Re: [patch] reduce log I/O latency V2

Lachlan McIlroy
2008-01-22T03:39:32Z

Looks good Dave. Shouldn't we also tag the second buffer in the
split log write case?

David Chinner wrote:
> Reduce log I/O latency
>
> To ensure that log I/O is issued as the highest priority I/O, set
> the I/O priority of the log I/O to the highest possible. This will
> ensure that log I/O is not held up behind bulk data or other
> metadata I/O as delaying log I/O can pause the entire transaction
> subsystem. Introduce a new buffer flag to allow us to tag the log
> buffers so we can discriminate when issuing the I/O.
>
> Version 2:
> - don't use realtime priority class
>
> Signed-off-by: Dave Chinner <dgc@sgi.com>
> ---
> fs/xfs/linux-2.6/xfs_buf.c | 3 +++
> fs/xfs/linux-2.6/xfs_buf.h | 5 ++++-
> fs/xfs/xfs_log.c | 2 ++
> 3 files changed, 9 insertions(+), 1 deletion(-)
>
> Index: 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_buf.c
> ===================================================================
> --- 2.6.x-xfs-new.orig/fs/xfs/linux-2.6/xfs_buf.c 2007-12-19 01:39:42.286538135 +1100
> +++ 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_buf.c 2007-12-19 11:56:47.977477993 +1100
> @@ -1198,6 +1198,9 @@ next_chunk:
>
> submit_io:
> if (likely(bio->bi_size)) {
> + /* log I/O should not be delayed by anything except realtime. */
> + if (bp->b_flags & XBF_LOG_BUFFER)
> + bio_set_prio(bio, IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, 0));
> submit_bio(rw, bio);
> if (size)
> goto next_chunk;
> Index: 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_buf.h
> ===================================================================
> --- 2.6.x-xfs-new.orig/fs/xfs/linux-2.6/xfs_buf.h 2007-12-19 01:11:16.871181483 +1100
> +++ 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_buf.h 2007-12-19 11:55:09.642183801 +1100
> @@ -53,7 +53,8 @@ typedef enum {
> XBF_DELWRI = (1 << 6), /* buffer has dirty pages */
> XBF_STALE = (1 << 7), /* buffer has been staled, do not find it */
> XBF_FS_MANAGED = (1 << 8), /* filesystem controls freeing memory */
> - XBF_ORDERED = (1 << 11), /* use ordered writes */
> + XBF_LOG_BUFFER = (1 << 9), /* Buffer issued by the log */
> + XBF_ORDERED = (1 << 11), /* use ordered writes */
> XBF_READ_AHEAD = (1 << 12), /* asynchronous read-ahead */
>
> /* flags used only as arguments to access routines */
> @@ -339,6 +340,8 @@ extern void xfs_buf_trace(xfs_buf_t *, c
> #define XFS_BUF_TARGET(bp) ((bp)->b_target)
> #define XFS_BUFTARG_NAME(target) xfs_buf_target_name(target)
>
> +#define XFS_BUF_SET_LOGBUF(bp) ((bp)->b_flags |= XBF_LOG_BUFFER)
> +
> static inline int xfs_bawrite(void *mp, xfs_buf_t *bp)
> {
> bp->b_fspriv3 = mp;
> Index: 2.6.x-xfs-new/fs/xfs/xfs_log.c
> ===================================================================
> --- 2.6.x-xfs-new.orig/fs/xfs/xfs_log.c 2007-12-19 01:11:16.871181483 +1100
> +++ 2.6.x-xfs-new/fs/xfs/xfs_log.c 2007-12-19 11:55:09.698176566 +1100
> @@ -1443,6 +1443,8 @@ xlog_sync(xlog_t *log,
> XFS_BUF_ZEROFLAGS(bp);
> XFS_BUF_BUSY(bp);
> XFS_BUF_ASYNC(bp);
> + XFS_BUF_SET_LOGBUF(bp);
> +
> /*
> * Do an ordered write for the log block.
> * Its unnecessary to flush the first split block in the log wrap case.
>