• Dave Chinner's avatar
    xfs: Improve scalability of busy extent tracking · ed3b4d6c
    Dave Chinner authored
    When we free a metadata extent, we record it in the per-AG busy
    extent array so that it is not re-used before the freeing
    transaction hits the disk. This array is fixed size, so when it
    overflows we make further allocation transactions synchronous
    because we cannot track more freed extents until those transactions
    hit the disk and are completed. Under heavy mixed allocation and
    freeing workloads with large log buffers, we can overflow this array
    quite easily.
    
    Further, the array is sparsely populated, which means that inserts
    need to search for a free slot, and array searches often have to
    search many more slots that are actually used to check all the
    busy extents. Quite inefficient, really.
    
    To enable this aspect of extent freeing to scale better, we need
    a structure that can grow dynamically. While in other areas of
    XFS we have used radix trees, the extents being freed are at random
    locations on disk so are better suited to being indexed by an rbtree.
    
    So, use a per-AG rbtree indexed by block number to track busy
    extents.  This incures a memory allocation when marking an extent
    busy, but should not occur too often in low memory situations. This
    should scale to an arbitrary number of extents so should not be a
    limitation for features such as in-memory aggregation of
    transactions.
    
    However, there are still situations where we can't avoid allocating
    busy extents (such as allocation from the AGFL). To minimise the
    overhead of such occurences, we need to avoid doing a synchronous
    log force while holding the AGF locked to ensure that the previous
    transactions are safely on disk before we use the extent. We can do
    this by marking the transaction doing the allocation as synchronous
    rather issuing a log force.
    
    Because of the locking involved and the ordering of transactions,
    the synchronous transaction provides the same guarantees as a
    synchronous log force because it ensures that all the prior
    transactions are already on disk when the synchronous transaction
    hits the disk. i.e. it preserves the free->allocate order of the
    extent correctly in recovery.
    
    By doing this, we avoid holding the AGF locked while log writes are
    in progress, hence reducing the length of time the lock is held and
    therefore we increase the rate at which we can allocate and free
    from the allocation group, thereby increasing overall throughput.
    
    The only problem with this approach is that when a metadata buffer is
    marked stale (e.g. a directory block is removed), then buffer remains
    pinned and locked until the log goes to disk. The issue here is that
    if that stale buffer is reallocated in a subsequent transaction, the
    attempt to lock that buffer in the transaction will hang waiting
    the log to go to disk to unlock and unpin the buffer. Hence if
    someone tries to lock a pinned, stale, locked buffer we need to
    push on the log to get it unlocked ASAP. Effectively we are trading
    off a guaranteed log force for a much less common trigger for log
    force to occur.
    
    Ideally we should not reallocate busy extents. That is a much more
    complex fix to the problem as it involves direct intervention in the
    allocation btree searches in many places. This is left to a future
    set of modifications.
    
    Finally, now that we track busy extents in allocated memory, we
    don't need the descriptors in the transaction structure to point to
    them. We can replace the complex busy chunk infrastructure with a
    simple linked list of busy extents. This allows us to remove a large
    chunk of code, making the overall change a net reduction in code
    size.
    Signed-off-by: default avatarDave Chinner <david@fromorbit.com>
    Reviewed-by: default avatarChristoph Hellwig <hch@lst.de>
    Signed-off-by: default avatarAlex Elder <aelder@sgi.com>
    ed3b4d6c
xfs_ag.h 9.83 KB