroup namespace device and inode numbers.
> - cgroup: cgroup pathname in the cgroupfs.
> - transaction: Transaction abort flags.
> - - overhead: Overhead percentage of sample
> - - overhead_sys: Overhead percentage of sample running in system mode
> - - overhead_us: Overhead percentage of sample running in user mode
> - - overhead_guest_sys: Overhead percentage of sample running in system mode
> + - overhead: CPU overhead percentage of sample.
> + - latency: latency (wall-clock) overhead percentage of sample.
> + See the `CPU and latency overheads' section for more details.
> + - overhead_sys: CPU overhead percentage of sample running in system mode
> + - overhead_us: CPU overhead percentage of sample running in user mode
> + - overhead_guest_sys: CPU overhead percentage of sample running in system mode
> on guest machine
> - - overhead_guest_us: Overhead percentage of sample running in user mode on
> + - overhead_guest_us: CPU overhead percentage of sample running in user mode on
> guest machine
> - sample: Number of sample
> - period: Raw number of event count of sample
> @@ -125,8 +138,8 @@ OPTIONS
> - weight2: Average value of event specific weight (2nd field of weight_struct).
> - weight3: Average value of event specific weight (3rd field of weight_struct).
>
> - By default, comm, dso and symbol keys are used.
> - (i.e. --sort comm,dso,symbol)
> + By default, overhead, comm, dso and symbol keys are used.
> + (i.e. --sort overhead,comm,dso,symbol).
>
> If --branch-stack option is used, following sort keys are also
> available:
> @@ -201,9 +214,9 @@ OPTIONS
> --fields=::
> Specify output field - multiple keys can be specified in CSV format.
> Following fields are available:
> - overhead, overhead_sys, overhead_us, overhead_children, sample, period,
> - weight1, weight2, weight3, ins_lat, p_stage_cyc and retire_lat. The
> - last 3 names are alias for the corresponding weights. When the weight
> + overhead, latency, overhead_sys, overhead_us, overhead_children, sample,
> + period, weight1, weight2, weight3, ins_lat, p_stage_cyc and retire_lat.
> + The last 3 names are alias for the corresponding weights. When the weight
> fields are used, they will show the average value of the weight.
>
> Also it can contain any sort key(s).
> @@ -289,7 +302,7 @@ OPTIONS
> Accumulate callchain of children to parent entry so that then can
> show up in the output. The output will have a new "Children" column
> and will be sorted on the data. It requires callchains are recorded.
> - See the `overhead calculation' section for more details. Enabled by
> + See the `Overhead calculation' section for more details. Enabled by
> default, disable with --no-children.
>
> --max-stack::
> @@ -442,9 +455,9 @@ OPTIONS
> --call-graph option for details.
>
> --percentage::
> - Determine how to display the overhead percentage of filtered entries.
> - Filters can be applied by --comms, --dsos and/or --symbols options and
> - Zoom operations on the TUI (thread, dso, etc).
> + Determine how to display the CPU and latency overhead percentage
> + of filtered entries. Filters can be applied by --comms, --dsos, --symbols
> + and/or --parallelism options and Zoom operations on the TUI (thread, dso, etc).
>
> "relative" means it's relative to filtered entries only so that the
> sum of shown entries will be always 100%. "absolute" means it retains
> @@ -627,6 +640,8 @@ include::itrace.txt[]
> --skip-empty::
> Do not print 0 results in the --stat output.
>
> +include::cpu-and-latency-overheads.txt[]
> +
> include::callchain-overhead-calculation.txt[]
>
> SEE ALSO
> diff --git a/tools/perf/Documentation/tips.txt b/tools/perf/Documentation/tips.txt
> index 67b326ba00407..f6f71e70ff2cb 100644
> --- a/tools/perf/Documentation/tips.txt
> +++ b/tools/perf/Documentation/tips.txt
> @@ -62,3 +62,6 @@ To show context switches in perf report sample context add --switch-events to pe
> To show time in nanoseconds in record/report add --ns
> To compare hot regions in two workloads use perf record -b -o file ... ; perf diff --stream file1 file2
> To compare scalability of two workload samples use perf diff -c ratio file1 file2
> +For latency profiling, try: perf record/report --latency
> +For parallelism histogram, try: perf report --hierarchy --sort latency,parallelism,comm,symbol

Ditto.

Thanks,
Namhyung


> +To analyze particular parallelism levels, try: perf report --latency --parallelism=32-64
> --
> 2.48.1.362.g079036d154-goog
>