• Chris Mason's avatar
    Btrfs: Add ordered async work queues · 4a69a410
    Chris Mason authored
    Btrfs uses kernel threads to create async work queues for cpu intensive
    operations such as checksumming and decompression.  These work well,
    but they make it difficult to keep IO order intact.
    
    A single writepages call from pdflush or fsync will turn into a number
    of bios, and each bio is checksummed in parallel.  Once the checksum is
    computed, the bio is sent down to the disk, and since we don't control
    the order in which the parallel operations happen, they might go down to
    the disk in almost any order.
    
    The code deals with this somewhat by having deep work queues for a single
    kernel thread, making it very likely that a single thread will process all
    the bios for a single inode.
    
    This patch introduces an explicitly ordered work queue.  As work structs
    are placed into the queue they are put onto the tail of a list.  They have
    three callbacks:
    
    ->func (cpu intensive processing here)
    ->ordered_func (order sensitive processing here)
    ->ordered_free (free the work struct, all processing is done)
    
    The work struct has three callbacks.  The func callback does the cpu intensive
    work, and when it completes the work struct is marked as done.
    
    Every time a work struct completes, the list is checked to see if the head
    is marked as done.  If so the ordered_func callback is used to do the
    order sensitive processing and the ordered_free callback is used to do
    any cleanup.  Then we loop back and check the head of the list again.
    
    This patch also changes the checksumming code to use the ordered workqueues.
    One a 4 drive array, it increases streaming writes from 280MB/s to 350MB/s.
    Signed-off-by: default avatarChris Mason <chris.mason@oracle.com>
    4a69a410
disk-io.h 3.84 KB