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Date: Mon, 3 Aug 2015 10:05:57 +0800
From: Xishi Qiu <qiuxishi@huawei.com>
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To: Dave Hansen <dave.hansen@intel.com>
CC: Andrew Morton <akpm@linux-foundation.org>, Mel Gorman <mgorman@suse.de>,
        Vlastimil Babka <vbabka@suse.cz>, Johannes Weiner <hannes@cmpxchg.org>,
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        <alexander.h.duyck@redhat.com>, <sasha.levin@oracle.com>,
        Linux MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm: add the block to the tail of the list in expand()
References: <55BB4027.7080200@huawei.com> <55BC0392.2070205@intel.com>
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On 2015/8/1 7:24, Dave Hansen wrote:

> On 07/31/2015 02:30 AM, Xishi Qiu wrote:
>> __free_one_page() will judge whether the the next-highest order is free,
>> then add the block to the tail or not. So when we split large order block, 
>> add the small block to the tail, it will reduce fragment.
> 
> It's an interesting idea, but what does it do in practice?  Can you
> measure a decrease in fragmentation?
> 
> Further, the comment above the function says:
>  * The order of subdivision here is critical for the IO subsystem.
>  * Please do not alter this order without good reasons and regression
>  * testing.
> 
> Has there been regression testing?
> 
> Also, this might not do very much good in practice.  If you are
> splitting a high-order page, you are doing the split because the
> lower-order lists are empty.  So won't that list_add() be to an empty

Hi Dave,

I made a mistake, you are right, all the lower-order lists are empty,
so it is no sense to add to the tail.

Thanks,
Xishi Qiu

> list most of the time?  Or does the __rmqueue_fallback()
> largest->smallest logic dominate?
> 
> .
>