From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756995AbcHYTw1 (ORCPT ); Thu, 25 Aug 2016 15:52:27 -0400 Received: from mx1.redhat.com ([209.132.183.28]:50844 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755120AbcHYTwZ (ORCPT ); Thu, 25 Aug 2016 15:52:25 -0400 Message-ID: <1472154243.2751.44.camel@redhat.com> Subject: Re: [PATCH] mm: Don't use radix tree writeback tags for pages in swap cache From: Rik van Riel To: "Huang, Ying" , Andrew Morton Cc: tim.c.chen@intel.com, dave.hansen@intel.com, andi.kleen@intel.com, aaron.lu@intel.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hugh Dickins , Shaohua Li , Minchan Kim , Mel Gorman , Tejun Heo , Wu Fengguang Date: Thu, 25 Aug 2016 15:44:03 -0400 In-Reply-To: <1472153230-14766-1-git-send-email-ying.huang@intel.com> References: <1472153230-14766-1-git-send-email-ying.huang@intel.com> Organization: Red Hat, Inc. Content-Type: text/plain; charset="UTF-8" Mime-Version: 1.0 Content-Transfer-Encoding: 8bit X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.31]); Thu, 25 Aug 2016 19:44:21 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2016-08-25 at 12:27 -0700, Huang, Ying wrote: > File pages use a set of radix tags (DIRTY, TOWRITE, WRITEBACK, etc.) > to > accelerate finding the pages with a specific tag in the radix tree > during inode writeback.  But for anonymous pages in the swap cache, > there is no inode writeback.  So there is no need to find the > pages with some writeback tags in the radix tree.  It is not > necessary > to touch radix tree writeback tags for pages in the swap cache. > > With this patch, the swap out bandwidth improved 22.3% (from ~1.2GB/s > to > ~ 1.48GBps) in the vm-scalability swap-w-seq test case with 8 > processes. > The test is done on a Xeon E5 v3 system.  The swap device used is a > RAM > simulated PMEM (persistent memory) device.  The improvement comes > from > the reduced contention on the swap cache radix tree lock.  To test > sequential swapping out, the test case uses 8 processes, which > sequentially allocate and write to the anonymous pages until RAM and > part of the swap device is used up. > > Details of comparison is as follow, > > base             base+patch > ---------------- -------------------------- >          %stddev     %change         %stddev >              \          |                \ >    1207402 ±  7%     +22.3%    1476578 ±  6%  vmstat.swap.so >    2506952 ±  2%     +28.1%    3212076 ±  7%  vm- > scalability.throughput >      10.86 ± 12%     -23.4%       8.31 ± 16%  perf-profile.cycles- > pp._raw_spin_lock_irq.__add_to_swap_cache.add_to_swap_cache.add_to_sw > ap.shrink_page_list >      10.82 ± 13%     -33.1%       7.24 ± 14%  perf-profile.cycles- > pp._raw_spin_lock_irqsave.__remove_mapping.shrink_page_list.shrink_in > active_list.shrink_zone_memcg >      10.36 ± 11%    -100.0%       0.00 ± -1%  perf-profile.cycles- > pp._raw_spin_lock_irqsave.__test_set_page_writeback.bdev_write_page._ > _swap_writepage.swap_writepage >      10.52 ± 12%    -100.0%       0.00 ± -1%  perf-profile.cycles- > pp._raw_spin_lock_irqsave.test_clear_page_writeback.end_page_writebac > k.page_endio.pmem_rw_page > > Cc: Hugh Dickins > Cc: Shaohua Li > Cc: Minchan Kim > Cc: Rik van Riel > Cc: Mel Gorman > Cc: Tejun Heo > Cc: Wu Fengguang > Cc: Dave Hansen > Signed-off-by: "Huang, Ying" > --- >  mm/page-writeback.c | 6 ++++-- >  1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c > index 82e7252..599d2f9 100644 > --- a/mm/page-writeback.c > +++ b/mm/page-writeback.c > @@ -2728,7 +2728,8 @@ int test_clear_page_writeback(struct page > *page) >   int ret; >   >   lock_page_memcg(page); > - if (mapping) { > + /* Pages in swap cache don't use writeback tags */ > + if (mapping && !PageSwapCache(page)) { I wonder if that should be a mapping_uses_tags(mapping) macro or similar, and a per-mapping flag? I suspect there will be another case coming up soon where we have a page cache radix tree, but no need for dirty/writeback/... tags. That use case would be DAX filesystems, where we do use a struct page, but that struct page points at persistent storage, and the tags are not necessary.