On Thu, Mar 01, 2018 at 04:09:50PM -0800, Andrew Morton wrote: > On Thu, 1 Mar 2018 14:28:45 +0800 Aaron Lu <aaron...@intel.com> wrote: > > > When a page is freed back to the global pool, its buddy will be checked > > to see if it's possible to do a merge. This requires accessing buddy's > > page structure and that access could take a long time if it's cache cold. > > > > This patch adds a prefetch to the to-be-freed page's buddy outside of > > zone->lock in hope of accessing buddy's page structure later under > > zone->lock will be faster. Since we *always* do buddy merging and check > > an order-0 page's buddy to try to merge it when it goes into the main > > allocator, the cacheline will always come in, i.e. the prefetched data > > will never be unused. > > > > In the meantime, there are two concerns: > > 1 the prefetch could potentially evict existing cachelines, especially > > for L1D cache since it is not huge; > > 2 there is some additional instruction overhead, namely calculating > > buddy pfn twice. > > > > For 1, it's hard to say, this microbenchmark though shows good result but > > the actual benefit of this patch will be workload/CPU dependant; > > For 2, since the calculation is a XOR on two local variables, it's expected > > in many cases that cycles spent will be offset by reduced memory latency > > later. This is especially true for NUMA machines where multiple CPUs are > > contending on zone->lock and the most time consuming part under zone->lock > > is the wait of 'struct page' cacheline of the to-be-freed pages and their > > buddies. > > > > Test with will-it-scale/page_fault1 full load: > > > > kernel Broadwell(2S) Skylake(2S) Broadwell(4S) Skylake(4S) > > v4.16-rc2+ 9034215 7971818 13667135 15677465 > > patch2/3 9536374 +5.6% 8314710 +4.3% 14070408 +3.0% 16675866 +6.4% > > this patch 10338868 +8.4% 8544477 +2.8% 14839808 +5.5% 17155464 +2.9% > > Note: this patch's performance improvement percent is against patch2/3. > > > > ... > > > > @@ -1150,6 +1153,18 @@ static void free_pcppages_bulk(struct zone *zone, > > int count, > > continue; > > > > list_add_tail(&page->lru, &head); > > + > > + /* > > + * We are going to put the page back to the global > > + * pool, prefetch its buddy to speed up later access > > + * under zone->lock. It is believed the overhead of > > + * calculating buddy_pfn here can be offset by reduced > > + * memory latency later. > > + */ > > + pfn = page_to_pfn(page); > > + buddy_pfn = __find_buddy_pfn(pfn, 0); > > + buddy = page + (buddy_pfn - pfn); > > + prefetch(buddy); > > What is the typical list length here? Maybe it's approximately the pcp > batch size which is typically 128 pages?
Most of time it is pcp->batch, unless when pcp's pages need to be all drained like in drain_local_pages(zone). The pcp->batch has a default value of 31 and its upper limit is 96 for x86_64. For this test, it is 31 here, I didn't manipulate /proc/sys/vm/percpu_pagelist_fraction to change it. With this said, the count here could be pcp->count when pcp's pages need to be all drained and though pcp->count's default value is (6*pcp->batch)=186, user can increase that value through the above mentioned procfs interface and the resulting pcp->count could be too big for prefetch. Ying also mentioned this today and suggested adding an upper limit here to avoid prefetching too much. Perhaps just prefetch the last pcp->batch pages if count here > pcp->batch? Since pcp->batch has an upper limit, we won't need to worry prefetching too much. > > If so, I'm a bit surprised that it is effective to prefetch 128 page > frames before using any them for real. I guess they'll fit in the L2 > cache. Thoughts?
