From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg0-f72.google.com (mail-pg0-f72.google.com [74.125.83.72]) by kanga.kvack.org (Postfix) with ESMTP id E634D6B0038 for ; Wed, 26 Apr 2017 08:42:14 -0400 (EDT) Received: by mail-pg0-f72.google.com with SMTP id c2so48215907pga.1 for ; Wed, 26 Apr 2017 05:42:14 -0700 (PDT) Received: from mga03.intel.com (mga03.intel.com. [134.134.136.65]) by mx.google.com with ESMTPS id o28si64834pgc.170.2017.04.26.05.42.12 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 26 Apr 2017 05:42:13 -0700 (PDT) From: "Huang\, Ying" Subject: Re: [PATCH -mm -v3] mm, swap: Sort swap entries before free References: <20170407064901.25398-1-ying.huang@intel.com> <20170418045909.GA11015@bbox> <87y3uwrez0.fsf@yhuang-dev.intel.com> <20170420063834.GB3720@bbox> <874lxjim7m.fsf@yhuang-dev.intel.com> <87tw5idjv9.fsf@yhuang-dev.intel.com> <20170424045213.GA11287@bbox> Date: Wed, 26 Apr 2017 20:42:10 +0800 In-Reply-To: <20170424045213.GA11287@bbox> (Minchan Kim's message of "Mon, 24 Apr 2017 13:52:13 +0900") Message-ID: <87y3un2vdp.fsf@yhuang-dev.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=ascii Sender: owner-linux-mm@kvack.org List-ID: To: Minchan Kim Cc: "Huang, Ying" , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hugh Dickins , Shaohua Li , Rik van Riel Minchan Kim writes: > On Fri, Apr 21, 2017 at 08:29:30PM +0800, Huang, Ying wrote: >> "Huang, Ying" writes: >> >> > Minchan Kim writes: >> > >> >> On Wed, Apr 19, 2017 at 04:14:43PM +0800, Huang, Ying wrote: >> >>> Minchan Kim writes: >> >>> >> >>> > Hi Huang, >> >>> > >> >>> > On Fri, Apr 07, 2017 at 02:49:01PM +0800, Huang, Ying wrote: >> >>> >> From: Huang Ying >> >>> >> >> >>> >> void swapcache_free_entries(swp_entry_t *entries, int n) >> >>> >> { >> >>> >> struct swap_info_struct *p, *prev; >> >>> >> @@ -1075,6 +1083,10 @@ void swapcache_free_entries(swp_entry_t *entries, int n) >> >>> >> >> >>> >> prev = NULL; >> >>> >> p = NULL; >> >>> >> + >> >>> >> + /* Sort swap entries by swap device, so each lock is only taken once. */ >> >>> >> + if (nr_swapfiles > 1) >> >>> >> + sort(entries, n, sizeof(entries[0]), swp_entry_cmp, NULL); >> >>> > >> >>> > Let's think on other cases. >> >>> > >> >>> > There are two swaps and they are configured by priority so a swap's usage >> >>> > would be zero unless other swap used up. In case of that, this sorting >> >>> > is pointless. >> >>> > >> >>> > As well, nr_swapfiles is never decreased so if we enable multiple >> >>> > swaps and then disable until a swap is remained, this sorting is >> >>> > pointelss, too. >> >>> > >> >>> > How about lazy sorting approach? IOW, if we found prev != p and, >> >>> > then we can sort it. >> >>> >> >>> Yes. That should be better. I just don't know whether the added >> >>> complexity is necessary, given the array is short and sort is fast. >> >> >> >> Huh? >> >> >> >> 1. swapon /dev/XXX1 >> >> 2. swapon /dev/XXX2 >> >> 3. swapoff /dev/XXX2 >> >> 4. use only one swap >> >> 5. then, always pointless sort. >> > >> > Yes. In this situation we will do unnecessary sorting. What I don't >> > know is whether the unnecessary sorting will hurt performance in real >> > life. I can do some measurement. >> >> I tested the patch with 1 swap device and 1 process to eat memory >> (remove the "if (nr_swapfiles > 1)" for test). I think this is the >> worse case because there is no lock contention. The memory freeing time >> increased from 1.94s to 2.12s (increase ~9.2%). So there is some >> overhead for some cases. I change the algorithm to something like >> below, >> >> void swapcache_free_entries(swp_entry_t *entries, int n) >> { >> struct swap_info_struct *p, *prev; >> int i; >> + swp_entry_t entry; >> + unsigned int prev_swp_type; >> >> if (n <= 0) >> return; >> >> + prev_swp_type = swp_type(entries[0]); >> + for (i = n - 1; i > 0; i--) { >> + if (swp_type(entries[i]) != prev_swp_type) >> + break; >> + } > > That's really what I want to avoid. For many swap usecases, > it adds unnecessary overhead. > >> + >> + /* Sort swap entries by swap device, so each lock is only taken once. */ >> + if (i) >> + sort(entries, n, sizeof(entries[0]), swp_entry_cmp, NULL); >> prev = NULL; >> p = NULL; >> for (i = 0; i < n; ++i) { >> - p = swap_info_get_cont(entries[i], prev); >> + entry = entries[i]; >> + p = swap_info_get_cont(entry, prev); >> if (p) >> - swap_entry_free(p, entries[i]); >> + swap_entry_free(p, entry); >> prev = p; >> } >> if (p) >> >> With this patch, the memory freeing time increased from 1.94s to 1.97s. >> I think this is good enough. Do you think so? > > What I mean is as follows(I didn't test it at all): > > With this, sort entries if we found multiple entries in current > entries. It adds some condition checks for non-multiple swap > usecase but it would be more cheaper than the sorting. > And it adds a [un]lock overhead for multiple swap usecase but > it should be a compromise for single-swap usecase which is more > popular. > How about the following solution? It can avoid [un]lock overhead and double lock issue for multiple swap user case and has good performance for one swap user case too. Best Regards, Huang, Ying