linux-kernel - Re: [PATCH] switch free memory back to MIGRATE

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Date:	Mon, 13 Jul 2009 11:18:01 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"mel@....ul.ie" <mel@....ul.ie>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH] switch free memory back to MIGRATE_MOVABLE

On Mon, Jul 13, 2009 at 11:08:14AM +0800, KOSAKI Motohiro wrote:
> > On Mon, Jul 13, 2009 at 10:58:03AM +0800, KAMEZAWA Hiroyuki wrote:
> > > On Mon, 13 Jul 2009 11:47:46 +0900 (JST)
> > > KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> wrote:
> > > 
> > > > > When page is back to buddy and its order is bigger than pageblock_order, we can
> > > > > switch its type to MIGRATE_MOVABLE. This can reduce fragmentation. The patch
> > > > > has obvious effect when read a block device and then drop caches.
> > > > > 
> > > > > Signed-off-by: Shaohua Li <shaohua.li@...el.com>
> > > > 
> > > > This patch change hot path, but there is no performance mesurement description.
> > > > Also, I don't like modification buddy core for only drop caches.
> > > > 
> > > Li, does this patch imply fallback of migration type doesn't work well ?
> > > What is the bad case ?
> > The page is initialized as migrate_movable, and then switch to reclaimable or
> > something else when fallback occurs, but its type remains even the page gets
> > freed. When the page gets freed, its type actually can be switch back to movable,
> > this is what the patch does.
> 
> This answer is not actual answer.
> Why do you think __rmqueue_fallback() doesn't works well? Do you have
> any test-case or found a bug by review?
I never said __rmqueue_fallback() doesn't work well. The page is already freed, switching
back the pageblock to movable might make next page allocation (non-movable) skip this
pageblock. So this could potentially reduce fragmentation and improve memory offline.
But your guys are right, I have no number if this will impact performance.

Thanks,
Shaohua
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