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Raghavendra K T wrote: > > > On 9/18/2025 3:14 AM, Andrew Morton wrote: >> On Wed, 17 Sep 2025 08:24:15 -0700 Ankur Arora >> wrote: >> >>> Change folio_zero_user() to clear contiguous page ranges instead of >>> clearing using the current page-at-a-time approach. Exposing the largest >>> feasible length can be useful in enabling processors to optimize based >>> on extent. >> >> This patch is something which MM developers might care to take a closer >> look at. >> >>> However, clearing in large chunks can have two problems: >>> >>>   - cache locality when clearing small folios (< MAX_ORDER_NR_PAGES) >>>     (larger folios don't have any expectation of cache locality). >>> >>>   - preemption latency when clearing large folios. >>> >>> Handle the first by splitting the clearing in three parts: the >>> faulting page and its immediate locality, its left and right >>> regions; with the local neighbourhood cleared last. >> >> Has this optimization been shown to be beneficial? >> >> If so, are you able to share some measurements? >> >> If not, maybe it should be removed? >> > > I reverted the effect of this patch by hard coding > > #define PAGE_CONTIG_NR 1 > > I see that benefit for voluntary kernel is lost without this change > > (for rep stosb) > > with PAGE_CONTIG_NR equivalent to 8MB > > Preempt mode: voluntary > > # Running 'mem/mmap' benchmark: > # function 'demand' (Demand loaded mmap()) > # Copying 64GB bytes ... > >       34.533414 GB/sec > > > with PAGE_CONTIG_NR equivalent to 4KB > > # Running 'mem/mmap' benchmark: > # function 'demand' (Demand loaded mmap()) > # Copying 64GB bytes ... > >       20.766059 GB/sec > > For now (barring David's recommendations), > feel free to add > > Reviewed-by: Raghavendra K T > > My reply was more towards the benefits of clearing multiple pages for non-preempt and voluntary kernel, than the effect of clearing neighborhood range at the end. Sorry if that was confusing :)