Re: [PATCH 3/9] writeback: bdi write bandwidth estimation

Andrea Righi
2011-07-01T15:20:17Z

On Wed, Jun 29, 2011 at 10:52:48PM +0800, Wu Fengguang wrote:
> The estimation value will start from 100MB/s and adapt to the real
> bandwidth in seconds.
>
> It tries to update the bandwidth only when disk is fully utilized.
> Any inactive period of more than one second will be skipped.
>
> The estimated bandwidth will be reflecting how fast the device can
> writeout when _fully utilized_, and won't drop to 0 when it goes idle.
> The value will remain constant at disk idle time. At busy write time, if
> not considering fluctuations, it will also remain high unless be knocked
> down by possible concurrent reads that compete for the disk time and
> bandwidth with async writes.
>
> The estimation is not done purely in the flusher because there is no
> guarantee for write_cache_pages() to return timely to update bandwidth.
>
> The bdi->avg_write_bandwidth smoothing is very effective for filtering
> out sudden spikes, however may be a little biased in long term.
>
> The overheads are low because the bdi bandwidth update only occurs at
> 200ms intervals.
>
> The 200ms update interval is suitable, becuase it's not possible to get
> the real bandwidth for the instance at all, due to large fluctuations.
>
> The NFS commits can be as large as seconds worth of data. One XFS
> completion may be as large as half second worth of data if we are going
> to increase the write chunk to half second worth of data. In ext4,
> fluctuations with time period of around 5 seconds is observed. And there
> is another pattern of irregular periods of up to 20 seconds on SSD tests.
>
> That's why we are not only doing the estimation at 200ms intervals, but
> also averaging them over a period of 3 seconds and then go further to do
> another level of smoothing in avg_write_bandwidth.
>
> CC: Li Shaohua <shaohua.li@intel.com>
> CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---

...

Another small time nitpick.

> +
> +static void bdi_update_bandwidth(struct backing_dev_info *bdi,
> + unsigned long start_time)
> +{
> + if (jiffies - bdi->bw_time_stamp <= MAX_PAUSE + MAX_PAUSE / 10)

if (time_is_after_eq_jiffies(bdi->bw_time_stamp + MAX_PAUSE +
MAX_PAUSE / 10)

> + return;
> + if (spin_trylock(&bdi->wb.list_lock)) {
> + __bdi_update_bandwidth(bdi, start_time);
> + spin_unlock(&bdi->wb.list_lock);
> + }
> +}
> +
> /*
> * 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
> @@ -491,6 +569,7 @@ static void balance_dirty_pages(struct a
> unsigned long pause = 1;
> bool dirty_exceeded = false;
> struct backing_dev_info *bdi = mapping->backing_dev_info;
> + unsigned long start_time = jiffies;
>
> for (;;) {
> nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
> @@ -545,6 +624,8 @@ static void balance_dirty_pages(struct a
> if (!bdi->dirty_exceeded)
> bdi->dirty_exceeded = 1;
>
> + bdi_update_bandwidth(bdi, start_time);
> +
> /* Note: nr_reclaimable denotes nr_dirty + nr_unstable.
> * Unstable writes are a feature of certain networked
> * filesystems (i.e. NFS) in which data may have been
>
>

-Andrea