• Wu Fengguang's avatar
    writeback: introduce max-pause and pass-good dirty limits · ffd1f609
    Wu Fengguang authored
    The max-pause limit helps to keep the sleep time inside
    balance_dirty_pages() within MAX_PAUSE=200ms. The 200ms max sleep means
    per task rate limit of 8pages/200ms=160KB/s when dirty exceeded, which
    normally is enough to stop dirtiers from continue pushing the dirty
    pages high, unless there are a sufficient large number of slow dirtiers
    (eg. 500 tasks doing 160KB/s will still sum up to 80MB/s, exceeding the
    write bandwidth of a slow disk and hence accumulating more and more dirty
    pages).
    
    The pass-good limit helps to let go of the good bdi's in the presence of
    a blocked bdi (ie. NFS server not responding) or slow USB disk which for
    some reason build up a large number of initial dirty pages that refuse
    to go away anytime soon.
    
    For example, given two bdi's A and B and the initial state
    
    	bdi_thresh_A = dirty_thresh / 2
    	bdi_thresh_B = dirty_thresh / 2
    	bdi_dirty_A  = dirty_thresh / 2
    	bdi_dirty_B  = dirty_thresh / 2
    
    Then A get blocked, after a dozen seconds
    
    	bdi_thresh_A = 0
    	bdi_thresh_B = dirty_thresh
    	bdi_dirty_A  = dirty_thresh / 2
    	bdi_dirty_B  = dirty_thresh / 2
    
    The (bdi_dirty_B < bdi_thresh_B) test is now useless and the dirty pages
    will be effectively throttled by condition (nr_dirty < dirty_thresh).
    This has two problems:
    (1) we lose the protections for light dirtiers
    (2) balance_dirty_pages() effectively becomes IO-less because the
        (bdi_nr_reclaimable > bdi_thresh) test won't be true. This is good
        for IO, but balance_dirty_pages() loses an important way to break
        out of the loop which leads to more spread out throttle delays.
    
    DIRTY_PASSGOOD_AREA can eliminate the above issues. The only problem is,
    DIRTY_PASSGOOD_AREA needs to be defined as 2 to fully cover the above
    example while this patch uses the more conservative value 8 so as not to
    surprise people with too many dirty pages than expected.
    
    The max-pause limit won't noticeably impact the speed dirty pages are
    knocked down when there is a sudden drop of global/bdi dirty thresholds.
    Because the heavy dirties will be throttled below 160KB/s which is slow
    enough. It does help to avoid long dirty throttle delays and especially
    will make light dirtiers more responsive.
    Signed-off-by: default avatarWu Fengguang <fengguang.wu@intel.com>
    ffd1f609
page-writeback.c 45.1 KB