On Fri, Mar 17, 2023 at 11:19 AM Chris Li <chrisl@xxxxxxxxxx> wrote: > > On Fri, Mar 17, 2023 at 03:19:09AM -0700, Yosry Ahmed wrote: > > > Now, we will not drop the swap cache even if the swap count becomes 0 if > > > swap space utility < 50%. Per my understanding, this avoid swap page > > > writing for read accesses. So I don't think we can change this directly > > > without necessary discussion firstly. > > > > > > Right. I am not sure I understand why we do this today, is it to save > > the overhead of allocating a new swap entry if the page is swapped out > > again soon? I am not sure I understand this statement "this avoid swap > > page > > writing for read accesses". > > When the page is swapped out again soon. If the swap slot has been recycled, > then the page need to assign to a new swap slot, most likely different than > the previous slot. Then the page write out would need to write that page > to the swap device, even though that swap device might already has the > same page data in the previous slot. > > If keeping the previous swap slot cache when swap space utility < 50%, > then the swap code can avoid writing out to the same slot with the same > data, if the page is not dirty (read only access). > > The saving is in 1) avoid allocating a new slot. 2) For read access, > avoid page io write the same data to the same slot. I see. Makes sense. Thanks, Chris. I guess in this case we shouldn't just unconditionally free the swap entry once the page is swapped in without giving it some thought. > > Chris
