• Wu Fengguang's avatar
    writeback: introduce smoothed global dirty limit · c42843f2
    Wu Fengguang authored
    The start of a heavy weight application (ie. KVM) may instantly knock
    down determine_dirtyable_memory() if the swap is not enabled or full.
    global_dirty_limits() and bdi_dirty_limit() will in turn get global/bdi
    dirty thresholds that are _much_ lower than the global/bdi dirty pages.
    
    balance_dirty_pages() will then heavily throttle all dirtiers including
    the light ones, until the dirty pages drop below the new dirty thresholds.
    During this _deep_ dirty-exceeded state, the system may appear rather
    unresponsive to the users.
    
    About "deep" dirty-exceeded: task_dirty_limit() assigns 1/8 lower dirty
    threshold to heavy dirtiers than light ones, and the dirty pages will
    be throttled around the heavy dirtiers' dirty threshold and reasonably
    below the light dirtiers' dirty threshold. In this state, only the heavy
    dirtiers will be throttled and the dirty pages are carefully controlled
    to not exceed the light dirtiers' dirty threshold. However if the
    threshold itself suddenly drops below the number of dirty pages, the
    light dirtiers will get heavily throttled.
    
    So introduce global_dirty_limit for tracking the global dirty threshold
    with policies
    
    - follow downwards slowly
    - follow up in one shot
    
    global_dirty_limit can effectively mask out the impact of sudden drop of
    dirtyable memory. It will be used in the next patch for two new type of
    dirty limits. Note that the new dirty limits are not going to avoid
    throttling the light dirtiers, but could limit their sleep time to 200ms.
    Signed-off-by: default avatarWu Fengguang <fengguang.wu@intel.com>
    c42843f2
fs-writeback.c 36.8 KB