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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
