linux-kernel - Re: 答复: [PATCH] mm: extend reuse_swap

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Message-ID: <20171102074917.y4uvfrzshtr7jahi@dhcp22.suse.cz>
Date:   Thu, 2 Nov 2017 08:49:17 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     zhouxianrong <zhouxianrong@...wei.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "rientjes@...gle.com" <rientjes@...gle.com>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "vegard.nossum@...cle.com" <vegard.nossum@...cle.com>,
        "aaron.lu@...el.com" <aaron.lu@...el.com>,
        Zhouxiyu <zhouxiyu@...wei.com>,
        "Duwei (Device OS)" <weidu.du@...wei.com>,
        fanghua <fanghua3@...wei.com>, hutj <hutj@...wei.com>,
        Won Ho Park <won.ho.park@...wei.com>
Subject: Re: 答复: [PATCH] mm: extend
 reuse_swap_page range as much as possible

On Thu 02-11-17 13:22:23, Minchan Kim wrote:
> On Thu, Nov 02, 2017 at 02:09:57AM +0000, zhouxianrong wrote:
> > <zhouxianrong@...wei.com> writes:
> > 
> > > From: zhouxianrong <zhouxianrong@...wei.com>
> > >
> > > origanlly reuse_swap_page requires that the sum of page's mapcount and 
> > > swapcount less than or equal to one.
> > > in this case we can reuse this page and avoid COW currently.
> > >
> > > now reuse_swap_page requires only that page's mapcount less than or 
> > > equal to one and the page is not dirty in swap cache. in this case we 
> > > do not care its swap count.
> > >
> > > the page without dirty in swap cache means that it has been written to 
> > > swap device successfully for reclaim before and then read again on a 
> > > swap fault. in this case the page can be reused even though its swap 
> > > count is greater than one and postpone the COW on other successive 
> > > accesses to the swap cache page later rather than now.
> > >
> > > i did this patch test in kernel 4.4.23 with arm64 and none huge 
> > > memory. it work fine.

this is not an appropriate justification

> > Why do you need this?  You saved copying one page from memory to memory
> > (COW) now, at the cost of reading a page from disk to memory later?
> > 
> > yes, accessing later does not always happen, there is probability for it, so postpone COW now.
> 
> So, it's trade-off. It means we need some number with some scenarios
> to prove it's better than as-is.
> It would help to drive reviewers/maintainer.

Absolutely agreed. We definitely need some numbers for different set of
workloads.
-- 
Michal Hocko
SUSE Labs