• Corrado Zoccolo's avatar
    cfq-iosched: enable idling for last queue on priority class · a6d44e98
    Corrado Zoccolo authored
    cfq can disable idling for queues in various circumstances.
    When workloads of different priorities are competing, if the higher
    priority queue has idling disabled, lower priority queues may steal
    its disk share. For example, in a scenario with an RT process
    performing seeky reads vs a BE process performing sequential reads,
    on an NCQ enabled hardware, with low_latency unset,
    the RT process will dispatch only the few pending requests every full
    slice of service for the BE process.
    
    The patch solves this issue by always performing idle on the last
    queue at a given priority class > idle. If the same process, or one
    that can pre-empt it (so at the same priority or higher), submits a
    new request within the idle window, the lower priority queue won't
    dispatch, saving the disk bandwidth for higher priority ones.
    
    Note: this doesn't touch the non_rotational + NCQ case (no hardware
    to test if this is a benefit in that case).
    Signed-off-by: default avatarCorrado Zoccolo <czoccolo@gmail.com>
    Signed-off-by: default avatarJens Axboe <jens.axboe@oracle.com>
    a6d44e98
cfq-iosched.c 73 KB