• Fengguang Wu's avatar
    writeback: speed up writeback of big dirty files · 8bc3be27
    Fengguang Wu authored
    After making dirty a 100M file, the normal behavior is to start the
    writeback for all data after 30s delays.  But sometimes the following
    happens instead:
    
    	- after 30s:    ~4M
    	- after 5s:     ~4M
    	- after 5s:     all remaining 92M
    
    Some analyze shows that the internal io dispatch queues goes like this:
    
    		s_io            s_more_io
    		-------------------------
    	1)	100M,1K         0
    	2)	1K              96M
    	3)	0               96M
    1) initial state with a 100M file and a 1K file
    
    2) 4M written, nr_to_write <= 0, so write more
    
    3) 1K written, nr_to_write > 0, no more writes(BUG)
    
    nr_to_write > 0 in (3) fools the upper layer to think that data have all
    been written out.  The big dirty file is actually still sitting in
    s_more_io.  We cannot simply splice s_more_io back to s_io as soon as s_io
    becomes empty, and let the loop in generic_sync_sb_inodes() continue: this
    may starve newly expired inodes in s_dirty.  It is also not an option to
    draw inodes from both s_more_io and s_dirty, an let the loop go on: this
    might lead to live locks, and might also starve other superblocks in sync
    time(well kupdate may still starve some superblocks, that's another bug).
    
    We have to return when a full scan of s_io completes.  So nr_to_write > 0
    does not necessarily mean that "all data are written".  This patch
    introduces a flag writeback_control.more_io to indicate that more io should
    be done.  With it the big dirty file no longer has to wait for the next
    kupdate invokation 5s later.
    
    In sync_sb_inodes() we only set more_io on super_blocks we actually
    visited.  This avoids the interaction between two pdflush deamons.
    
    Also in __sync_single_inode() we don't blindly keep requeuing the io if the
    filesystem cannot progress.  Failing to do so may lead to 100% iowait.
    Tested-by: default avatarMike Snitzer <snitzer@gmail.com>
    Signed-off-by: default avatarFengguang Wu <wfg@mail.ustc.edu.cn>
    Cc: Michael Rubin <mrubin@google.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    8bc3be27
fs-writeback.c 22.4 KB