writeback: balance_dirty_pages(): reduce calls to global_page_state
Reducing the number of times balance_dirty_pages calls global_page_state reduces the cache references and so improves write performance on a variety of workloads. 'perf stats' of simple fio write tests shows the reduction in cache access. Where the test is fio 'write,mmap,600Mb,pre_read' on AMD AthlonX2 with 3Gb memory (dirty_threshold approx 600 Mb) running each test 10 times, dropping the fasted & slowest values then taking the average & standard deviation average (s.d.) in millions (10^6) 2.6.31-rc8 648.6 (14.6) +patch 620.1 (16.5) Achieving this reduction is by dropping clip_bdi_dirty_limit as it rereads the counters to apply the dirty_threshold and moving this check up into balance_dirty_pages where it has already read the counters. Also by rearrange the for loop to only contain one copy of the limit tests allows the pdflush test after the loop to use the local copies of the counters rather than rereading them. In the common case with no throttling it now calls global_page_state 5 fewer times and bdi_stat 2 fewer. Fengguang: This patch slightly changes behavior by replacing clip_bdi_dirty_limit() with the explicit check (nr_reclaimable + nr_writeback >= dirty_thresh) to avoid exceeding the dirty limit. Since the bdi dirty limit is mostly accurate we don't need to do routinely clip. A simple dirty limit check would be enough. The check is necessary because, in principle we should throttle everything calling balance_dirty_pages() when we're over the total limit, as said by Peter. We now set and clear dirty_exceeded not only based on bdi dirty limits, but also on the global dirty limit. The global limit check is added in place of clip_bdi_dirty_limit() for safety and not intended as a behavior change. The bdi limits should be tight enough to keep all dirty pages under the global limit at most time; occasional small exceeding should be OK though. The change makes the logic more obvious: the global limit is the ultimate goal and shall be always imposed. We may now start background writeback work based on outdated conditions. That's safe because the bdi flush thread will (and have to) double check the states. It reduces overall overheads because the test based on old states still have good chance to be right. [akpm@linux-foundation.org] fix uninitialized dirty_exceeded Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com> Cc: Jan Kara <jack@suse.cz> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Christoph Hellwig <hch@infradead.org> Cc: Dave Chinner <david@fromorbit.com> Cc: Jens Axboe <axboe@kernel.dk> 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
Linus Torvalds
Родитель
a292dfa017
Коммит
e50e37201a
|
|
@ -253,32 +253,6 @@ static void bdi_writeout_fraction(struct backing_dev_info *bdi,
|
|||
}
|
||||
}
|
||||
|
||||
/*
|
||||
* Clip the earned share of dirty pages to that which is actually available.
|
||||
* This avoids exceeding the total dirty_limit when the floating averages
|
||||
* fluctuate too quickly.
|
||||
*/
|
||||
static void clip_bdi_dirty_limit(struct backing_dev_info *bdi,
|
||||
unsigned long dirty, unsigned long *pbdi_dirty)
|
||||
{
|
||||
unsigned long avail_dirty;
|
||||
|
||||
avail_dirty = global_page_state(NR_FILE_DIRTY) +
|
||||
global_page_state(NR_WRITEBACK) +
|
||||
global_page_state(NR_UNSTABLE_NFS) +
|
||||
global_page_state(NR_WRITEBACK_TEMP);
|
||||
|
||||
if (avail_dirty < dirty)
|
||||
avail_dirty = dirty - avail_dirty;
|
||||
else
|
||||
avail_dirty = 0;
|
||||
|
||||
avail_dirty += bdi_stat(bdi, BDI_RECLAIMABLE) +
|
||||
bdi_stat(bdi, BDI_WRITEBACK);
|
||||
|
||||
*pbdi_dirty = min(*pbdi_dirty, avail_dirty);
|
||||
}
|
||||
|
||||
static inline void task_dirties_fraction(struct task_struct *tsk,
|
||||
long *numerator, long *denominator)
|
||||
{
|
||||
|
|
@ -469,7 +443,6 @@ get_dirty_limits(unsigned long *pbackground, unsigned long *pdirty,
|
|||
bdi_dirty = dirty * bdi->max_ratio / 100;
|
||||
|
||||
*pbdi_dirty = bdi_dirty;
|
||||
clip_bdi_dirty_limit(bdi, dirty, pbdi_dirty);
|
||||
task_dirty_limit(current, pbdi_dirty);
|
||||
}
|
||||
}
|
||||
|
|
@ -491,7 +464,7 @@ static void balance_dirty_pages(struct address_space *mapping,
|
|||
unsigned long bdi_thresh;
|
||||
unsigned long pages_written = 0;
|
||||
unsigned long pause = 1;
|
||||
|
||||
bool dirty_exceeded = false;
|
||||
struct backing_dev_info *bdi = mapping->backing_dev_info;
|
||||
|
||||
for (;;) {
|
||||
|
|
@ -509,10 +482,35 @@ static void balance_dirty_pages(struct address_space *mapping,
|
|||
global_page_state(NR_UNSTABLE_NFS);
|
||||
nr_writeback = global_page_state(NR_WRITEBACK);
|
||||
|
||||
bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
|
||||
bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
|
||||
/*
|
||||
* In order to avoid the stacked BDI deadlock we need
|
||||
* to ensure we accurately count the 'dirty' pages when
|
||||
* the threshold is low.
|
||||
*
|
||||
* Otherwise it would be possible to get thresh+n pages
|
||||
* reported dirty, even though there are thresh-m pages
|
||||
* actually dirty; with m+n sitting in the percpu
|
||||
* deltas.
|
||||
*/
|
||||
if (bdi_thresh < 2*bdi_stat_error(bdi)) {
|
||||
bdi_nr_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
|
||||
bdi_nr_writeback = bdi_stat_sum(bdi, BDI_WRITEBACK);
|
||||
} else {
|
||||
bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
|
||||
bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
|
||||
}
|
||||
|
||||
if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
|
||||
/*
|
||||
* The bdi thresh is somehow "soft" limit derived from the
|
||||
* global "hard" limit. The former helps to prevent heavy IO
|
||||
* bdi or process from holding back light ones; The latter is
|
||||
* the last resort safeguard.
|
||||
*/
|
||||
dirty_exceeded =
|
||||
(bdi_nr_reclaimable + bdi_nr_writeback >= bdi_thresh)
|
||||
|| (nr_reclaimable + nr_writeback >= dirty_thresh);
|
||||
|
||||
if (!dirty_exceeded)
|
||||
break;
|
||||
|
||||
/*
|
||||
|
|
@ -540,34 +538,10 @@ static void balance_dirty_pages(struct address_space *mapping,
|
|||
if (bdi_nr_reclaimable > bdi_thresh) {
|
||||
writeback_inodes_wb(&bdi->wb, &wbc);
|
||||
pages_written += write_chunk - wbc.nr_to_write;
|
||||
get_dirty_limits(&background_thresh, &dirty_thresh,
|
||||
&bdi_thresh, bdi);
|
||||
trace_wbc_balance_dirty_written(&wbc, bdi);
|
||||
if (pages_written >= write_chunk)
|
||||
break; /* We've done our duty */
|
||||
}
|
||||
|
||||
/*
|
||||
* In order to avoid the stacked BDI deadlock we need
|
||||
* to ensure we accurately count the 'dirty' pages when
|
||||
* the threshold is low.
|
||||
*
|
||||
* Otherwise it would be possible to get thresh+n pages
|
||||
* reported dirty, even though there are thresh-m pages
|
||||
* actually dirty; with m+n sitting in the percpu
|
||||
* deltas.
|
||||
*/
|
||||
if (bdi_thresh < 2*bdi_stat_error(bdi)) {
|
||||
bdi_nr_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
|
||||
bdi_nr_writeback = bdi_stat_sum(bdi, BDI_WRITEBACK);
|
||||
} else if (bdi_nr_reclaimable) {
|
||||
bdi_nr_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
|
||||
bdi_nr_writeback = bdi_stat(bdi, BDI_WRITEBACK);
|
||||
}
|
||||
|
||||
if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
|
||||
break;
|
||||
if (pages_written >= write_chunk)
|
||||
break; /* We've done our duty */
|
||||
|
||||
trace_wbc_balance_dirty_wait(&wbc, bdi);
|
||||
__set_current_state(TASK_INTERRUPTIBLE);
|
||||
io_schedule_timeout(pause);
|
||||
|
|
@ -581,8 +555,7 @@ static void balance_dirty_pages(struct address_space *mapping,
|
|||
pause = HZ / 10;
|
||||
}
|
||||
|
||||
if (bdi_nr_reclaimable + bdi_nr_writeback < bdi_thresh &&
|
||||
bdi->dirty_exceeded)
|
||||
if (!dirty_exceeded && bdi->dirty_exceeded)
|
||||
bdi->dirty_exceeded = 0;
|
||||
|
||||
if (writeback_in_progress(bdi))
|
||||
|
|
@ -597,9 +570,7 @@ static void balance_dirty_pages(struct address_space *mapping,
|
|||
* background_thresh, to keep the amount of dirty memory low.
|
||||
*/
|
||||
if ((laptop_mode && pages_written) ||
|
||||
(!laptop_mode && ((global_page_state(NR_FILE_DIRTY)
|
||||
+ global_page_state(NR_UNSTABLE_NFS))
|
||||
> background_thresh)))
|
||||
(!laptop_mode && (nr_reclaimable > background_thresh)))
|
||||
bdi_start_background_writeback(bdi);
|
||||
}
|
||||
|
||||
|
|
|
|||
Загрузка…
Ссылка в новой задаче