linux-kernel - Re: [PATCH] mm, swap: Make VMA based swap readahead configurable

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Message-ID: <20171002072345.sd5zcbk4y34hh3mk@dhcp22.suse.cz>
Date:   Mon, 2 Oct 2017 09:23:45 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
        Rik van Riel <riel@...hat.com>, Shaohua Li <shli@...nel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Dave Hansen <dave.hansen@...el.com>
Subject: Re: [PATCH] mm, swap: Make VMA based swap readahead configurable

On Thu 28-09-17 09:02:20, Huang, Ying wrote:
> Hi, Michal,
> 
> Michal Hocko <mhocko@...nel.org> writes:
> 
> > On Wed 27-09-17 23:10:08, Minchan Kim wrote:
> >> On Wed, Sep 27, 2017 at 03:50:34PM +0200, Michal Hocko wrote:
> >> > On Wed 27-09-17 22:41:17, Minchan Kim wrote:
> >> > > On Wed, Sep 27, 2017 at 03:22:41PM +0200, Michal Hocko wrote:
> >> > [...]
> >> > > > simply cannot disable swap readahead when page-cluster is 0?
> >> > > 
> >> > > That's was what I want really but Huang want to use two readahead
> >> > > algorithms in parallel so he wanted to keep two separated disable
> >> > > knobs.
> >> > 
> >> > If it breaks existing and documented behavior then it is a clear
> >> > regression and it should be fixed. I do not see why this should be
> >> > disputable at all.
> >> 
> >> Indeed but Huang doesn't think so. He has thought it's not a regression.
> >> Frankly speaking, I'm really bored of discussing with it.
> >> https://marc.info/?l=linux-mm&m=150526413319763&w=2
> >
> > Then send a patch explaining why you consider this a regression with
> > some numbers backing it and I will happily ack it.
> 
> I still think there may be a performance regression for some users
> because of the change of the algorithm and the knobs, and the
> performance regression can be resolved via setting the new knob.  But I
> don't think there will be a functionality regression.  Do you agree?

I am not sure I understand. One thing is clear though. Your change has
introduced a regression as described by Minchan. And that has to be
resolved no matter what. You cannot expect users will tweak the system
to resolve it or configure their systems in a specific way.

-- 
Michal Hocko
SUSE Labs