list_lru Optimizations for UEK7 U1

The Oracle Linux team has incorporated a major memory-saving feature into the UEK7 U1 release. The Linux kernel utilizes a least-recently-used list (list_lru) to track inodes and dentries. The list_lru grows proportionally with the number of memory cgroups on the system. On systems running a large number of containers, the list_lru can consume gigabytes of RAM.

For UEK7 U1, Kamalesh Babulal backported an upstream patchset [1] that improves the list_lru behavior. The list_lru tracks every memcg struct on every superblock, and in previous releases this information was made available to every memory cgroup – whether it was needed it or not. Many containers don’t utilize this information, and thus, this memory is often wasted. This patchset modifies the list_lru logic to delay the availability of the list_lru in the cgroup until the first time it’s needed.

To highlight the significance of this patchset, we ran a simple test on a 32-cpu Intel Xen machine with 188G of RAM. We sequentially created and deleted 400 lxc containers.

kmalloc-32 object count

For the above example, this patchset reduced the number of kmalloc-32 objects from 7.2 million down to approximately 83,000, a savings of ~227 MB and roughly 87x fewer kmalloc-32 objects!

The list_lru improvement is available and always on in the UEK7 U1 kernel.

[1] https://lore.kernel.org/linux-mm/20211220085649.8196-1-songmuchun@bytedance.com/