Background Memory Compaction

Vlastimil Babka remarked on the fact that memory compaction only occurs at certain times, such as the kernel swap daemon, kswapd, balancing a zone of memory after failing to find a large enough region of free memory to satisfy a user request. Compaction solves this problem by grouping allocated memory together and leaving empty space grouped together as well.

Vlastimil acknowledged that there were other times when Linux might compact memory, but he wasn't satisfied. If the system waited until an allocation request came in before compacting the memory it needed to satisfy that request, this would create latency problems for larger allocation requests. Vlastimil proposed that, "To improve the situation, we need an equivalent of kswapd, but for compaction. E.g. a background thread which responds to fragmentation and the need for high-order allocations (including hugepages) somewhat proactively."

He considered and discarded the various existing threads that could be used. He felt that extending kswapd to include memory compaction would complicate its design too much, and extending khugepaged would have the drawback of being tied to THP (Transparent HugePages) configurations. Transparent HugePages is an abstraction layer that sits on top of normal memory allocation and deals in very large blocks of memory. Extending khugepaged would fail to compact memory that was allocated in any other way.

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