Here is an interesting test to verify the theory with balance_dirty_pages() tracing. On a partition that can do ~60MB/s, a sparse file is created and 4 rsync tasks with different write bandwidth started: dd if=/dev/zero of=/mnt/1T bs=1M count=1 seek=1024000 echo 1 > /debug/tracing/events/writeback/balance_dirty_pages/enable rsync localhost:/mnt/1T /mnt/a --bwlimit 10000& rsync localhost:/mnt/1T /mnt/A --bwlimit 10000& rsync localhost:/mnt/1T /mnt/b --bwlimit 20000& rsync localhost:/mnt/1T /mnt/c --bwlimit 30000& Trace outputs within 0.1 second, grouped by tasks: rsync-3824 [004] 15002.076447: balance_dirty_pages: bdi=btrfs-2 weight=15% limit=130876 gap=5340 dirtied=192 pause=20 rsync-3822 [003] 15002.091701: balance_dirty_pages: bdi=btrfs-2 weight=15% limit=130777 gap=5113 dirtied=192 pause=20 rsync-3821 [006] 15002.004667: balance_dirty_pages: bdi=btrfs-2 weight=30% limit=129570 gap=3714 dirtied=64 pause=8 rsync-3821 [006] 15002.012654: balance_dirty_pages: bdi=btrfs-2 weight=30% limit=129589 gap=3733 dirtied=64 pause=8 rsync-3821 [006] 15002.021838: balance_dirty_pages: bdi=btrfs-2 weight=30% limit=129604 gap=3748 dirtied=64 pause=8 rsync-3821 [004] 15002.091193: balance_dirty_pages: bdi=btrfs-2 weight=29% limit=129583 gap=3983 dirtied=64 pause=8 rsync-3821 [004] 15002.102729: balance_dirty_pages: bdi=btrfs-2 weight=29% limit=129594 gap=3802 dirtied=64 pause=8 rsync-3821 [000] 15002.109252: balance_dirty_pages: bdi=btrfs-2 weight=29% limit=129619 gap=3827 dirtied=64 pause=8 rsync-3823 [002] 15002.009029: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128762 gap=2842 dirtied=64 pause=12 rsync-3823 [002] 15002.021598: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128813 gap=3021 dirtied=64 pause=12 rsync-3823 [003] 15002.032973: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128805 gap=2885 dirtied=64 pause=12 rsync-3823 [003] 15002.048800: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128823 gap=2967 dirtied=64 pause=12 rsync-3823 [003] 15002.060728: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128821 gap=3221 dirtied=64 pause=12 rsync-3823 [000] 15002.073152: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128825 gap=3225 dirtied=64 pause=12 rsync-3823 [005] 15002.090111: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128782 gap=3214 dirtied=64 pause=12 rsync-3823 [004] 15002.102520: balance_dirty_pages: bdi=btrfs-2 weight=39% limit=128764 gap=3036 dirtied=64 pause=12 The data vividly show that - the heaviest writer is throttled a bit (weight=39%) - the lighter writers run at full speed (weight=15%,15%,30%) rsync is smart enough to compensate for the in-kernel pause time Signed-off-by: Wu Fengguang --- include/trace/events/writeback.h | 61 +++++++++++++++++++++++++++-- mm/page-writeback.c | 6 ++ 2 files changed, 64 insertions(+), 3 deletions(-) --- linux-next.orig/include/trace/events/writeback.h 2010-12-08 22:44:20.000000000 +0800 +++ linux-next/include/trace/events/writeback.h 2010-12-08 22:44:26.000000000 +0800 @@ -147,11 +147,66 @@ DEFINE_EVENT(wbc_class, name, \ DEFINE_WBC_EVENT(wbc_writeback_start); DEFINE_WBC_EVENT(wbc_writeback_written); DEFINE_WBC_EVENT(wbc_writeback_wait); -DEFINE_WBC_EVENT(wbc_balance_dirty_start); -DEFINE_WBC_EVENT(wbc_balance_dirty_written); -DEFINE_WBC_EVENT(wbc_balance_dirty_wait); DEFINE_WBC_EVENT(wbc_writepage); +#define BDP_PERCENT(a, b, c) ((__entry->a - __entry->b) * 100 * c + \ + __entry->bdi_limit/2) / (__entry->bdi_limit|1) +TRACE_EVENT(balance_dirty_pages, + + TP_PROTO(struct backing_dev_info *bdi, + long bdi_dirty, + long bdi_limit, + long task_limit, + long pages_dirtied, + long pause), + + TP_ARGS(bdi, bdi_dirty, bdi_limit, task_limit, + pages_dirtied, pause), + + TP_STRUCT__entry( + __array(char, bdi, 32) + __field(long, bdi_dirty) + __field(long, bdi_limit) + __field(long, task_limit) + __field(long, pages_dirtied) + __field(long, pause) + ), + + TP_fast_assign( + strlcpy(__entry->bdi, dev_name(bdi->dev), 32); + __entry->bdi_dirty = bdi_dirty; + __entry->bdi_limit = bdi_limit; + __entry->task_limit = task_limit; + __entry->pages_dirtied = pages_dirtied; + __entry->pause = pause * 1000 / HZ; + ), + + + /* + * [..............soft throttling range............] + * ^ |<=========== bdi_gap =========>| + * (background+dirty)/2 |<== task_gap ==>| + * -------------------|-------+----------------|--------------| + * (bdi_limit * 7/8)^ ^bdi_dirty ^task_limit ^bdi_limit + * + * Reasonable large gaps help produce smooth pause times. + */ + TP_printk("bdi=%s bdi_dirty=%lu bdi_limit=%lu task_limit=%lu " + "task_weight=%ld%% task_gap=%ld%% bdi_gap=%ld%% " + "pages_dirtied=%lu pause=%lu", + __entry->bdi, + __entry->bdi_dirty, + __entry->bdi_limit, + __entry->task_limit, + /* task weight: proportion of recent dirtied pages */ + BDP_PERCENT(bdi_limit, task_limit, TASK_SOFT_DIRTY_LIMIT), + BDP_PERCENT(task_limit, bdi_dirty, TASK_SOFT_DIRTY_LIMIT), + BDP_PERCENT(bdi_limit, bdi_dirty, BDI_SOFT_DIRTY_LIMIT), + __entry->pages_dirtied, + __entry->pause + ) +); + DECLARE_EVENT_CLASS(writeback_congest_waited_template, TP_PROTO(unsigned int usec_timeout, unsigned int usec_delayed), --- linux-next.orig/mm/page-writeback.c 2010-12-08 22:44:26.000000000 +0800 +++ linux-next/mm/page-writeback.c 2010-12-08 22:44:26.000000000 +0800 @@ -666,6 +666,12 @@ static void balance_dirty_pages(struct a pause = clamp_val(pause, 1, HZ/10); pause: + trace_balance_dirty_pages(bdi, + bdi_dirty, + bdi_thresh, + task_thresh, + pages_dirtied, + pause); bdi_update_write_bandwidth(bdi, &bw_time, &bw_written); __set_current_state(TASK_INTERRUPTIBLE); io_schedule_timeout(pause); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/