From 240c879f20a605346705be24253bc9fc6fa8a106 Mon Sep 17 00:00:00 2001
From: Minchan Kim <minchan.kim@gmail.com>
Date: Thu, 13 Jan 2011 15:46:27 -0800
Subject: writeback: avoid unnecessary determine_dirtyable_memory call
I think determine_dirtyable_memory() is a rather costly function since it
need many atomic reads for gathering zone/global page state. But when we
use vm_dirty_bytes && dirty_background_bytes, we don't need that costly
calculation.
This patch eliminates such unnecessary overhead.
NOTE : newly added if condition might add overhead in normal path.
But it should be _really_ small because anyway we need the
access both vm_dirty_bytes and dirty_background_bytes so it is
likely to hit the cache.
[akpm@linux-foundation.org: fix used-uninitialised warning]
Signed-off-by: Minchan Kim <minchan.kim@gmail.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
mm/page-writeback.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)
(limited to 'mm/page-writeback.c')
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 28763b8bdbdd..2cb01f6ec5d0 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -410,9 +410,12 @@ void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
{
unsigned long background;
unsigned long dirty;
- unsigned long available_memory = determine_dirtyable_memory();
+ unsigned long uninitialized_var(available_memory);
struct task_struct *tsk;
+ if (!vm_dirty_bytes || !dirty_background_bytes)
+ available_memory = determine_dirtyable_memory();
+
if (vm_dirty_bytes)
dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
else
--
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