Hey Michal, Thanks for taking the time to review! On Thu, Jan 18, 2024 at 9:40 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Thu 18-01-24 20:03:46, Lance Yang wrote: > [...] > > before we discuss the semantic, let's focus on the usecase. > > > Use Cases > > > > An immediate user of this new functionality is the Go runtime heap allocator > > that manages memory in hugepage-sized chunks. In the past, whether it was a > > newly allocated chunk through mmap() or a reused chunk released by > > madvise(MADV_DONTNEED), the allocator attempted to eagerly back memory with > > huge pages using madvise(MADV_HUGEPAGE)[2] and madvise(MADV_COLLAPSE)[3] > > respectively. However, both approaches resulted in performance issues; for > > both scenarios, there could be entries into direct reclaim and/or compaction, > > leading to unpredictable stalls[4]. Now, the allocator can confidently use > > process_madvise(MADV_F_COLLAPSE_LIGHT) to attempt the allocation of huge pages. > > IIUC the primary reason is the cost of the huge page allocation which > can be really high if the memory is heavily fragmented and it is called > synchronously from the process directly, correct? Can that be worked Yes, that's correct. > around by process_madvise and performing the operation from a different > context? Are there any other reasons to have a different mode? In latency-sensitive scenarios, some applications aim to enhance performance by utilizing huge pages as much as possible. At the same time, in case of allocation failure, they prefer a quick return without triggering direct memory reclamation and compaction. > > I mean I can think of a more relaxed (opportunistic) MADV_COLLAPSE - > e.g. non blocking one to make sure that the caller doesn't really block > on resource contention (be it locks or memory availability) because that > matches our non-blocking interface in other areas but having a LIGHT > operation sounds really vague and the exact semantic would be > implementation specific and might change over time. Non-blocking has a > clear semantic but it is not really clear whether that is what you > really need/want. Could you provide me with some suggestions regarding the naming of a more relaxed (opportunistic) MADV_COLLAPSE? Thanks again for your review and your suggestion! Lance > > > [1] https://github.com/torvalds/linux/commit/7d8faaf155454f8798ec56404faca29a82689c77 > > [2] https://github.com/golang/go/commit/8fa9e3beee8b0e6baa7333740996181268b60a3a > > [3] https://github.com/golang/go/commit/9f9bb26880388c5bead158e9eca3be4b3a9bd2af > > [4] https://github.com/golang/go/issues/63334 > > > > [v1] https://lore.kernel.org/lkml/20240117050217.43610-1-ioworker0@xxxxxxxxx/ > -- > Michal Hocko > SUSE Labs
