Traditionally, all RAM is DRAM. Some DRAM might be closer/faster than
others, but a byte of media has about the same cost whether it is close
or far. But, with new memory tiers such as High-Bandwidth Memory or
Persistent Memory, there is a choice between fast/expensive and
We use the existing reclaim mechanisms for moving cold data out of
fast/expensive tiers. It works well for that. However, reclaim does
not work well for moving hot data which might be stuck in a slow tier
since the pages near the top of the LRU are the most recently accessed
only if there’s regular memory pressure on the slow/cheap tiers.
Fortunately, NUMA Balancing can find recently-accessed pages
regardless of memory pressure. We have repurposed it from being used
for location-based optimization to being used for tier-based
optimization. We have also optimized it for better hot data
identification, such as to find frequently-accessed pages instead of
recently-accessed pages, etc.
We will show our findings so far, and discuss the remaining problems,
potential solutions, and alternatives.
The patchset email threads are as follows,
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