From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751997AbdDNBgX (ORCPT ); Thu, 13 Apr 2017 21:36:23 -0400 Received: from mga06.intel.com ([134.134.136.31]:49708 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751255AbdDNBgV (ORCPT ); Thu, 13 Apr 2017 21:36:21 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.37,195,1488873600"; d="scan'208";a="1155740825" From: "Huang\, Ying" To: Andrew Morton Cc: "Huang\, Ying" , , , Hugh Dickins , Shaohua Li , Minchan Kim , Rik van Riel Subject: Re: [PATCH -mm -v3] mm, swap: Sort swap entries before free References: <20170407064901.25398-1-ying.huang@intel.com> <20170407144346.b2e5d3c8364767eb2b4118ed@linux-foundation.org> Date: Fri, 14 Apr 2017 09:36:18 +0800 In-Reply-To: <20170407144346.b2e5d3c8364767eb2b4118ed@linux-foundation.org> (Andrew Morton's message of "Fri, 7 Apr 2017 14:43:46 -0700") Message-ID: <878tn3db3h.fsf@yhuang-dev.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/25.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; charset=ascii Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Andrew Morton writes: > On Fri, 7 Apr 2017 14:49:01 +0800 "Huang, Ying" wrote: > >> To reduce the lock contention of swap_info_struct->lock when freeing >> swap entry. The freed swap entries will be collected in a per-CPU >> buffer firstly, and be really freed later in batch. During the batch >> freeing, if the consecutive swap entries in the per-CPU buffer belongs >> to same swap device, the swap_info_struct->lock needs to be >> acquired/released only once, so that the lock contention could be >> reduced greatly. But if there are multiple swap devices, it is >> possible that the lock may be unnecessarily released/acquired because >> the swap entries belong to the same swap device are non-consecutive in >> the per-CPU buffer. >> >> To solve the issue, the per-CPU buffer is sorted according to the swap >> device before freeing the swap entries. Test shows that the time >> spent by swapcache_free_entries() could be reduced after the patch. >> >> Test the patch via measuring the run time of swap_cache_free_entries() >> during the exit phase of the applications use much swap space. The >> results shows that the average run time of swap_cache_free_entries() >> reduced about 20% after applying the patch. > > "20%" is useful info, but it is much better to present the absolute > numbers, please. If it's "20% of one nanosecond" then the patch isn't > very interesting. If it's "20% of 35 seconds" then we know we have > more work to do. I added memory freeing timing capability to vm-scalability test suite. The result shows the memory freeing time reduced from 2.64s to 2.31s (about -12.5%). Best Regards, Huang, Ying > If there is indeed still a significant problem here then perhaps it > would be better to move the percpu swp_entry_t buffer into the > per-device structure swap_info_struct, so it becomes "per cpu, per > device". That way we should be able to reduce contention further. > > Or maybe we do something else - it all depends upon the significance of > this problem, which is why a full description of your measurements is > useful.