linux-kernel - Re: [PATCH] vmscan: skip freeing memory from zones with lots free

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Date:	Mon, 08 Dec 2008 15:25:10 -0500
From:	Rik van Riel <riel@...hat.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH] vmscan: skip freeing memory from zones with lots free

KOSAKI Motohiro wrote:

> +	for (o = order; o < MAX_ORDER; o++) {
> +		if (z->free_area[o].nr_free)
> +			return 1;

Since page breakup and coalescing always manipulates .nr_free,
I wonder if it would make sense to pack the nr_free variables
in their own cache line(s), so we have fewer cache misses when
going through zone_watermark_ok() ?

That would end up looking something like this:

(whitespace mangled because it doesn't make sense to apply
just this thing, anyway)

Index: linux-2.6.28-rc7/include/linux/mmzone.h
===================================================================
--- linux-2.6.28-rc7.orig/include/linux/mmzone.h        2008-12-02 
15:04:33.000000000 -0500
+++ linux-2.6.28-rc7/include/linux/mmzone.h     2008-12-08 
15:24:25.000000000 -0500
@@ -58,7 +58,6 @@ static inline int get_pageblock_migratet

  struct free_area {
         struct list_head        free_list[MIGRATE_TYPES];
-       unsigned long           nr_free;
  };

  struct pglist_data;
@@ -296,6 +295,7 @@ struct zone {
         seqlock_t               span_seqlock;
  #endif
         struct free_area        free_area[MAX_ORDER];
+       struct nr_free          [MAX_ORDER];

  #ifndef CONFIG_SPARSEMEM
         /*


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