linux-kernel - Re: [PATCH -mm -v3] mm, swap: Sort swap entries before free

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Date:   Tue, 2 May 2017 14:48:58 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
        Shaohua Li <shli@...nel.org>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH -mm -v3] mm, swap: Sort swap entries before free

Hi Huang,

On Tue, May 02, 2017 at 01:35:24PM +0800, Huang, Ying wrote:
> Hi, Minchan,
> 
> Minchan Kim <minchan@...nel.org> writes:
> 
> > On Fri, Apr 28, 2017 at 09:35:37PM +0800, Huang, Ying wrote:
> >> In fact, during the test, I found the overhead of sort() is comparable
> >> with the performance difference of adding likely()/unlikely() to the
> >> "if" in the function.
> >
> > Huang,
> >
> > This discussion is started from your optimization code:
> >
> >         if (nr_swapfiles > 1)
> >                 sort();
> >
> > I don't have such fast machine so cannot test it. However, you added
> > such optimization code in there so I guess it's *worth* to review so
> > with spending my time, I pointed out what you are missing and
> > suggested a idea to find a compromise.
> 
> Sorry for wasting your time and Thanks a lot for your review and
> suggestion!
> 
> When I started talking this with you, I found there is some measurable
> overhead of sort().  But later when I done more tests, I found the
> measurable overhead is at the same level of likely()/unlikely() compiler
> notation.  So you help me to find that, Thanks again!
> 
> > Now you are saying sort is so fast so no worth to add more logics
> > to avoid the overhead?
> > Then, please just drop that if condition part and instead, sort
> > it unconditionally.
> 
> Now, because we found the overhead of sort() is low, I suggest to put
> minimal effort to avoid it.  Like the original implementation,
> 
>          if (nr_swapfiles > 1)
>                  sort();

It might confuse someone in future and would make him/her send a patch
to fix like we discussed. If the logic is not clear and doesn't have
measureable overhead, just leave it which is more simple/clear.

> 
> Or, we can make nr_swapfiles more correct as Tim suggested (tracking
> the number of the swap devices during swap on/off).

It might be better option but it's still hard to justify the patch
because you said it's hard to measure. Such optimiztion patch should
be from numbers.