Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752910AbdHGJzS (ORCPT ); Mon, 7 Aug 2017 05:55:18 -0400 Received: from mx2.suse.de ([195.135.220.15]:47084 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1752698AbdHGJzR (ORCPT ); Mon, 7 Aug 2017 05:55:17 -0400 Date: Mon, 7 Aug 2017 11:55:15 +0200 From: Jan Kara To: "Huang, Ying" Cc: Andrew Morton , 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 Subject: Re: [PATCH -mm] mm: Clear to access sub-page last when clearing huge page Message-ID: <20170807095515.GA6470@quack2.suse.cz> References: <20170807072131.8343-1-ying.huang@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170807072131.8343-1-ying.huang@intel.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3633 Lines: 86 On Mon 07-08-17 15:21:31, 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% Hum, the improvement looks impressive enough that it is probably worth the bother. But please add at least a brief explanation why you do stuff in this more complicated way to a comment in clear_huge_page() so that people don't have to look it up in the changelog. Otherwise the patch looks good to me so feel free to add: Acked-by: Jan Kara Honza > @@ -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); > } > } > > -- > 2.11.0 > -- Jan Kara SUSE Labs, CR