• Chris Mason's avatar
    Btrfs: add a priority queue to the async thread helpers · d313d7a3
    Chris Mason authored
    Btrfs is using WRITE_SYNC_PLUG to send down synchronous IOs with a
    higher priority.  But, the checksumming helper threads prevent it
    from being fully effective.
    
    There are two problems.  First, a big queue of pending checksumming
    will delay the synchronous IO behind other lower priority writes.  Second,
    the checksumming uses an ordered async work queue.  The ordering makes sure
    that IOs are sent to the block layer in the same order they are sent
    to the checksumming threads.  Usually this gives us less seeky IO.
    
    But, when we start mixing IO priorities, the lower priority IO can delay
    the higher priority IO.
    
    This patch solves both problems by adding a high priority list to the async
    helper threads, and a new btrfs_set_work_high_prio(), which is used
    to make put a new async work item onto the higher priority list.
    
    The ordering is still done on high priority IO, but all of the high
    priority bios are ordered separately from the low priority bios.  This
    ordering is purely an IO optimization, it is not involved in data
    or metadata integrity.
    Signed-off-by: default avatarChris Mason <chris.mason@oracle.com>
    d313d7a3
async-thread.c 13 KB