From: SeongJae Park <sjpark@xxxxxxxxx> Hello, Very interesting work, thank you for sharing this :) On Tue, 13 Apr 2021 00:56:17 -0600 Yu Zhao <yuzhao@xxxxxxxxxx> wrote: > What's new in v2 > ================ > Special thanks to Jens Axboe for reporting a regression in buffered > I/O and helping test the fix. Is the discussion open? If so, could you please give me a link? > > This version includes the support of tiers, which represent levels of > usage from file descriptors only. Pages accessed N times via file > descriptors belong to tier order_base_2(N). Each generation contains > at most MAX_NR_TIERS tiers, and they require additional MAX_NR_TIERS-2 > bits in page->flags. In contrast to moving across generations which > requires the lru lock, moving across tiers only involves an atomic > operation on page->flags and therefore has a negligible cost. A > feedback loop modeled after the well-known PID controller monitors the > refault rates across all tiers and decides when to activate pages from > which tiers, on the reclaim path. > > This feedback model has a few advantages over the current feedforward > model: > 1) It has a negligible overhead in the buffered I/O access path > because activations are done in the reclaim path. > 2) It takes mapped pages into account and avoids overprotecting pages > accessed multiple times via file descriptors. > 3) More tiers offer better protection to pages accessed more than > twice when buffered-I/O-intensive workloads are under memory > pressure. > > The fio/io_uring benchmark shows 14% improvement in IOPS when randomly > accessing Samsung PM981a in the buffered I/O mode. Improvement under memory pressure, right? How much pressure? [...] > > Differential scans via page tables > ---------------------------------- > Each differential scan discovers all pages that have been referenced > since the last scan. Specifically, it walks the mm_struct list > associated with an lruvec to scan page tables of processes that have > been scheduled since the last scan. Does this means it scans only virtual address spaces of processes and therefore pages in the page cache that are not mmap()-ed will not be scanned? > The cost of each differential scan > is roughly proportional to the number of referenced pages it > discovers. Unless address spaces are extremely sparse, page tables > usually have better memory locality than the rmap. The end result is > generally a significant reduction in CPU usage, for workloads using a > large amount of anon memory. When and how frequently it scans? Thanks, SeongJae Park [...]
