Using Linux Perf

Linux Perf provides low level CPU profiling with JavaScript, native and OS level frames.

Important: this tutorial is only available on Linux.

How To

Linux Perf is usually available through the linux-tools-common package. Through either --perf-basic-prof or --perf-basic-prof-only-functions we are able to start a Node.js application supporting perf_events.

--perf-basic-prof will always write to a file (/tmp/perf-PID.map), which can lead to infinite disk growth. If that’s a concern either use the module: linux-perf or --perf-basic-prof-only-functions.

The main difference between both is that --perf-basic-prof-only-functions produces less output, it is a viable option for production profiling.

# Launch the application an get the PID
$ node --perf-basic-prof-only-functions index.js &
[1] 3870

Then record events based in the desired frequency:

$ sudo perf record -F 99 -p 3870 -g

In this phase, you may want to use a load test in the application in order to generate more records for a reliable analysis. When the job is done, close the perf process by sending a SIGINT (Ctrl-C) to the command.

The perf will generate a file inside the /tmp folder, usually called /tmp/perf-PID.map (in above example: /tmp/perf-3870.map) containing the traces for each function called.

To aggregate those results in a specific file execute:

$ sudo perf script > perfs.out
$ cat ./perfs.out
node 3870 25147.878454:          1 cycles:
        ffffffffb5878b06 native_write_msr+0x6 ([kernel.kallsyms])
        ffffffffb580d9d5 intel_tfa_pmu_enable_all+0x35 ([kernel.kallsyms])
        ffffffffb5807ac8 x86_pmu_enable+0x118 ([kernel.kallsyms])
        ffffffffb5a0a93d perf_pmu_enable.part.0+0xd ([kernel.kallsyms])
        ffffffffb5a10c06 __perf_event_task_sched_in+0x186 ([kernel.kallsyms])
        ffffffffb58d3e1d finish_task_switch+0xfd ([kernel.kallsyms])
        ffffffffb62d46fb __sched_text_start+0x2eb ([kernel.kallsyms])
        ffffffffb62d4b92 schedule+0x42 ([kernel.kallsyms])
        ffffffffb62d87a9 schedule_hrtimeout_range_clock+0xf9 ([kernel.kallsyms])
        ffffffffb62d87d3 schedule_hrtimeout_range+0x13 ([kernel.kallsyms])
        ffffffffb5b35980 ep_poll+0x400 ([kernel.kallsyms])
        ffffffffb5b35a88 do_epoll_wait+0xb8 ([kernel.kallsyms])
        ffffffffb5b35abe __x64_sys_epoll_wait+0x1e ([kernel.kallsyms])
        ffffffffb58044c7 do_syscall_64+0x57 ([kernel.kallsyms])
        ffffffffb640008c entry_SYSCALL_64_after_hwframe+0x44 ([kernel.kallsyms])
....

The raw output can be a bit hard to understand so typically the raw file is used to generate flamegraphs for a better visualization.

Example nodejs flamegraph

To generate a flamegraph from this result, follow this tutorial from step 6.

Because perf output is not a Node.js specific tool, it might have issues with how JavaScript code is optimized in Node.js. See perf output issues for a futher reference.

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