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

David Chinner
2008-01-22T03:58:12Z

On Tue, Jan 22, 2008 at 02:42:13PM +1100, Lachlan McIlroy wrote:
> Looks good Dave. Shouldn't we also tag the second buffer in the
> split log write case?

Hmm - I thought it was being tagged. looks like I missed a reject
on my last set of patch re-orderings - updated patch attached.

---

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.

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 | 3 +++
3 files changed, 10 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 2008-01-18 19:13:07.499131222 +1100
+++ 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_buf.c 2008-01-22 14:56:58.738098626 +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 2008-01-18 19:13:07.503130710 +1100
+++ 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_buf.h 2008-01-18 19:13:29.448320166 +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 2008-01-18 19:13:07.519128661 +1100
+++ 2.6.x-xfs-new/fs/xfs/xfs_log.c 2008-01-22 14:56:57.550673091 +1100
@@ -1462,6 +1462,8 @@ 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.
@@ -1499,6 +1501,7 @@ xlog_sync(xlog_t *log,
XFS_BUF_ZEROFLAGS(bp);
XFS_BUF_BUSY(bp);
XFS_BUF_ASYNC(bp);
+ XFS_BUF_SET_LOGBUF(bp);
if (log->l_mp->m_flags & XFS_MOUNT_BARRIER)
XFS_BUF_ORDERED(bp);
dptr = XFS_BUF_PTR(bp);