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? 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?
