Enhance reassignbuf(). When a buffer cannot be time-optimally inserted

    into vnode dirtyblkhd we append it to the list instead of prepend it to
    the list in order to maintain a 'forward' locality of reference, which
    is arguably better then 'reverse'.  The original algorithm did things this
    way to but at a huge time cost.

    Enhance the append interlock for NFS writes to handle intr/soft mounts
    better.

    Fix the hysteresis for NFS async daemon I/O requests to reduce the
    number of unnecessary context switches.

    Modify handling of NFS mount options.  Any given user option that is
    too high now defaults to the kernel maximum for that option rather then
    the kernel default for that option.

Reviewed by:	 Alfred Perlstein <bright@wintelcom.net>

1 files changed, 19 insertions, 2 deletions

diff --git a/sys/kern/vfs_export.c b/sys/kern/vfs_export.c
index 76bd5843c369..059ca2a4787a 100644
--- a/sys/kern/vfs_export.c
+++ b/sys/kern/vfs_export.c
@@ -1208,6 +1208,7 @@ reassignbuf(bp, newvp)
 		tbp = TAILQ_FIRST(listheadp);
 		if (tbp == NULL ||
 		    bp->b_lblkno == 0 ||
+		    (bp->b_lblkno > 0 && tbp->b_lblkno < 0) ||
 		    (bp->b_lblkno > 0 && bp->b_lblkno < tbp->b_lblkno)) {
 			TAILQ_INSERT_HEAD(listheadp, bp, b_vnbufs);
 			++reassignbufsortgood;
@@ -1217,14 +1218,30 @@ reassignbuf(bp, newvp)
 		} else if (reassignbufmethod == 1) {
 			/*
 			 * New sorting algorithm, only handle sequential case,
-			 * otherwise guess.
+			 * otherwise append to end (but before metadata)
 			 */
 			if ((tbp = gbincore(newvp, bp->b_lblkno - 1)) != NULL &&
 			    (tbp->b_xflags & BX_VNDIRTY)) {
+				/*
+				 * Found the best place to insert the buffer
+				 */
 				TAILQ_INSERT_AFTER(listheadp, tbp, bp, b_vnbufs);
 				++reassignbufsortgood;
 			} else {
-				TAILQ_INSERT_HEAD(listheadp, bp, b_vnbufs);
+				/*
+				 * Missed, append to end, but before meta-data.
+				 * We know that the head buffer in the list is
+				 * not meta-data due to prior conditionals.
+				 *
+				 * Indirect effects:  NFS second stage write
+				 * tends to wind up here, giving maximum 
+				 * distance between the unstable write and the
+				 * commit rpc.
+				 */
+				tbp = TAILQ_LAST(listheadp, buflists);
+				while (tbp && tbp->b_lblkno < 0)
+					tbp = TAILQ_PREV(tbp, buflists, b_vnbufs);
+				TAILQ_INSERT_AFTER(listheadp, tbp, bp, b_vnbufs);
 				++reassignbufsortbad;
 			}
 		} else {