forked from OSchip/llvm-project
51 lines
1.6 KiB
Markdown
51 lines
1.6 KiB
Markdown
# Code Heatmaps
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BOLT has gained the ability to print code heatmaps based on
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sampling-based LBR profiles generated by `perf`. The output is produced
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in colored ASCII to be displayed in a color-capable terminal. It looks
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something like this:
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Heatmaps can be generated for BOLTed and non-BOLTed binaries. You can
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use them to compare the code layout before and after optimizations.
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To generate a heatmap, start with running your app under `perf`:
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```bash
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$ perf record -e cycles:u -j any,u -- <executable with args>
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```
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or if you want to monitor the existing process(es):
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```bash
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$ perf record -e cycles:u -j any,u [-p PID|-a] -- sleep <interval>
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```
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Note that at the moment running with LBR (`-j any,u` or `-b`) is
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a requirement.
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Once the run is complete, and `perf.data` is generated, run BOLT in
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a heatmap mode:
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```bash
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$ llvm-bolt heatmap -p perf.data <executable>
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```
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By default the heatmap will be dumped to *stdout*. You can change it
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with `-o <heatmapfile>` option. Each character/block in the heatmap
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shows the execution data accumulated for corresponding 64 bytes of
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code. You can change this granularity with a `-block-size` option.
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E.g. set it to 4096 to see code usage grouped by 4K pages.
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Other useful options are:
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```bash
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-line-size=<uint> - number of entries per line (default 256)
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-max-address=<uint> - maximum address considered valid for heatmap (default 4GB)
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```
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If you prefer to look at the data in a browser (or would like to share
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it that way), then you can use an HTML conversion tool. E.g.:
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```bash
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$ aha -b -f <heatmapfile> > <heatmapfile>.html
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```
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