• David Chinner's avatar
    [XFS] Move AIL pushing into it's own thread · 249a8c11
    David Chinner authored
    When many hundreds to thousands of threads all try to do simultaneous
    transactions and the log is in a tail-pushing situation (i.e. full), we
    can get multiple threads walking the AIL list and contending on the AIL
    The AIL push is, in effect, a simple I/O dispatch algorithm complicated by
    the ordering constraints placed on it by the transaction subsystem. It
    really does not need multiple threads to push on it - even when only a
    single CPU is pushing the AIL, it can push the I/O out far faster that
    pretty much any disk subsystem can handle.
    So, to avoid contention problems stemming from multiple list walkers, move
    the list walk off into another thread and simply provide a "target" to
    push to. When a thread requires a push, it sets the target and wakes the
    push thread, then goes to sleep waiting for the required amount of space
    to become available in the log.
    This mechanism should also be a lot fairer under heavy load as the waiters
    will queue in arrival order, rather than queuing in "who completed a push
    first" order.
    Also, by moving the pushing to a separate thread we can do more
    effectively overload detection and prevention as we can keep context from
    loop iteration to loop iteration. That is, we can push only part of the
    list each loop and not have to loop back to the start of the list every
    time we run. This should also help by reducing the number of items we try
    to lock and/or push items that we cannot move.
    Note that this patch is not intended to solve the inefficiencies in the
    AIL structure and the associated issues with extremely large list
    contents. That needs to be addresses separately; parallel access would
    cause problems to any new structure as well, so I'm only aiming to isolate
    the structure from unbounded parallelism here.
    SGI-PV: 972759
    SGI-Modid: xfs-linux-melb:xfs-kern:30371a
    Signed-off-by: default avatarDavid Chinner <dgc@sgi.com>
    Signed-off-by: default avatarLachlan McIlroy <lachlan@sgi.com>
xfs_trans_priv.h 2.31 KB