• Wu Fengguang's avatar
    writeback: dirty ratelimit - think time compensation · 83712358
    Wu Fengguang authored
    Compensate the task's think time when computing the final pause time,
    so that ->dirty_ratelimit can be executed accurately.
    
            think time := time spend outside of balance_dirty_pages()
    
    In the rare case that the task slept longer than the 200ms period time
    (result in negative pause time), the sleep time will be compensated in
    the following periods, too, if it's less than 1 second.
    
    Accumulated errors are carefully avoided as long as the max pause area
    is not hitted.
    
    Pseudo code:
    
            period = pages_dirtied / task_ratelimit;
            think = jiffies - dirty_paused_when;
            pause = period - think;
    
    1) normal case: period > think
    
            pause = period - think
            dirty_paused_when = jiffies + pause
            nr_dirtied = 0
    
                                 period time
                  |===============================>|
                      think time      pause time
                  |===============>|==============>|
            ------|----------------|---------------|------------------------
            dirty_paused_when   jiffies
    
    2) no pause case: period <= think
    
            don't pause; reduce future pause time by:
            dirty_paused_when += period
            nr_dirtied = 0
    
                               period time
                  |===============================>|
                                      think time
                  |===================================================>|
            ------|--------------------------------+-------------------|----
            dirty_paused_when                                       jiffies
    Acked-by: default avatarJan Kara <jack@suse.cz>
    Acked-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
    Signed-off-by: default avatarWu Fengguang <fengguang.wu@intel.com>
    83712358
writeback.h 13.3 KB