linux-kernel - Re: [PATCH -v3 01/10] mm, swap: Make swap cluster size same of THP size on x86

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Date:   Thu, 08 Sep 2016 13:51:47 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     "Huang, Ying" <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     tim.c.chen@...el.com, dave.hansen@...el.com, andi.kleen@...el.com,
        aaron.lu@...el.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
        Shaohua Li <shli@...nel.org>, Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH -v3 01/10] mm, swap: Make swap cluster size same of THP
 size on x86_64

On 09/07/2016 10:16 PM, Huang, Ying wrote:
> From: Huang Ying <ying.huang@...el.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.
> 
> For other architectures which want THP swap support, 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.

This patch needs to be split into two separate ones

(1) Add THP_SWAP_CLUSTER config option
(2) Enable CONFIG_THP_SWAP_CLUSTER for X86_64

The first patch should explain the proposal and the second patch
should have 86_64 arch specific details, regressions etc as already
been explained in the commit message.