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

[Huang\, Ying]

Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:

> On Thu,  1 Sep 2016 08:16:54 -0700 "Huang, Ying" <ying.huang@xxxxxxxxx> wrote:
>
>> From: Huang Ying <ying.huang@xxxxxxxxx>
>> 
>> In this patch, the size of the swap cluster is changed to that of the
>> THP (Transparent Huge Page) on x86_64 architecture (512).  This is for
>> the THP swap support on x86_64.  Where one swap cluster will be used to
>> hold the contents of each THP swapped out.  And some information of the
>> swapped out THP (such as compound map count) will be recorded in the
>> swap_cluster_info data structure.
>> 
>> In effect,  this will enlarge swap  cluster size by 2  times.  Which may
>> make  it harder  to find  a  free cluster  when the  swap space  becomes
>> fragmented.   So  that,  this  may  reduce  the  continuous  swap  space
>> allocation and sequential write in theory.  The performance test in 0day
>> show no regressions caused by this.
>> 
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -196,7 +196,7 @@ static void discard_swap_cluster(struct swap_info_struct *si,
>>  	}
>>  }
>>  
>> -#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.

Best Regards,
Huang, Ying