147 lines
4.2 KiB
Markdown
147 lines
4.2 KiB
Markdown
# JIT Profiling with perf
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## Maybe preferable: profiling with `samply`
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`samply` makes profiling JIT a lot easier and should be preferred if running
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macOS or Linux. Documentation is at [JIT profiling with
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`samply`](jit_profiling_with_samply.md).
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## Introduction
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`perf` is a performance profiling tool available on Linux that is
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capable of measuring performance events such as cycles, instructions executed,
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cache misses, etc and providing assembly and source code annotation. It is
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possible to collect performance profiles of the SpiderMonkey JIT using perf on
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Linux and also annotate the generated assembly with the IR opcodes that were
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used during compilation as shown below.
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## Build setup
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JIT profiling with perf jitdump is **enabled by default for nightly builds** in the shell and browser.
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If you wish to disable jitdump support in your build, then you can include the following build flag:
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```
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ac_add_options --disable-jitdump
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```
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## Environment Variables
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Environment variables that must be defined for perf JIT profiling:
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`PERF_SPEW_DIR`: Location of jitdump output files. Making this directory a tmpfs filesystem could help reduce overhead.\
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`IONPERF`: Valid options include: `func`, `src`, `ir`, `ir-ops`.
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`IONPERF=func` will disable all annotation and only function names will be available. It is the fastest option.\
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`IONPERF=ir` will enable IR annotation.\
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`IONPERF=ir-ops` will enable IR annotation with operand support. **Requires --enable-jitspew** and adds additional overhead to "ir".\
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`IONPERF=ir-graph` will output the full IR for interactive visualization. **Requires --enable-jitspew** and adds additional overhead to "ir".\
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`IONPERF=src` will enable source code annotation **only if** perf can read the source file locally. Only really works well in the JS shell.
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## Profiling the JS shell
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Profiling the JS shell requires the following commands but is very straight forward.
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Begin by removing any pre-existing jitdump files:
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`rm -rf output` or `rm -f jitted-*.so jit.data perf.data jit-*.dump jitdump-*.txt`
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Next define environment variables:
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```
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export IONPERF=ir
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export PERF_SPEW_DIR=output
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```
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Run your test case with perf attached:
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```
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perf record -g -k 1 /home/denis/src/mozilla-central/obj-js/dist/bin/js test.js
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```
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Inject the jitdump files into your perf.data file:
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```
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perf inject -j -i perf.data -o jit.data
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```
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View the profile:
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```
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perf report --no-children --call-graph=graph,0 -i jit.data
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```
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All of the above commands can be put into a single shell script.
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## Profiling the Browser
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Profiling the browser is less straight forward than the shell, but the only main difference is that perf must attach to the content process while it is running.
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Begin by removing any pre-existing jitdump files:
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`rm -rf output` or `rm -f jitted-*.so jit.data perf.data jit-*.dump jitdump-*.txt`
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Next define environment variables:
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```
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export IONPERF=ir
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export PERF_SPEW_DIR=output
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export MOZ_DISABLE_CONTENT_SANDBOX=1
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```
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Run the Firefox browser
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```
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~/mozilla-central/obj-opt64/dist/bin/firefox -profile ~/mozilla-central/obj-opt64/tmp/profile-default &
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```
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Navigate to the test case, but do not start it yet. Then hover over the tab to get the content process PID.
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Attach perf to begin profiling:
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```
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perf record -g -k 1 -p <pid>
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```
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Close the browser when finished benchmarking.
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Inject the jitdump files into your perf.data file:
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```
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perf inject -j -i perf.data -o jit.data
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```
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View the profile (--call-graph=graph,0 shows all call stacks instead of the default threshold of >= 0.5%):
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```
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perf report --no-children --call-graph=graph,0 -i jit.data
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```
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## Additional Information
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Some Linux distributions offer a "libc6-prof" package that includes frame pointers. This can help resolve symbols and call stacks that involve libc calls.
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On Ubuntu, you can install this with:
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```
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sudo apt-get install libc6-prof
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```
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libc6-prof can be used with `LD_LIBRARY_PATH=/lib/libc6-prof/x86_64-linux-gnu`
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It may also be useful to have access to kernel addresses during profiling. These can be exposed with:
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```
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sudo sh -c "echo 0 > /proc/sys/kernel/kptr_restrict"
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```
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The max stack depth is 127 by default. This is often too few. It can be increased with:
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```
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sudo sh -c "echo 4000 > /proc/sys/kernel/perf_event_max_stack"
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```
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