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authorWu Fengguang <fengguang.wu@intel.com>2010-08-11 14:17:40 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2010-08-12 08:43:30 -0700
commit1babe18385d3976043c04237ce837f3736197eb4 (patch)
treec766bb0022ec5188cd7e991fc1f9ad51687e8aca /mm
parent16c4042f08919f447d6b2a55679546c9b97c7264 (diff)
writeback: add comment to the dirty limit functions
Document global_dirty_limits() and bdi_dirty_limit(). Signed-off-by: Wu Fengguang <fengguang.wu@intel.com> Cc: Christoph Hellwig <hch@infradead.org> Cc: Dave Chinner <david@fromorbit.com> Cc: Jens Axboe <axboe@kernel.dk> 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>
Diffstat (limited to 'mm')
-rw-r--r--mm/page-writeback.c31
1 files changed, 28 insertions, 3 deletions
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 1ea13ef350a8..20890d80c7ef 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -261,11 +261,18 @@ static inline void task_dirties_fraction(struct task_struct *tsk,
}
/*
- * scale the dirty limit
+ * task_dirty_limit - scale down dirty throttling threshold for one task
*
* task specific dirty limit:
*
* dirty -= (dirty/8) * p_{t}
+ *
+ * To protect light/slow dirtying tasks from heavier/fast ones, we start
+ * throttling individual tasks before reaching the bdi dirty limit.
+ * Relatively low thresholds will be allocated to heavy dirtiers. So when
+ * dirty pages grow large, heavy dirtiers will be throttled first, which will
+ * effectively curb the growth of dirty pages. Light dirtiers with high enough
+ * dirty threshold may never get throttled.
*/
static unsigned long task_dirty_limit(struct task_struct *tsk,
unsigned long bdi_dirty)
@@ -390,6 +397,15 @@ unsigned long determine_dirtyable_memory(void)
return x + 1; /* Ensure that we never return 0 */
}
+/**
+ * global_dirty_limits - background-writeback and dirty-throttling thresholds
+ *
+ * Calculate the dirty thresholds based on sysctl parameters
+ * - vm.dirty_background_ratio or vm.dirty_background_bytes
+ * - vm.dirty_ratio or vm.dirty_bytes
+ * The dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
+ * runtime tasks.
+ */
void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
{
unsigned long background;
@@ -424,8 +440,17 @@ void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty)
*pdirty = dirty;
}
-unsigned long bdi_dirty_limit(struct backing_dev_info *bdi,
- unsigned long dirty)
+/**
+ * bdi_dirty_limit - @bdi's share of dirty throttling threshold
+ *
+ * Allocate high/low dirty limits to fast/slow devices, in order to prevent
+ * - starving fast devices
+ * - piling up dirty pages (that will take long time to sync) on slow devices
+ *
+ * The bdi's share of dirty limit will be adapting to its throughput and
+ * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
+ */
+unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty)
{
u64 bdi_dirty;
long numerator, denominator;

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