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authorDave Chinner <david@fromorbit.com>2010-05-21 06:07:08 +0400
committerAlex Elder <aelder@sgi.com>2010-05-24 19:34:00 +0400
commited3b4d6cdc81e8feefdbfa3c584614be301b6d39 (patch)
tree5b8cd5735dfbc5eb834f96d25a8eb587186715be /fs/xfs/xfs_ag.h
parent955833cf2ad0aa39b336e853cad212d867199984 (diff)
downloadlinux-ed3b4d6cdc81e8feefdbfa3c584614be301b6d39.tar.xz
xfs: Improve scalability of busy extent tracking
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: Dave Chinner <david@fromorbit.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Alex Elder <aelder@sgi.com>
Diffstat (limited to 'fs/xfs/xfs_ag.h')
-rw-r--r--fs/xfs/xfs_ag.h24
1 files changed, 15 insertions, 9 deletions
diff --git a/fs/xfs/xfs_ag.h b/fs/xfs/xfs_ag.h
index abb8222b88c9..401f364ad36c 100644
--- a/fs/xfs/xfs_ag.h
+++ b/fs/xfs/xfs_ag.h
@@ -175,14 +175,20 @@ typedef struct xfs_agfl {
} xfs_agfl_t;
/*
- * Busy block/extent entry. Used in perag to mark blocks that have been freed
- * but whose transactions aren't committed to disk yet.
+ * Busy block/extent entry. Indexed by a rbtree in perag to mark blocks that
+ * have been freed but whose transactions aren't committed to disk yet.
+ *
+ * Note that we use the transaction ID to record the transaction, not the
+ * transaction structure itself. See xfs_alloc_busy_insert() for details.
*/
-typedef struct xfs_perag_busy {
- xfs_agblock_t busy_start;
- xfs_extlen_t busy_length;
- struct xfs_trans *busy_tp; /* transaction that did the free */
-} xfs_perag_busy_t;
+struct xfs_busy_extent {
+ struct rb_node rb_node; /* ag by-bno indexed search tree */
+ struct list_head list; /* transaction busy extent list */
+ xfs_agnumber_t agno;
+ xfs_agblock_t bno;
+ xfs_extlen_t length;
+ xlog_tid_t tid; /* transaction that created this */
+};
/*
* Per-ag incore structure, copies of information in agf and agi,
@@ -216,7 +222,8 @@ typedef struct xfs_perag {
xfs_agino_t pagl_leftrec;
xfs_agino_t pagl_rightrec;
#ifdef __KERNEL__
- spinlock_t pagb_lock; /* lock for pagb_list */
+ spinlock_t pagb_lock; /* lock for pagb_tree */
+ struct rb_root pagb_tree; /* ordered tree of busy extents */
atomic_t pagf_fstrms; /* # of filestreams active in this AG */
@@ -226,7 +233,6 @@ typedef struct xfs_perag {
int pag_ici_reclaimable; /* reclaimable inodes */
#endif
int pagb_count; /* pagb slots in use */
- xfs_perag_busy_t pagb_list[XFS_PAGB_NUM_SLOTS]; /* unstable blocks */
} xfs_perag_t;
/*