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

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Date:   Tue, 02 May 2017 13:35:24 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     "Huang\, Ying" <ying.huang@...el.com>,
        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, 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();

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

Best Regards,
Huang, Ying