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

From: Huang\, Ying
Date: Tue May 02 2017 - 02:08:31 EST

Minchan Kim <minchan@xxxxxxxxxx> writes:

> Hi Huang,
> On Tue, May 02, 2017 at 01:35:24PM +0800, Huang, Ying wrote:
>> Hi, Minchan,
>> Minchan Kim <minchan@xxxxxxxxxx> 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.

Because the added code is minimal and cheap, I tend to keep it and add
some comments to avoid confusion. For example,

* Although nr_swapfiles isn't absolute correct, but the overhead of sort()
* is so low that it isn't necessary to optimize further.

>> 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.


Best Regards,
Huang, Ying