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    writeback: bdi write bandwidth estimation · e98be2d5
    Wu Fengguang 提交于
    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, because 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: NWu Fengguang <fengguang.wu@intel.com>
    e98be2d5
page-writeback.c 42.1 KB