Landing via Andrew Morton’s patch series today in the Linux 5.15 kernel is handling for demoting pages during memory reclaim, which can be used for punting cold pages off to slower, tiered memory devices (like Intel persistent memory) when under system memory pressure.
Intel engineers have been working on this migrating of pages to slower memory tiers the past few months and is now part of Linux 5.15. While the code can demote pages to slower memory tiers, currently there isn’t any means of promoting pages back into faster DRAM when capacity is available — there are other patches currently working on this promotion handling.
The current behavior with Linux right now is when the system memory (RAM) fills up under memory pressure, some of the DRAM contents will be tossed out. For recent and future servers with tiered memory like using Intel Optane DC persistent memory, the Linux kernel may eventually fall over to start using that persistent memory if necessary for allocations but not in any intelligent manner.
Intel engineers found this functionality could improve the PostgreSQL performance by as much as 22% on tiered memory servers with persistent memory.
With Linux 5.15 is now the notion of demoting pages during reclaim. This page migration on reclaim allows for the kernel to migrate pages from the primary system RAM over to slower tiers of memory when that fast tier is under memory pressure. This demoted reclaim is done prior to any swapping to disk and should be more desirable than just wiping out portions of the system memory when there is persistent memory available albeit slower.
- For memory tiered servers, Linux 5.15 has a significant improvement.
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