Code Heatmaps
BOLT has gained the ability to print code heatmaps based on sampling-based profiles generated by perf, either with LBR data or not. The output is produced in colored ASCII to be displayed in a color-capable terminal. It looks something like this:
Heatmaps can be generated for BOLTed and non-BOLTed binaries. You can use them to compare the code layout before and after optimizations.
To generate a heatmap, start with running your app under perf:
$ perf record -e cycles:u -j any,u -- <executable with args>
or if you want to monitor the existing process(es):
$ perf record -e cycles:u -j any,u [-p PID|-a] -- sleep <interval>
Running with LBR (-j any,u or -b) is recommended. Heatmaps can be generated from basic events by using the llvm-bolt-heatmap option -nl (no LBR) but such heatmaps do not have the coverage provided by LBR and may only be useful for finding event hotspots at larger code block granularities.
Once the run is complete, and perf.data is generated, run llvm-bolt-heatmap:
$ llvm-bolt-heatmap -p perf.data <executable>
By default the heatmap will be dumped to stdout. You can change it with -o <heatmapfile> option.
If you prefer to look at the data in a browser (or would like to share it that way), then you can use an HTML conversion tool. E.g.:
$ aha -b -f <heatmapfile> > <heatmapfile>.html
Background on heatmaps:
A heatmap is effectively a histogram that is rendered into a grid for better visualization. In theory we can generate a heatmap using any binary and a perf profile.
Each block/character in the heatmap shows the execution data accumulated for corresponding 64 bytes of code. You can change this granularity with a -block-size option. E.g. set it to 4096 to see code usage grouped by 4K pages.
When a block is shown as a dot, it means that no samples were found for that address. When it is shown as a letter, it indicates a captured sample on a particular text section of the binary. To show a mapping between letters and text sections in the legend, use -print-mappings. When a sampled address does not belong to any of the text sections, the characters ‘o’ or ‘O’ will be shown.
The legend shows by default the ranges in the heatmap according to the number of samples per block. A color is assigned per range, except the first two ranges that distinguished by lower and upper case letters.
On the Y axis, each row/line starts with an actual address of the binary. Consecutive lines in the heatmap advance by the same amount, with the binary size covered by a line dependent on the block size and the line size. An empty new line is inserted for larger gaps between samples.
On the X axis, the horizontally emitted hex numbers can help estimate where in the line the samples lie, but they cannot be combined to provide a full address, as they are relative to both the bucket and line sizes.
In the example below, the highlighted 0x100 column is not an offset to each row's address, but instead, it points to the middle of the line. For the generation, the default bucket size was used with a line size of 128.
Some useful options are:
-line-size=<uint> - number of entries per line (default 256) -max-address=<uint> - maximum address considered valid for heatmap (default 4GB) -print-mappings - print mappings in the legend, between characters/blocks and text sections (default false)