--- /dev/null
+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'
+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.
--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).
--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.
--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.
+
--latency::
Show latency-centric profile rather than the default
CPU-consumption-centric profile
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
- 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
- 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:
--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).
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::
--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
--skip-empty::
Do not print 0 results in the --stat output.
+include::cpu-and-latency-overheads.txt[]
+
include::callchain-overhead-calculation.txt[]
SEE ALSO
projects / linux-block.git / commitdiff