PERF-TOP(1)

📦
version 7.3.1 · 42 versions
, in arch rolling

🦋 PERF-TOP(1) 🦋

okay bestie, this is perf-top from the perf Manual — the system profiling tool that's basically a live vibe check for your performance counters. No cap, it's giving main-character energy for your CPU. 💅✨

💖 NAME 💖

perf-top - System profiling tool.

🌸 SYNOPSIS 🌸

perf top [-e <EVENT> | --event=EVENT] [<options>]

✨ DESCRIPTION ✨

bestie, this command literally generates and displays a performance counter profile in real time. It's a whole live feed of what your system is doing, and yes, it is iconic. 🔥

🎀 OPTIONS 🎀

  • -a, --all-cpus — System-wide collection. (This is the default, bestie. It's already doing the most.) 💅
  • -c <count>, --count=<count> — Event period to sample.
  • -C <cpu-list>, --cpu=<cpu> — Monitor only on the list of CPUs provided. Multiple CPUs can be provided as a comma-separated list with no space: 0,1. Ranges of CPUs are specified with -: 0-2. Default is to monitor all CPUS.
  • -d <seconds>, --delay=<seconds> — Number of seconds to delay between refreshes.
  • -e <event>, --event=<event> — Select the PMU event. Selection can be a symbolic event name (use perf list to list all events) or a raw PMU event in the form of rN where N is a hexadecimal value that represents the raw register encoding with the layout of the event control registers as described by entries in /sys/bus/event_source/devices/cpu/format/*.
  • --filter=<filter> — Event filter. This option should follow an event selector (-e). For syntax see perf-record(1).
  • -E <entries>, --entries=<entries> — Display this many functions.
  • -f <count>, --count-filter=<count> — Only display functions with more events than this.
  • --group-sort-idx — Sort the output by the event at the index n in group. If n is invalid, sort by the first event. It can support multiple groups with different amount of events.
⚠️ WARNING: This should be used on grouped events.
  • -F <freq>, --freq=<freq> — Profile at this frequency. Use max to use the currently maximum allowed frequency, i.e. the value in the kernel.perf_event_max_sample_rate sysctl.
  • -i, --inherit — Child tasks do not inherit counters.
  • -k <path>, --vmlinux=<path> — Path to vmlinux. Required for annotation functionality.
  • --ignore-vmlinux — Ignore vmlinux files.
  • --kallsyms=<file> — kallsyms pathname.
  • -m <pages>, --mmap-pages=<pages> — Number of mmap data pages (must be a power of two) or size specification in bytes with appended unit character - B/K/M/G. The size is rounded up to the nearest power-of-two page value.
  • -p <pid>, --pid=<pid> — Profile events on existing Process ID (comma separated list).
  • -t <tid>, --tid=<tid> — Profile events on existing thread ID (comma separated list).
  • -u, --uid= — Record events in threads owned by uid. Name or number.
  • -r <priority>, --realtime=<priority> — Collect data with this RT SCHED_FIFO priority.
  • --sym-annotate=<symbol> — Annotate this symbol.
  • -K, --hide_kernel_symbols — Hide kernel symbols.
  • -U, --hide_user_symbols — Hide user symbols.
  • --demangle-kernel — Demangle kernel symbols.
  • -D, --dump-symtab — Dump the symbol table used for profiling.
  • -v, --verbose — Be more verbose (show counter open errors, etc).
  • -z, --zero — Zero history across display updates.
  • -s, --sort — Sort by key(s): pid, comm, dso, symbol, parent, srcline, weight, local_weight, abort, in_tx, transaction, overhead, sample, period. Please see description of --sort in the perf-report man page.
  • --fields= — Specify output field - multiple keys can be specified in CSV format. Following fields are available: overhead, overhead_sys, overhead_us, overhead_children, sample and period. Also it can contain any sort key(s).
By default, every sort keys not specified in --field will be appended automatically.
  • -n, --show-nr-samples — Show a column with the number of samples.
  • --show-total-period — Show a column with the sum of periods.
  • --dsos — Only consider symbols in these dsos. This option will affect the percentage of the overhead column. See --percentage for more info.
  • --comms — Only consider symbols in these comms. This option will affect the percentage of the overhead column. See --percentage for more info.
  • --symbols — Only consider these symbols. This option will affect the percentage of the overhead column. See --percentage for more info.
  • -M, --disassembler-style= — Set disassembler style for objdump.
  • --addr2line=<path> — Path to addr2line binary.
  • --objdump=<path> — Path to objdump binary.
  • --prefix=PREFIX, --prefix-strip=N — Remove first N entries from source file path names in executables and add PREFIX. This allows to display source code compiled on systems with different file system layout.
  • --source — Interleave source code with assembly code. Enabled by default, disable with --no-source.
  • --asm-raw — Show raw instruction encoding of assembly instructions.
  • -g — Enables call-graph (stack chain/backtrace) recording.
  • --call-graph [mode,type,min[,limit],order[,key][,branch]] — Setup and enable call-graph (stack chain/backtrace) recording, implies -g. See --call-graph section in perf-record and perf-report man pages for details.
  • --children — 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 -g/--call-graph option enabled. See the ‘overhead calculation’ section for more details. Enabled by default, disable with --no-children.
  • --max-stack — Set the stack depth limit when parsing the callchain, anything beyond the specified depth will be ignored. This is a trade-off between information loss and faster processing especially for workloads that can have a very long callchain stack.
Default: /proc/sys/kernel/perf_event_max_stack when present, 127 otherwise.
  • --ignore-callees=<regex> — Ignore callees of the function(s) matching the given regex. This has the effect of collecting the callers of each such function into one place in the call-graph tree.
  • --percent-limit — Do not show entries which have an overhead under that percent. (Default: 0).
  • --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).
"relative" means it's relative to filtered entries only so that the sum of shown entries will be always 100%. "absolute" means it retains the original value before and after the filter is applied.
  • -w, --column-widths=<width[,width...]> — Force each column width to the provided list, for large terminal readability. 0 means no limit (default behavior).
  • --proc-map-timeout — When processing pre-existing threads /proc/XXX/mmap, it may take a long time, because the file may be huge. A time out is needed in such cases. This option sets the time out limit. The default value is 500 ms.
  • -b, --branch-any — Enable taken branch stack sampling. Any type of taken branch may be sampled. This is a shortcut for --branch-filter any. See --branch-filter for more infos.
  • -j, --branch-filter — Enable taken branch stack sampling. Each sample captures a series of consecutive taken branches. The number of branches captured with each sample depends on the underlying hardware, the type of branches of interest, and the executed code. It is possible to select the types of branches captured by enabling filters. For a full list of modifiers please see the perf record manpage.

The option requires at least one branch type among any, any_call, any_ret, ind_call, cond. The privilege levels may be omitted, in which case, the privilege levels of the associated event are applied to the branch filter. Both kernel (k) and hypervisor (hv) privilege levels are subject to permissions. When sampling on multiple events, branch stack sampling is enabled for all the sampling events. The sampled branch type is the same for all events. The various filters must be specified as a comma separated list: --branch-filter any_ret,u,k. Note that this feature may not be available on all processors.

  • --branch-history — Add the addresses of sampled taken branches to the callstack. This allows to examine the path the program took to each sample.
  • --raw-trace — When displaying traceevent output, do not use print fmt or plugins.
  • -H, --hierarchy — Enable hierarchical output. In the hierarchy mode, each sort key groups samples based on the criteria and then sub-divide it using the lower level sort key.

For example, in normal output:

perf report -s dso,sym
#
# Overhead  Shared Object      Symbol
# ........  .................  ...........
    50.00%  [kernel.kallsyms]  [k] kfunc1
    20.00%  perf               [.] foo
    15.00%  [kernel.kallsyms]  [k] kfunc2
    10.00%  perf               [.] bar
     5.00%  libc.so            [.] libcall

In hierarchy output:

perf report -s dso,sym --hierarchy
#
#   Overhead  Shared Object / Symbol
# ..........  ......................
    65.00%    [kernel.kallsyms]
      50.00%    [k] kfunc1
      15.00%    [k] kfunc2
    30.00%    perf
      20.00%    [.] foo
      10.00%    [.] bar
     5.00%    libc.so
       5.00%    [.] libcall
  • --overwrite — Enable this to use just the most recent records, which helps in high core count machines such as Knights Landing/Mill, but right now is disabled by default as the pausing used in this technique is leading to loss of metadata events such as PERF_RECORD_MMAP which makes perf top unable to resolve samples, leading to lots of unknown samples appearing on the UI. Enable this if you are in such machines and profiling a workload that doesn’t create short lived threads and/or doesn’t use many executable mmap operations. Work is being planned to solve this situation, till then, this will remain disabled by default.
  • --force — Don’t do ownership validation.
  • --num-thread-synthesize — The number of threads to run when synthesizing events for existing processes. By default, the number of threads equals to the number of online CPUs.
  • --namespaces — Record events of type PERF_RECORD_NAMESPACES and display it with the cgroup_id sort key.
  • -G name, --cgroup name — okay so this one means monitor only in the container (cgroup) called name. Heads up: this option is available only in per-cpu mode, and the cgroup filesystem must be mounted. All threads belonging to container name are monitored when they run on the monitored CPUs. Multiple cgroups can be provided — each cgroup is applied to the corresponding event, i.e., first cgroup to first event, second cgroup to second event and so on. It is possible to provide an empty cgroup (monitor all the time) using, e.g., -G foo,,bar. Cgroups must have corresponding events, i.e., they always refer to events defined earlier on the command line. If the user wants to track multiple events for a specific cgroup, the user can use -e e1 -e e2 -G foo,foo or just use -e e1 -e e2 -G foo.
  • --all-cgroups — Record events of type PERF_RECORD_CGROUP and display it with the cgroup sort key. 💅
  • --switch-on EVENT_NAME — Only consider events after this event is found.

E.g.:

Find out where broadcast packets are handled

perf probe -L icmp_rcv

Insert a probe there:

perf probe icmp_rcv:59

Start perf top and ask it to only consider the cycles events when a
broadcast packet arrives This will show a menu with two entries and
will start counting when a broadcast packet arrives:

perf top -e cycles,probe:icmp_rcv --switch-on=probe:icmp_rcv

Alternatively one can ask for a group and then two overhead columns
will appear, the first for cycles and the second for the switch-on event.

perf top -e '{cycles,probe:icmp_rcv}' --switch-on=probe:icmp_rcv

This may be interesting to measure a workload only after some initialization
phase is over, i.e. insert a perf probe at that point and use the above
examples replacing probe:icmp_rcv with the just-after-init probe.
  • --switch-off EVENT_NAME — Stop considering events after this event is found.
  • --show-on-off-events — Show the --switch-on/off events too. This has no effect in perf top now but probably we’ll make the default not to show the switch-on/off events on the --group mode and if there is only one event besides the off/on ones, go straight to the histogram browser, just like perf top with no events explicitly specified does.
  • --stitch-lbr — Show callgraph with stitched LBRs, which may have more complete callgraph. The option must be used with --call-graph lbr recording. Disabled by default. In common cases with call stack overflows, it can recreate better call stacks than the default lbr call stack output. But this approach is not foolproof. There can be cases where it creates incorrect call stacks from incorrect matches. The known limitations include exception handing such as setjmp/longjmp will have calls/returns not match. 😭
  • --pfm-events events — Select a PMU event using libpfm4 syntax (see http://perfmon2.sf.net) including support for event filters. For example --pfm-events inst_retired:any_p:u:c=1:i. More than one event can be passed to the option using the comma separator. Hardware events and generic hardware events cannot be mixed together. The latter must be used with the -e option. The -e option and this one can be mixed and matched. Events can be grouped using the {} notation. ✨

✨ INTERACTIVE PROMPTING KEYS ✨

  • [d] — Display refresh delay.
  • [e] — Number of entries to display.
  • [E] — Event to display when multiple counters are active.
  • [f] — Profile display filter (>= hit count).
  • [F] — Annotation display filter (>= % of total).
  • [s] — Annotate symbol.
  • [S] — Stop annotation, return to full profile display.
  • [K] — Hide kernel symbols.
  • [U] — Hide user symbols.
  • [z] — Toggle event count zeroing across display updates.
  • [qQ] — Quit.

Pressing any unmapped key displays a menu, and prompts for input. 🫶

🦋 OVERHEAD CALCULATION 🦋

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%.

The children overhead is calculated by adding all period values of the child functions so that it can show the total overhead of the higher level functions even if they don’t directly execute much. Children here means functions that are called from another (parent) function.

It might be confusing that the sum of all the children overhead values exceeds 100% since each of them is already an accumulation of self overhead of its child functions. But with this enabled, users can find which function has the most overhead even if samples are spread over the children.

Consider the following example; there are three functions like below.

void foo(void) {
    /* do something */
}

void bar(void) {
    /* do something */
    foo();
}

int main(void) {
    bar()
    return 0;
}

In this case foo is a child of bar, and bar is an immediate child of main so foo also is a child of main. In other words, main is a parent of foo and bar, and bar is a parent of foo.

Suppose all samples are recorded in foo and bar only. When it’s recorded with callchains the output will show something like below in the usual (self-overhead-only) output of perf report:

Overhead  Symbol
........  .....................
  60.00%  foo
          |
          --- foo
              bar
              main
              __libc_start_main

  40.00%  bar
          |
          --- bar
              main
              __libc_start_main

When the --children option is enabled, the self overhead values of child functions (i.e. foo and bar) are added to the parents to calculate the children overhead. In this case the report could be displayed as:

Children      Self  Symbol
........  ........  ....................
 100.00%     0.00%  __libc_start_main
          |
          --- __libc_start_main

 100.00%     0.00%  main
          |
          --- main
              __libc_start_main

 100.00%    40.00%  bar
          |
          --- bar
              main
              __libc_start_main

  60.00%    60.00%  foo
          |
          --- foo
              bar
              main
              __libc_start_main

In the above output, the self overhead of foo (60%) was add to the children overhead of bar, main and __libc_start_main. Likewise, the self overhead of bar (40%) was added to the children overhead of main and __libc_start_main.

So __libc_start_main and main are shown first since they have same (100%) children overhead (even though they have zero self overhead) and they are the parents of foo and bar. 💖

Since v3.16 the children overhead is shown by default and the output is sorted by its values. The children overhead is disabled by specifying --no-children option on the command line or by adding report.children = false or top.children = false in the perf config file.

🌈 SEE ALSO 🌈

perf-stat(1), perf-list(1), perf-report(1) — go read your sisters, bestie, they ate. 👑

💖 cute little footer sign-off: source/version perf, date 10/31/2026, page PERF-TOP(1) — mwah, go profile something iconic! 💅✨

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