On Thu, Apr 7, 2022 at 3:04 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote: > > On Wed, Apr 6, 2022 at 8:29 PM Barry Song <21cnbao@xxxxxxxxx> wrote: > > > > On Wed, Mar 9, 2022 at 3:48 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote: > > > > > > Searching the rmap for PTEs mapping each page on an LRU list (to test > > > and clear the accessed bit) can be expensive because pages from > > > different VMAs (PA space) are not cache friendly to the rmap (VA > > > space). For workloads mostly using mapped pages, the rmap has a high > > > CPU cost in the reclaim path. > > > > > > This patch exploits spatial locality to reduce the trips into the > > > rmap. When shrink_page_list() walks the rmap and finds a young PTE, a > > > new function lru_gen_look_around() scans at most BITS_PER_LONG-1 > > > adjacent PTEs. On finding another young PTE, it clears the accessed > > > bit and updates the gen counter of the page mapped by this PTE to > > > (max_seq%MAX_NR_GENS)+1. > > > > Hi Yu, > > It seems an interesting feature to save the cost of rmap. but will it lead to > > possible judging of cold pages as hot pages? > > In case a page is mapped by 20 processes, and it has been accessed > > by 5 of them, when we look around one of the 5 processes, the page > > will be young and this pte is cleared. but we still have 4 ptes which are not > > cleared. then we don't access the page for a long time, but the 4 uncleared > > PTEs will still make the page "hot" since they are not cleared, we will find > > the page is hot either due to look-arounding the 4 processes or rmapping > > the page later? > > Why are the remaining 4 accessed PTEs skipped? The rmap should check > all the 20 PTEs. for example page A is the neighbour of page B in process 1, when we do rmap for B, we look-around and clear A's pte in process 1. but A's ptes are still set in process 2,3,4,5. > > Even if they were skipped, it doesn't matter. The same argument could > be made for the rest of 1 millions minus 1 pages that have been timely > scanned, on a 4GB laptop. The fundamental principle (assumption) of > MGLRU is never about making the best choices. Nothing can because it's > impossible to predict the future that well, given the complexity of > today's workloads, not on a phone, definitely not on a server that > runs mixed types of workloads. The primary goal is to avoid the worst > choices at a minimum (scanning) cost. The second goal is to pick good > ones at an acceptable cost, which probably are a half of all possible > choices. thanks barry
