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

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Date:   Thu, 28 Sep 2017 09:02:20 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Minchan Kim <minchan@...nel.org>,
        "Huang\, Ying" <ying.huang@...el.com>,
        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

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?

Best Regards,
Huang, Ying

>> So I passed the decision to Andrew.
>> http://lkml.kernel.org/r/<20170913014019.GB29422@...x>
>> 
>> The config option idea is compromise approach although I don't like it
>> and still believe it's simple clear *regression* so 0 page-cluster
>> should keep the swap readahead disabled.
>
> It is not a compromise. The regression is still there for many users
> potentially (just consider zram distribution kernel users...).