Re: [PATCH -mm -v6 1/9] mm, swap: Make swap cluster size same of THP size on x86_64

[Huang\, Ying]

Hi, Matthew,

Matthew Wilcox <willy@xxxxxxxxxxxxx> writes:

> On Wed, Mar 08, 2017 at 03:26:05PM +0800, Huang, Ying wrote:
>> 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.
>> 
>> For other architectures which want THP swap support,
>> ARCH_USES_THP_SWAP_CLUSTER need to be selected in the Kconfig file for
>> the architecture.
>> 
>> In effect, this will enlarge swap cluster size by 2 times on x86_64.
>> 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
>> shows no regressions caused by this.
>
> Well ... if there are no regressions found, why not change it
> unconditionally?  The value '256' seems relatively arbitrary (I bet it
> was tuned by some doofus with a 486, 8MB RAM and ST506 hard drive ...
> it certainly hasn't changed since git started in 2005)
>
> Might be worth checking with the PowerPC people to see if their larger
> pages causes this smaller patch to perform badly:

I found the huge page size is large not only on PowerPC, for example, on
MIPS, the PMD_SHIFT could be from 21 to 29, depends on configuration.  I
don't know the situation for the other architectures.  So I thought it
may be better to let the architecture developers to determine whether to
make the change and under which configuration.

> diff --git a/mm/swapfile.c b/mm/swapfile.c
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -199,7 +199,7 @@ static void discard_swap_cluster(struct swap_info_struct *si,
>  	}
>  }
>  
> -#define SWAPFILE_CLUSTER	256
> +#define SWAPFILE_CLUSTER	HPAGE_PMD_NR
>  #define LATENCY_LIMIT		256
>  
>  static inline void cluster_set_flag(struct swap_cluster_info *info,

Best Regards,
Huang, Ying