linux-kernel - Re: [RFC PATCH 1/5] mm/page_alloc: always add freeing page at the tail of the buddy list

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Date:   Wed, 26 Oct 2016 13:37:40 +0900
From:   Joonsoo Kim <iamjoonsoo.kim@....com>
To:     Xishi Qiu <qiuxishi@...wei.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/5] mm/page_alloc: always add freeing page at the
 tail of the buddy list

On Mon, Oct 17, 2016 at 05:21:54PM +0800, Xishi Qiu wrote:
> On 2016/10/13 16:08, js1304@...il.com wrote:
> 
> > From: Joonsoo Kim <iamjoonsoo.kim@....com>
> > 
> > Currently, freeing page can stay longer in the buddy list if next higher
> > order page is in the buddy list in order to help coalescence. However,
> > it doesn't work for the simplest sequential free case. For example, think
> > about the situation that 8 consecutive pages are freed in sequential
> > order.
> > 
> > page 0: attached at the head of order 0 list
> > page 1: merged with page 0, attached at the head of order 1 list
> > page 2: attached at the tail of order 0 list
> > page 3: merged with page 2 and then merged with page 0, attached at
> >  the head of order 2 list
> > page 4: attached at the head of order 0 list
> > page 5: merged with page 4, attached at the tail of order 1 list
> > page 6: attached at the tail of order 0 list
> > page 7: merged with page 6 and then merged with page 4. Lastly, merged
> >  with page 0 and we get order 3 freepage.
> > 
> > With excluding page 0 case, there are three cases that freeing page is
> > attached at the head of buddy list in this example and if just one
> > corresponding ordered allocation request comes at that moment, this page
> > in being a high order page will be allocated and we would fail to make
> > order-3 freepage.
> > 
> > Allocation usually happens in sequential order and free also does. So, it
> > would be important to detect such a situation and to give some chance
> > to be coalesced.
> > 
> > I think that simple and effective heuristic about this case is just
> > attaching freeing page at the tail of the buddy list unconditionally.
> > If freeing isn't merged during one rotation, it would be actual
> > fragmentation and we don't need to care about it for coalescence.
> > 
> 
> Hi Joonsoo,
> 
> I find another two places to reduce fragmentation.
> 
> 1)
> __rmqueue_fallback
> 	steal_suitable_fallback
> 		move_freepages_block
> 			move_freepages
> 				list_move
> If we steal some free pages, we will add these page at the head of start_migratetype list,
> this will cause more fixed migratetype, because this pages will be allocated more easily.
> So how about use list_move_tail instead of list_move?

Yeah... I don't think deeply but, at a glance, it would be helpful.

> 
> 2)
> __rmqueue_fallback
> 	expand
> 		list_add
> How about use list_add_tail instead of list_add? If add the tail, then the rest of pages
> will be hard to be allocated and we can merge them again as soon as the page freed.

I guess that it has no effect. When we do __rmqueue_fallback() and
expand(), we don't have any freepage on this or more order. So,
list_add or list_add_tail will show the same result.

Thanks.