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

[Huang\, Ying]

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.

OK.

Best Regards,
Huang, Ying