linux-kernel - Re: [resend][PATCH] mm: Restore zone->all

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Date:	Fri, 15 Jan 2010 11:30:35 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	David Rientjes <rientjes@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Huang Shijie <shijie8@...il.com>
Subject: Re: [resend][PATCH] mm: Restore zone->all_unreclaimable to
 independence word

On Thu, 14 Jan 2010 15:19:59 -0800
Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Thu, 14 Jan 2010 16:32:29 +0800
> Wu Fengguang <fengguang.wu@...el.com> wrote:
> 
> > On Thu, Jan 14, 2010 at 03:14:10PM +0800, KOSAKI Motohiro wrote:
> > > > On Thu, 14 Jan 2010, KOSAKI Motohiro wrote:
> > > > 
> > > > > commit e815af95 (change all_unreclaimable zone member to flags) chage
> > > > > all_unreclaimable member to bit flag. but It have undesireble side
> > > > > effect.
> > > > > free_one_page() is one of most hot path in linux kernel and increasing
> > > > > atomic ops in it can reduce kernel performance a bit.
> > > > > 
> > > > > Thus, this patch revert such commit partially. at least
> > > > > all_unreclaimable shouldn't share memory word with other zone flags.
> > > > > 
> > > > 
> > > > I still think you need to quantify this; saying you don't have a large 
> > > > enough of a machine that will benefit from it isn't really a rationale for 
> > > > the lack of any data supporting your claim.  We should be basing VM 
> > > > changes on data, not on speculation that there's a measurable impact 
> > > > here.
> > > > 
> > > > Perhaps you could ask a colleague or another hacker to run a benchmark for 
> > > > you so that the changelog is complete?
> > > 
> > > ok, fair. although I dislike current unnecessary atomic-ops.
> > > I'll pending this patch until get good data.
> > 
> > I think it's a reasonable expectation to help large boxes.
> > 
> > What we can do now, is to measure if it hurts mainline SMP
> > boxes. If not, we are set on doing the patch :)
> 
> yup, the effects of the change might be hard to measure.  Not that one
> shouldn't try!
> 
> But sometimes we just have to do a best-effort change based upon theory
> and past experience.
> 
> Speaking of which...
> 
> : --- a/include/linux/mmzone.h
> : +++ b/include/linux/mmzone.h
> : @@ -341,6 +341,7 @@ struct zone {
> :  
> :  	unsigned long		pages_scanned;	   /* since last reclaim */
> :  	unsigned long		flags;		   /* zone flags, see below */
> : +	int                     all_unreclaimable; /* All pages pinned */
> :  
> :  	/* Zone statistics */
> :  	atomic_long_t		vm_stat[NR_VM_ZONE_STAT_ITEMS];
> 
> Was that the best place to put the field?  It adds four bytes of
> padding to the zone, hence is suboptimal from a cache utilisation point
> of view.
> 
> It might also be that we can place this field closed in memory to other
> fields which are being manipulated at the same time as
> all_unreclaimable, hm?
> 
How about the same line where zone->lock is ?

Thanks,
-Kame

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