linux-kernel - Re: [PATCH -v2 01/10] swap: Change SWAPFILE

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Message-ID: <87a8fqca2e.fsf@yhuang-mobile.sh.intel.com>
Date:   Fri, 02 Sep 2016 13:37:29 -0700
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     "Huang\, Ying" <ying.huang@...el.com>, <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 -v2 01/10] swap: Change SWAPFILE_CLUSTER to 512

Andrew Morton <akpm@...ux-foundation.org> writes:

> On Thu, 01 Sep 2016 16:04:57 -0700 "Huang\, Ying" <ying.huang@...el.com> 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