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SubjectRe: [RFC PATCH] v2 mm: balance_dirty_pages. reduce calls to global_page_state to reduce cache references
On Fri, Sep 04, 2009 at 07:05:30PM +0800, Richard Kennedy wrote:
> 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.
>
> This version includes the changes suggested by
> Wu Fengguang <fengguang.wu@intel.com>

It seems that an redundant pages_written test can be reduced by

--- linux.orig/mm/page-writeback.c 2009-09-06 11:44:39.000000000 +0800
+++ linux/mm/page-writeback.c 2009-09-06 11:44:42.000000000 +0800
@@ -526,10 +526,6 @@ static void balance_dirty_pages(struct a
(background_thresh + dirty_thresh) / 2)
break;

- /* done enough? */
- if (pages_written >= write_chunk)
- break;
-
if (!bdi->dirty_exceeded)
bdi->dirty_exceeded = 1;

@@ -547,7 +543,7 @@ static void balance_dirty_pages(struct a
pages_written += write_chunk - wbc.nr_to_write;
/* don't wait if we've done enough */
if (pages_written >= write_chunk)
- continue;
+ break;
}
congestion_wait(BLK_RW_ASYNC, HZ/10);
}
Otherwise the patch looks good to me. Thank you for the nice work!

Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>

> Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
> ----
> Thanks to everybody for the feedback & suggestions.
> This patch is against 2.6.31-rc8
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 81627eb..9581359 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -260,32 +260,6 @@ 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)
> {
> @@ -478,7 +452,6 @@ 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);
> }
> }
> @@ -499,7 +472,7 @@ static void balance_dirty_pages(struct address_space *mapping)
> unsigned long bdi_thresh;
> unsigned long pages_written = 0;
> unsigned long write_chunk = sync_writeback_pages();
> -
> + int dirty_exceeded;
> struct backing_dev_info *bdi = mapping->backing_dev_info;
>
> for (;;) {
> @@ -512,16 +485,36 @@ static void balance_dirty_pages(struct address_space *mapping)
> };
>
> get_dirty_limits(&background_thresh, &dirty_thresh,
> - &bdi_thresh, bdi);
> + &bdi_thresh, bdi);
>
> nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
> - global_page_state(NR_UNSTABLE_NFS);
> - nr_writeback = global_page_state(NR_WRITEBACK);
> + global_page_state(NR_UNSTABLE_NFS);
> + nr_writeback = global_page_state(NR_WRITEBACK) +
> + global_page_state(NR_WRITEBACK_TEMP);
>
> - 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 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)
> + dirty_exceeded =
> + (bdi_nr_reclaimable + bdi_nr_writeback >= bdi_thresh)
> + || (nr_reclaimable + nr_writeback >= dirty_thresh);
> +
> + if (!dirty_exceeded)
> break;
>
> /*
> @@ -530,7 +523,11 @@ static void balance_dirty_pages(struct address_space *mapping)
> * when the bdi limits are ramping up.
> */
> if (nr_reclaimable + nr_writeback <
> - (background_thresh + dirty_thresh) / 2)
> + (background_thresh + dirty_thresh) / 2)
> + break;
> +
> + /* done enough? */
> + if (pages_written >= write_chunk)
> break;
>
> if (!bdi->dirty_exceeded)
> @@ -548,38 +545,14 @@ static void balance_dirty_pages(struct address_space *mapping)
> if (bdi_nr_reclaimable > bdi_thresh) {
> writeback_inodes(&wbc);
> pages_written += write_chunk - wbc.nr_to_write;
> - get_dirty_limits(&background_thresh, &dirty_thresh,
> - &bdi_thresh, bdi);
> - }
> -
> - /*
> - * 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);
> + /* don't wait if we've done enough */
> + if (pages_written >= write_chunk)
> + continue;
> }
> -
> - if (bdi_nr_reclaimable + bdi_nr_writeback <= bdi_thresh)
> - break;
> - if (pages_written >= write_chunk)
> - break; /* We've done our duty */
> -
> congestion_wait(BLK_RW_ASYNC, 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))
> @@ -593,10 +566,8 @@ static void balance_dirty_pages(struct address_space *mapping)
> * In normal mode, we start background writeout at the lower
> * 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)))
> + if ((laptop_mode && pages_written) || (!laptop_mode &&
> + (nr_reclaimable > background_thresh)))
> pdflush_operation(background_writeout, 0);
> }
>
>


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