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SubjectRe: [resend][PATCH] mm: Restore zone->all_unreclaimable to independence word
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> 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 ?

Sure. page allocator obviously touch zone->lock at first.
Incremental patch is here.


---
include/linux/mmzone.h | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 4f0c6f1..0df3749 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -314,6 +314,7 @@ struct zone {
* free areas of different sizes
*/
spinlock_t lock;
+ int all_unreclaimable; /* All pages pinned */
#ifdef CONFIG_MEMORY_HOTPLUG
/* see spanned/present_pages for more description */
seqlock_t span_seqlock;
@@ -341,7 +342,6 @@ 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];
--
1.6.5.2






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