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confirmed. If >>> only a single process has pages on a slow memory tier, for example, then >>> flexible tuning of the scan period and size ensures we do not scan >>> needlessly. The scan period can be tuned to be more responsive (down to >>> 400ms in this proposal) depending on how many accesses we have on the >>> last scan; similarly, it can be much less responsive (up to 5s) if memory >>> is not found to be accessed. >>> >>> I also asked if scanning can be disabled entirely, Raghu clarified that >>> it cannot be. >>> >> >> We have a sysfs tunable (kmmscand/scan_enabled) to enable/disable the >> whole scanning at a global level but not at process level granularity. >> > > Thanks Raghu for the clarification. I think during discussion that there > was a preference to make this multi-threaded so we didn't rely on a single > kmmscand thread, perhaps this would be (at minimum) one kmmscand thread > per NUMA node? > Correct. From my side a bit more thought on: - whether we need kthread for CXL nodes too? - How to share the scanning between kthreads etc.. needed. >>> Wei Xu asked if the scan period should be interpreted as the minimal >>> interval between scans because kmmscand is single threaded and there are >>> many processes. Raghu confirmed this is correct, the minimal delay. >>> Even if the scan period is 400ms, in reality it could be multiple seconds >>> based on load. >>> >>> Liam Howlett asked how we could have two scans colliding in a time >>> segment. Raghu noted if we are able to complete the last scan in less >>> time than 400ms, then we have this delay to avoid continuously scanning >>> that results in increased cpu overhead. Liam further asked if processes >>> opt into a scan or out of the scan, Raghu noted we always scan every >>> process. John Hubbard suggested that we have per-process control. >> >> +1 for prctl() >> >> Also I want to add that, I will get data on: >> >> what is the min and max time required to finish the entire scan for the >> current micro-benchmark and one of the real workload (such as Redis/ >> Rocksdb...), so that we can check if we are meeting the deadline of >> scanning with single kthread. >> > > Do we want more fine-grained per-process control other than just the > ability to opt out entire processes? There may be situations where we > want to always serve latency tolerant jobs from CXL extended memory, we > don't care to ever promote its memory, but I also think there will be > processes that are between the two extremes (latency critical and latency > tolerant). > > I think careful consideration needs to be given to how we handle > per-process policy for multi-tenant systems that have different levels of > latency sensitivity. If kmmscand becomes the standard way of doing page > promotion in the kernel, the userspace API to inform it of these policy > decisions is going to be key. There have been approaches where this was > primarily driven by BPF that has to solve the same challenge. > Very good point. How to defer/skip? Should we provide a numeric value to determine latency sensitivity range? This can be provided perhaps along with a normal enable/disable. >>> Wei noted an important point about separating hot page detection and >>> promotion, which don't actually need to be coupled at all. This uses >>> page table scanning while future support may not need to leverage this at >>> all. We'd very much like to avoid multiple promotion solutions for >>> different ways to track page hotness. >>> >>> I strongly supported this because I believe for CXL, at least within the >>> next three years, that memory hotness will likely not be derived from >>> page table Accessed bit scanning. Zi Yan agreed. >>> >>> The promotion path may also want to be much less aggressive than on first >>> access. Raghu showed many improvements, including handling short lived >>> processes, more accurate hot page detection using timestamp, etc. >> >> Some of these TODOs can be implemented in next version. >> > > Thanks! Are you planning on sending out another RFC patch series soon or > are you interested in publishing this on git.kernel.org or github? There > may be an opportunity for others to send you pull requests into the series > of patches while we discuss. > Good idea. will do. Perhaps a simple changes that is needed immediately as next RFC + github (Will explore on this internally how it is done here.) sooner. >>> ----->o----- >>> I followed up on a discussion point early in the talk about whether this >>> should be virtual address scanning like the current approach, walking >>> mm_struct's, or the alternative approach which would be physical address >>> scanning. >>> >>> Raghu sees this as a fully alternative approach such as what DAMON uses >>> that is based on rmap. The only advantage appears to be avoiding >>> scanning on top tier memory completely. >> >> Having a clarity here would help. Both the approaches have its own pros >> and cons. >> >> Need to also explore on using / Reusing DMAON/ MGLRU.. to the extent possible >> based on the approach. >> > > Yeah, I definitely think this is a key point to discuss early on. Gregory > had indicated that unmapped file cache is one of the key downsides to > using only virtual memory scanning. > > While things like the CHMU are still on the way, I think there's benefit > to making incremental progress from what we currently have available (NUMA > Balancing) before we get there. Agree. Regards - Raghu