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This feature has now been accepted and merged in the upstream kernel and will be part of kernel release 5.9. This post has been updated to match the upstream version of this feature.

In my previous post, I described how on-demand compaction scheme hurts hugepage allocation latencies on Linux. To improve the situation, I have been working on Proactive Compaction for the Linux kernel, which tries to reduce higher-order allocation latencies by compacting memory in the background.

Some drivers needs to allocate almost all memory as hugepages to reduce (on-device or CPU) TLB pressure. However, on a running system, higher order allocations can fail if the memory is fragmented. Linux kernel can do on-demand compaction as we request more hugepages but this style of compaction incurs very high latency.

To show the effect of on-demand compaction on hugepage allocation latency, I created a test program “frag” which allocates almost all available system memory followed by freeing $\frac{3}{4}$ of pages from each hugepage-sized aligned chunk. This allocation pattern results in ~300% fragmented address space w.r.t order 9 i.e. physical mappings of our VA space is spread over 3x the number of hugepage-aligned chunks than what is ideally required.