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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>
This commit is contained in:
Wu Fengguang 2010-08-11 14:17:40 -07:00 committed by Linus Torvalds
parent 16c4042f08
commit 1babe18385
1 changed files with 28 additions and 3 deletions
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31
mm/page-writeback.c
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@ -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;