Re: [PATCH -v2 01/10] swap: Change SWAPFILE_CLUSTER to 512

[Huang\, Ying]

Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:

> On Thu, 01 Sep 2016 16:04:57 -0700 "Huang\, Ying" <ying.huang@xxxxxxxxx> wrote:
>
>> >>  }
>> >>  
>> >> -#define SWAPFILE_CLUSTER	256
>> >> +#define SWAPFILE_CLUSTER	512
>> >>  #define LATENCY_LIMIT		256
>> >>  
>> >
>> > What happens to architectures which have different HPAGE_SIZE and/or
>> > PAGE_SIZE?
>> 
>> For the architecture with HPAGE_SIZE / PAGE_SIZE == 512 (for example
>> x86_64), the huge page swap optimizing will be turned on.  For other
>> architectures, it will be turned off as before.
>> 
>> This mostly because I don't know whether it is a good idea to turn on
>> THP swap optimizing for the architectures other than x86_64.  For
>> example, it appears that the huge page size is 8M (1<<23) on SPARC.  But
>> I don't know whether 8M is too big for a swap cluster.  And it appears
>> that the huge page size could be as large as 512M on MIPS.
>
> This doesn't sounds very organized.  If some architecture with some
> config happens to have HPAGE_SIZE / PAGE_SIZE == 512 then the feature
> will be turned on; otherwise it will be turned off.  Nobody will even
> notice that it happened.
>
> Would it not be better to do
>
> #ifdef CONFIG_SOMETHING
> #define SWAPFILE_CLUSTER (HPAGE_SIZE / PAGE_SIZE)
> #else
> #define SWAPFILE_CLUSTER 256
> #endif
>
> and, by using CONFIG_SOMETHING in the other appropriate places, enable
> the feature in the usual fashion?

Yes.  That is better.  I will change it in the next version.

Best Regards,
Huang, Ying