The estimation value will start from 100MB/s and adapt to the real bandwidth in seconds. It's pretty accurate for common filesystems. As the first use case, it replaces the fixed 100MB/s value used for throttle bandwidth calculation in balance_dirty_pages(). The overheads won't be high because the bdi bandwidth update only occurs in >10ms intervals. Initially it's only estimated in balance_dirty_pages() because this is the most reliable place to get reasonable large bandwidth -- the bdi is normally fully utilized when bdi_thresh is reached. Then Shaohua recommends to also do it in the flusher thread, to keep the value updated when there are only periodic/background writeback and no tasks throttled. The estimation cannot be done purely in the flusher thread because it's not sufficient for NFS. NFS writeback won't block at get_request_wait(), so tend to complete quickly. Another problem is, slow devices may take dozens of seconds to write the initial 64MB chunk (write_bandwidth starts with 100MB/s, this translates to 64MB nr_to_write). So it may take more than 1 minute to adapt to the smallish bandwidth if the bandwidth is only updated in the flusher thread. CC: Li Shaohua Signed-off-by: Wu Fengguang --- fs/fs-writeback.c | 5 ++++ include/linux/backing-dev.h | 2 + include/linux/writeback.h | 3 ++ mm/backing-dev.c | 1 mm/page-writeback.c | 41 +++++++++++++++++++++++++++++++++- 5 files changed, 51 insertions(+), 1 deletion(-) --- linux-next.orig/include/linux/backing-dev.h 2010-12-08 22:44:24.000000000 +0800 +++ linux-next/include/linux/backing-dev.h 2010-12-08 22:44:24.000000000 +0800 @@ -75,6 +75,8 @@ struct backing_dev_info { struct percpu_counter bdi_stat[NR_BDI_STAT_ITEMS]; struct prop_local_percpu completions; + unsigned long write_bandwidth_update_time; + int write_bandwidth; int dirty_exceeded; unsigned int min_ratio; --- linux-next.orig/mm/backing-dev.c 2010-12-08 22:44:24.000000000 +0800 +++ linux-next/mm/backing-dev.c 2010-12-08 22:44:24.000000000 +0800 @@ -660,6 +660,7 @@ int bdi_init(struct backing_dev_info *bd goto err; } + bdi->write_bandwidth = 100 << 20; bdi->dirty_exceeded = 0; err = prop_local_init_percpu(&bdi->completions); --- linux-next.orig/fs/fs-writeback.c 2010-12-08 22:44:22.000000000 +0800 +++ linux-next/fs/fs-writeback.c 2010-12-08 22:44:24.000000000 +0800 @@ -635,6 +635,8 @@ static long wb_writeback(struct bdi_writ .range_cyclic = work->range_cyclic, }; unsigned long oldest_jif; + unsigned long bw_time; + s64 bw_written = 0; long wrote = 0; long write_chunk; struct inode *inode; @@ -668,6 +670,8 @@ static long wb_writeback(struct bdi_writ write_chunk = LONG_MAX; wbc.wb_start = jiffies; /* livelock avoidance */ + bdi_update_write_bandwidth(wb->bdi, &bw_time, &bw_written); + for (;;) { /* * Stop writeback when nr_pages has been consumed @@ -702,6 +706,7 @@ static long wb_writeback(struct bdi_writ else writeback_inodes_wb(wb, &wbc); trace_wbc_writeback_written(&wbc, wb->bdi); + bdi_update_write_bandwidth(wb->bdi, &bw_time, &bw_written); work->nr_pages -= write_chunk - wbc.nr_to_write; wrote += write_chunk - wbc.nr_to_write; --- linux-next.orig/mm/page-writeback.c 2010-12-08 22:44:24.000000000 +0800 +++ linux-next/mm/page-writeback.c 2010-12-08 22:44:24.000000000 +0800 @@ -515,6 +515,41 @@ out: return 1 + int_sqrt(dirty_thresh - dirty_pages); } +void bdi_update_write_bandwidth(struct backing_dev_info *bdi, + unsigned long *bw_time, + s64 *bw_written) +{ + unsigned long written; + unsigned long elapsed; + unsigned long bw; + unsigned long w; + + if (*bw_written == 0) + goto snapshot; + + elapsed = jiffies - *bw_time; + if (elapsed < HZ/100) + return; + + /* + * When there lots of tasks throttled in balance_dirty_pages(), they + * will each try to update the bandwidth for the same period, making + * the bandwidth drift much faster than the desired rate (as in the + * single dirtier case). So do some rate limiting. + */ + if (jiffies - bdi->write_bandwidth_update_time < elapsed) + goto snapshot; + + written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]) - *bw_written; + bw = (HZ * PAGE_CACHE_SIZE * written + elapsed/2) / elapsed; + w = min(elapsed / (HZ/100), 128UL); + bdi->write_bandwidth = (bdi->write_bandwidth * (1024-w) + bw * w) >> 10; + bdi->write_bandwidth_update_time = jiffies; +snapshot: + *bw_written = percpu_counter_read(&bdi->bdi_stat[BDI_WRITTEN]); + *bw_time = jiffies; +} + /* * balance_dirty_pages() must be called by processes which are generating dirty * data. It looks at the number of dirty pages in the machine and will force @@ -535,6 +570,8 @@ static void balance_dirty_pages(struct a unsigned long pause = 0; bool dirty_exceeded = false; struct backing_dev_info *bdi = mapping->backing_dev_info; + unsigned long bw_time; + s64 bw_written = 0; for (;;) { /* @@ -583,7 +620,7 @@ static void balance_dirty_pages(struct a goto pause; } - bw = 100 << 20; /* use static 100MB/s for the moment */ + bw = bdi->write_bandwidth; bw = bw * (bdi_thresh - bdi_dirty); bw = bw / (bdi_thresh / TASK_SOFT_DIRTY_LIMIT + 1); @@ -592,8 +629,10 @@ static void balance_dirty_pages(struct a pause = clamp_val(pause, 1, HZ/10); pause: + bdi_update_write_bandwidth(bdi, &bw_time, &bw_written); __set_current_state(TASK_INTERRUPTIBLE); io_schedule_timeout(pause); + bdi_update_write_bandwidth(bdi, &bw_time, &bw_written); /* * The bdi thresh is somehow "soft" limit derived from the --- linux-next.orig/include/linux/writeback.h 2010-12-08 22:44:22.000000000 +0800 +++ linux-next/include/linux/writeback.h 2010-12-08 22:44:24.000000000 +0800 @@ -138,6 +138,9 @@ void global_dirty_limits(unsigned long * unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty, unsigned long dirty_pages); +void bdi_update_write_bandwidth(struct backing_dev_info *bdi, + unsigned long *bw_time, + s64 *bw_written); void page_writeback_init(void); void balance_dirty_pages_ratelimited_nr(struct address_space *mapping, -- 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/