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

[Vivek Goyal]

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.

What IO scheduler have you used for testing? CFQ now a days almost chokes
async requests in presence of lots of sync IO. Have you done some testing
with that scenario and see how quickly you adjust to that change.

/me is trying to wrap his head around all the smoothing and bandwidth
calculation functions. Wished there was more explanation to it.

Thanks
Vivek
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