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SubjectRe: [resend][PATCH] mm: Restore zone->all_unreclaimable to independence word
On Thu, 14 Jan 2010 15:19:59 -0800
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Thu, 14 Jan 2010 16:32:29 +0800
> Wu Fengguang <fengguang.wu@intel.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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