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Date: Thu, 1 Sep 2016 14:22:46 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: "Huang, Ying" <ying.huang@intel.com>
Cc: tim.c.chen@intel.com, dave.hansen@intel.com, andi.kleen@intel.com,
        aaron.lu@intel.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Hugh Dickins <hughd@google.com>, Shaohua Li <shli@kernel.org>,
        Minchan Kim <minchan@kernel.org>, Rik van Riel <riel@redhat.com>
Subject: Re: [PATCH -v2 01/10] swap: Change SWAPFILE_CLUSTER to 512
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On Thu,  1 Sep 2016 08:16:54 -0700 "Huang, Ying" <ying.huang@intel.com> wrote:

> From: Huang Ying <ying.huang@intel.com>
> 
> 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?