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[140.211.169.12]) by mx.google.com with ESMTPS id j27si3945148wrd.9.2017.08.09.14.25.03 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 09 Aug 2017 14:25:04 -0700 (PDT) Date: Wed, 9 Aug 2017 14:25:01 -0700 From: Andrew Morton Subject: Re: [PATCH -mm] mm: Clear to access sub-page last when clearing huge page Message-Id: <20170809142501.1286e8818359fd95b5794abd@linux-foundation.org> In-Reply-To: <20170807072131.8343-1-ying.huang@intel.com> References: <20170807072131.8343-1-ying.huang@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: "Huang, Ying" Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrea Arcangeli , "Kirill A. Shutemov" , Nadia Yvette Chambers , Michal Hocko , Jan Kara , Matthew Wilcox , Hugh Dickins , Minchan Kim , Shaohua Li On Mon, 7 Aug 2017 15:21:31 +0800 "Huang, Ying" wrote: > From: Huang Ying > > Huge page helps to reduce TLB miss rate, but it has higher cache > footprint, sometimes this may cause some issue. For example, when > clearing huge page on x86_64 platform, the cache footprint is 2M. But > on a Xeon E5 v3 2699 CPU, there are 18 cores, 36 threads, and only 45M > LLC (last level cache). That is, in average, there are 2.5M LLC for > each core and 1.25M LLC for each thread. If the cache pressure is > heavy when clearing the huge page, and we clear the huge page from the > begin to the end, it is possible that the begin of huge page is > evicted from the cache after we finishing clearing the end of the huge > page. And it is possible for the application to access the begin of > the huge page after clearing the huge page. > > To help the above situation, in this patch, when we clear a huge page, > the order to clear sub-pages is changed. In quite some situation, we > can get the address that the application will access after we clear > the huge page, for example, in a page fault handler. Instead of > clearing the huge page from begin to end, we will clear the sub-pages > farthest from the the sub-page to access firstly, and clear the > sub-page to access last. This will make the sub-page to access most > cache-hot and sub-pages around it more cache-hot too. If we cannot > know the address the application will access, the begin of the huge > page is assumed to be the the address the application will access. > > With this patch, the throughput increases ~28.3% in vm-scalability > anon-w-seq test case with 72 processes on a 2 socket Xeon E5 v3 2699 > system (36 cores, 72 threads). The test case creates 72 processes, > each process mmap a big anonymous memory area and writes to it from > the begin to the end. For each process, other processes could be seen > as other workload which generates heavy cache pressure. At the same > time, the cache miss rate reduced from ~33.4% to ~31.7%, the > IPC (instruction per cycle) increased from 0.56 to 0.74, and the time > spent in user space is reduced ~7.9% > > Thanks Andi Kleen to propose to use address to access to determine the > order of sub-pages to clear. > > The hugetlbfs access address could be improved, will do that in > another patch. I agree with what others said, plus... > @@ -4374,9 +4374,31 @@ void clear_huge_page(struct page *page, > } > > might_sleep(); > - for (i = 0; i < pages_per_huge_page; i++) { > + VM_BUG_ON(clamp(addr_hint, addr, addr + > + (pages_per_huge_page << PAGE_SHIFT)) != addr_hint); > + n = (addr_hint - addr) / PAGE_SIZE; > + if (2 * n <= pages_per_huge_page) { > + base = 0; > + l = n; > + for (i = pages_per_huge_page - 1; i >= 2 * n; i--) { > + cond_resched(); > + clear_user_highpage(page + i, addr + i * PAGE_SIZE); > + } > + } else { > + base = 2 * n - pages_per_huge_page; > + l = pages_per_huge_page - n; > + for (i = 0; i < base; i++) { > + cond_resched(); > + clear_user_highpage(page + i, addr + i * PAGE_SIZE); > + } > + } > + for (i = 0; i < l; i++) { > + cond_resched(); > + clear_user_highpage(page + base + i, > + addr + (base + i) * PAGE_SIZE); > cond_resched(); > - clear_user_highpage(page + i, addr + i * PAGE_SIZE); > + clear_user_highpage(page + base + 2 * l - 1 - i, > + addr + (base + 2 * l - 1 - i) * PAGE_SIZE); Please document this design with a carefully written code comment. For example, why was "2 * n" chosen? What is it trying to achieve? Also, the final clearing loop "for (i = 0; i < l; i++)" might cause eviction of data which was cached in the previous loop. Perhaps some additional gains will be made by clearing the hugepage in a left-right-left-right "start from the ends and work inwards" manner, if you see what I mean. So the 4k pages immediately surrounding addr_hint are the most-recently-cleared. Although accesses to the data at lower addresses than addr_hint are probably somewhat rare (and may be nonexistent in your synthetic test case). -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org