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writeback: avoid unnecessary determine_dirtyable_memory call

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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>
This commit is contained in:
Minchan Kim 2011-01-13 15:46:27 -08:00 committed by Linus Torvalds
parent ecb256f815
commit 240c879f20
1 changed files with 4 additions and 1 deletions
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5
mm/page-writeback.c
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@ -410,9 +410,12 @@ void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
{ {
unsigned long background; unsigned long background;
unsigned long dirty; unsigned long dirty;
unsigned long available_memory = determine_dirtyable_memory(); unsigned long uninitialized_var(available_memory);
struct task_struct *tsk; struct task_struct *tsk;
if (!vm_dirty_bytes || !dirty_background_bytes)
available_memory = determine_dirtyable_memory();
if (vm_dirty_bytes) if (vm_dirty_bytes)
dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE); dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
else else