linux-kernel - Re: [PATCH v3] mm: swap: properly update readahead statistics in unuse_pte

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Message-ID: <20200418085145.GA147473@xps-13>
Date:   Sat, 18 Apr 2020 10:51:45 +0200
From:   Andrea Righi <andrea.righi@...onical.com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Minchan Kim <minchan@...nel.org>,
        Anchal Agarwal <anchalag@...zon.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm: swap: properly update readahead statistics in
 unuse_pte_range()

On Fri, Apr 17, 2020 at 01:18:37PM +0800, Huang, Ying wrote:
> Andrew Morton <akpm@...ux-foundation.org> writes:
> 
> > On Fri, 17 Apr 2020 11:01:22 +0800 "Huang\, Ying" <ying.huang@...el.com> wrote:
> >
> >> Andrea Righi <andrea.righi@...onical.com> writes:
> >> 
> >> > In unuse_pte_range() we blindly swap-in pages without checking if the
> >> > swap entry is already present in the swap cache.
> >> >
> >> > By doing this, the hit/miss ratio used by the swap readahead heuristic
> >> > is not properly updated and this leads to non-optimal performance during
> >> > swapoff.
> >> 
> >> It's more important to describe why we need this patch in the patch
> >> description.  So, please add some information about your use case.  And
> >> please focus on the technical part instead of the business part.
> >
> > Confused.  I thought the changelog was quite good.  If "business part"
> > means "end user effect of the patch" then that's a very important
> > thing.
> 
> Previously, Andrea has described their use case in the cloud environment
> to hiberate the guest and swapoff after resuming.  So swapoff
> performance is important for them.  I think that should be included.
> For the business part, I mean something like "Ubuntu used in AWS EC2", I
> think that isn't important for the patch description.

I just sent a v4 of this patch adding "conclusion" section in the
description to better explain the purpose of this patch. Let me know if
you have any comment on that.

Thanks,
-Andrea