• Jens Axboe's avatar
    block: hook up writeback throttling · 87760e5e
    Jens Axboe authored
    Enable throttling of buffered writeback to make it a lot
    more smooth, and has way less impact on other system activity.
    Background writeback should be, by definition, background
    activity. The fact that we flush huge bundles of it at the time
    means that it potentially has heavy impacts on foreground workloads,
    which isn't ideal. We can't easily limit the sizes of writes that
    we do, since that would impact file system layout in the presence
    of delayed allocation. So just throttle back buffered writeback,
    unless someone is waiting for it.
    The algorithm for when to throttle takes its inspiration in the
    CoDel networking scheduling algorithm. Like CoDel, blk-wb monitors
    the minimum latencies of requests over a window of time. In that
    window of time, if the minimum latency of any request exceeds a
    given target, then a scale count is incremented and the queue depth
    is shrunk. The next monitoring window is shrunk accordingly. Unlike
    CoDel, if we hit a window that exhibits good behavior, then we
    simply increment the scale count and re-calculate the limits for that
    scale value. This prevents us from oscillating between a
    close-to-ideal value and max all the time, instead remaining in the
    windows where we get good behavior.
    Unlike CoDel, blk-wb allows the scale count to to negative. This
    happens if we primarily have writes going on. Unlike positive
    scale counts, this doesn't change the size of the monitoring window.
    When the heavy writers finish, blk-bw quickly snaps back to it's
    stable state of a zero scale count.
    The patch registers a sysfs entry, 'wb_lat_usec'. This sets the latency
    target to me met. It defaults to 2 msec for non-rotational storage, and
    75 msec for rotational storage. Setting this value to '0' disables
    blk-wb. Generally, a user would not have to touch this setting.
    We don't enable WBT on devices that are managed with CFQ, and have
    a non-root block cgroup attached. If we have a proportional share setup
    on this particular disk, then the wbt throttling will interfere with
    that. We don't have a strong need for wbt for that case, since we will
    rely on CFQ doing that for us.
    Signed-off-by: default avatarJens Axboe <axboe@fb.com>
blk-settings.c 27.5 KB