Speaker
Description
Hardware platforms continue to expose useful and actionable memory access information to the OS in various ways. Sources of such information include CPU-level instruction/op sampling mechanisms (like AMD IBS and ARM SPE), PMU-based precise sampling (like Intel PEBS), and device-side facilities such as the CXL Hotness Monitoring Unit (HMU). These platform-provided hints can be used by sub-systems like NUMA balancing, reclaim and hot page promotion in tiered memory systems to make better placement decisions, especially on systems with CXL-attached or accelerator memory where the cost of a bad placement is high.
The IBS Memory Profiler [1] on future AMD processors is one concrete example of such a hint source. It is a second, lightweight IBS instance on the chip, dedicated to memory access profiling, and provides per-access information (virtual/physical address, source/target NUMA node) at low overhead. This talk intends to share the experience of working with the IBS Memory Profiler and to look at the ways and challenges in making such a hardware source usable by the kernel in general.
For any such platform-provided hint to be useful, a sub-system that can collate and act on hotness information from multiple sources in a producer-agnostic manner is needed. The pghot subsystem [2] is being developed for this purpose. It moves the hot page promotion engine out of NUMA balancing into a sub-system of its own and exposes a uniform interface so that multiple producers (NUMA hint faults, AMD IBS, CXL HMU, etc.) can feed into a single promotion policy. pghot has been discussed at LPC 2023 [3], LSF/MM/BPF 2025 [4] and LPC 2025, and is a recurring topic on the biweekly Linux Memory Hotness and Promotion community call. In this talk, pghot is treated as the enabling base infrastructure and not as the main subject.
The talk will also touch upon the producer-side ABI that pghot should commit to, where filtering and aging of the access information should reside, the coexistence with NUMA balancing-driven sampling on capable hardware, and a set of workloads and metrics for fairly comparing different hotness sources.
[1] AMD IBS Memory Profiler documentation: https://docs.amd.com/v/u/en-US/69205_1.00_AMD64_IBS_PUB
[2] pghot v7 patchset: https://lore.kernel.org/linux-mm/20260504060924.344313-1-bharata@amd.com/
[3] Using hardware hints for optimal page placement, LPC 2023: https://lpc.events/event/17/contributions/1513/
[4] Unifying sources of page hotness information, LSF/MM/BPF 2025: https://lore.kernel.org/linux-mm/20250319124552.0000344a@huawei.com/