Why the heatmap is not showing the expected access patterns?

TL; DR: try --draw_range all option of damo report heatmap if it shows not what you expected.

damo report heatmap outputs sometimes show no expected access pattern. It is sometimes just entirely black, or shows some access pattern but not what the user expected. This post is for explaining the reason and how you can work around.

The problem usually happens when user tries to draw the heatmap for access pattern that recorded for a virtual address space of a process. Each virtual address space is very huge. And usually those have two wide regions that not really mapped to physical memory. That’s because the stack and the heap are mapped from the two ends of the address space, and do so for mmap()-ed regions from the middle of the space. The resulting mapping looks like below:

<heap>
<BIG UNMAPPED REGION 1>
<uppermost mmap()-ed region>
(small mmap()-ed regions and munmap()-ed regions)
<lowermost mmap()-ed region>
<BIG UNMAPPED REGION 2>
<stack>

If we try to draw the heatmap for the entire address space, the two wide unmapped regions will cover most of the graph. And it will only show black, because the unmapped wide regions will never get accessed.

To draw heatmap for only the address ranges that meaningful accesses were made, damo report heatmap allows users to specify what address ranges they want draw heampa for, using --address_range option.

Because it is not easy to know to what address range specific data objects of the program is mapped, the command also provides a few features for helping it.

damo report heatmap --guide or damo record_info shows high level information about the record file. The three address ranges excepting the two huge gaps are included there. For example:

$ sudo ./damo record_info --info ./damon.data
target_id:0
time: 67900017466000-67959790343999 (59.773 s)
region   0: 00000093925306740736-00000093925678731264 (354.758 MiB)
region   1: 00000139965707698176-00000139965814927360 (102.262 MiB)
region   2: 00000140730699452416-00000140730699587584 (132.000 KiB)

To help easier selection, damo report heatmap also provides --draw_range option. As of this writing, the option allows two inputs, namely hottest and all. If all is passed, damo will draw heatmaps for all the mapped regions. If hottest is passed to the option, damo will find a mapped region that most access has happened, and draw the heatmap for only the hottest region.

As of this writing, damo report heatmap works as --draw_range hottest is given by default. Note that that the default behavior could be changed in future. Also, --draw_range option will be available from v2.7.2.

Wrapup

So, if damo report heatmap output is different from what you expected, please try running it again with --draw_range all. If you don’t want to draw all the maps but hottest one, please use --draw_range hottest. Or, if you want to draw the map for only specific region, use --guide option for high level information that will help you what region you really want to understand in dtail, and use --address_range option.