• Chris Mason's avatar
    Btrfs: unplug in the async bio submission threads · bedf762b
    Chris Mason authored
    
    
    Btrfs pages being written get set to writeback, and then may go through
    a number of steps before they hit the block layer.  This includes compression,
    checksumming and async bio submission.
    
    The end result is that someone who writes a page and then does
    wait_on_page_writeback is likely to unplug the queue before the bio they
    cared about got there.
    
    We could fix this by marking bios sync, or by doing more frequent unplugs,
    but this commit just changes the async bio submission code to unplug
    after it has processed all the bios for a device.  The async bio submission
    does a fair job of collection bios, so this shouldn't be a huge problem
    for reducing merging at the elevator.
    
    For streaming O_DIRECT writes on a 5 drive array, it boosts performance
    from 386MB/s to 460MB/s.
    
    Thanks to Hisashi Hifumi for helping with this work.
    Signed-off-by: default avatarChris Mason <chris.mason@oracle.com>
    bedf762b