• Christoph Hellwig's avatar
    xfs: on-stack delayed write buffer lists · 43ff2122
    Christoph Hellwig authored
    Queue delwri buffers on a local on-stack list instead of a per-buftarg one,
    and write back the buffers per-process instead of by waking up xfsbufd.
    
    This is now easily doable given that we have very few places left that write
    delwri buffers:
    
     - log recovery:
    	Only done at mount time, and already forcing out the buffers
    	synchronously using xfs_flush_buftarg
    
     - quotacheck:
    	Same story.
    
     - dquot reclaim:
    	Writes out dirty dquots on the LRU under memory pressure.  We might
    	want to look into doing more of this via xfsaild, but it's already
    	more optimal than the synchronous inode reclaim that writes each
    	buffer synchronously.
    
     - xfsaild:
    	This is the main beneficiary of the change.  By keeping a local list
    	of buffers to write we reduce latency of writing out buffers, and
    	more importably we can remove all the delwri list promotions which
    	were hitting the buffer cache hard under sustained metadata loads.
    
    The implementation is very straight forward - xfs_buf_delwri_queue now gets
    a new list_head pointer that it adds the delwri buffers to, and all callers
    need to eventually submit the list using xfs_buf_delwi_submit or
    xfs_buf_delwi_submit_nowait.  Buffers that already are on a delwri list are
    skipped in xfs_buf_delwri_queue, assuming they already are on another delwri
    list.  The biggest change to pass down the buffer list was done to the AIL
    pushing. Now that we operate on buffers the trylock, push and pushbuf log
    item methods are merged into a single push routine, which tries to lock the
    item, and if possible add the buffer that needs writeback to the buffer list.
    This leads to much simpler code than the previous split but requires the
    individual IOP_PUSH instances to unlock and reacquire the AIL around calls
    to blocking routines.
    
    Given that xfsailds now also handle writing out buffers, the conditions for
    log forcing and the sleep times needed some small changes.  The most
    important one is that we consider an AIL busy as long we still have buffers
    to push, and the other one is that we do increment the pushed LSN for
    buffers that are under flushing at this moment, but still count them towards
    the stuck items for restart purposes.  Without this we could hammer on stuck
    items without ever forcing the log and not make progress under heavy random
    delete workloads on fast flash storage devices.
    
    [ Dave Chinner:
    	- rebase on previous patches.
    	- improved comments for XBF_DELWRI_Q handling
    	- fix XBF_ASYNC handling in queue submission (test 106 failure)
    	- rename delwri submit function buffer list parameters for clarity
    	- xfs_efd_item_push() should return XFS_ITEM_PINNED ]
    Signed-off-by: default avatarChristoph Hellwig <hch@lst.de>
    Reviewed-by: default avatarDave Chinner <dchinner@redhat.com>
    Reviewed-by: default avatarMark Tinguely <tinguely@sgi.com>
    Signed-off-by: default avatarBen Myers <bpm@sgi.com>
    43ff2122
xfs_trans.h 18.2 KB