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authorDave Chinner <david@fromorbit.com>2010-05-21 12:07:08 +1000
committerAlex Elder <aelder@sgi.com>2010-05-24 10:34:00 -0500
commited3b4d6cdc81e8feefdbfa3c584614be301b6d39 (patch)
tree5b8cd5735dfbc5eb834f96d25a8eb587186715be /fs/xfs/xfs_trans.h
parent955833cf2ad0aa39b336e853cad212d867199984 (diff)
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_trans.h')
-rw-r--r--fs/xfs/xfs_trans.h35
1 files changed, 2 insertions, 33 deletions
diff --git a/fs/xfs/xfs_trans.h b/fs/xfs/xfs_trans.h
index c62beee0921..ff7e9e6eee8 100644
--- a/fs/xfs/xfs_trans.h
+++ b/fs/xfs/xfs_trans.h
@@ -813,6 +813,7 @@ struct xfs_log_item_desc;
struct xfs_mount;
struct xfs_trans;
struct xfs_dquot_acct;
+struct xfs_busy_extent;
typedef struct xfs_log_item {
struct list_head li_ail; /* AIL pointers */
@@ -872,34 +873,6 @@ typedef struct xfs_item_ops {
#define XFS_ITEM_PUSHBUF 3
/*
- * This structure is used to maintain a list of block ranges that have been
- * freed in the transaction. The ranges are listed in the perag[] busy list
- * between when they're freed and the transaction is committed to disk.
- */
-
-typedef struct xfs_log_busy_slot {
- xfs_agnumber_t lbc_ag;
- ushort lbc_idx; /* index in perag.busy[] */
-} xfs_log_busy_slot_t;
-
-#define XFS_LBC_NUM_SLOTS 31
-typedef struct xfs_log_busy_chunk {
- struct xfs_log_busy_chunk *lbc_next;
- uint lbc_free; /* free slots bitmask */
- ushort lbc_unused; /* first unused */
- xfs_log_busy_slot_t lbc_busy[XFS_LBC_NUM_SLOTS];
-} xfs_log_busy_chunk_t;
-
-#define XFS_LBC_MAX_SLOT (XFS_LBC_NUM_SLOTS - 1)
-#define XFS_LBC_FREEMASK ((1U << XFS_LBC_NUM_SLOTS) - 1)
-
-#define XFS_LBC_INIT(cp) ((cp)->lbc_free = XFS_LBC_FREEMASK)
-#define XFS_LBC_CLAIM(cp, slot) ((cp)->lbc_free &= ~(1 << (slot)))
-#define XFS_LBC_SLOT(cp, slot) (&((cp)->lbc_busy[(slot)]))
-#define XFS_LBC_VACANCY(cp) (((cp)->lbc_free) & XFS_LBC_FREEMASK)
-#define XFS_LBC_ISFREE(cp, slot) ((cp)->lbc_free & (1 << (slot)))
-
-/*
* This is the type of function which can be given to xfs_trans_callback()
* to be called upon the transaction's commit to disk.
*/
@@ -950,8 +923,7 @@ typedef struct xfs_trans {
unsigned int t_items_free; /* log item descs free */
xfs_log_item_chunk_t t_items; /* first log item desc chunk */
xfs_trans_header_t t_header; /* header for in-log trans */
- unsigned int t_busy_free; /* busy descs free */
- xfs_log_busy_chunk_t t_busy; /* busy/async free blocks */
+ struct list_head t_busy; /* list of busy extents */
unsigned long t_pflags; /* saved process flags state */
} xfs_trans_t;
@@ -1025,9 +997,6 @@ int _xfs_trans_commit(xfs_trans_t *,
void xfs_trans_cancel(xfs_trans_t *, int);
int xfs_trans_ail_init(struct xfs_mount *);
void xfs_trans_ail_destroy(struct xfs_mount *);
-xfs_log_busy_slot_t *xfs_trans_add_busy(xfs_trans_t *tp,
- xfs_agnumber_t ag,
- xfs_extlen_t idx);
extern kmem_zone_t *xfs_trans_zone;