Limit the commit size to 1/8 dirty control scope, so that the arrival of
one commit will not knock the overall dirty pages off the scope.

CC: Trond Myklebust <redacted>
Signed-off-by: Wu Fengguang <redacted>
---
 fs/nfs/write.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

After patch, there are still drop offs from the control scope,

http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/NFS/nfs-1dd-1M-8p-2945M-20%25-2.6.38-rc6-dt6+-2011-02-22-21-09/balance_dirty_pages-pages.png

due to bursty arrival of commits:

http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/NFS/nfs-1dd-1M-8p-2945M-20%25-2.6.38-rc6-dt6+-2011-02-22-21-09/nfs-commit.png

--- linux-next.orig/fs/nfs/write.c	2011-03-02 20:39:01.000000000 +0800
+++ linux-next/fs/nfs/write.c	2011-03-02 20:39:01.000000000 +0800

@@ -1492,9 +1492,10 @@ static int nfs_commit_unstable_pages(str
 
 	if (wbc->sync_mode == WB_SYNC_NONE) {
 		/* Don't commit yet if this is a non-blocking flush and there
-		 * are a lot of outstanding writes for this mapping.
+		 * are a lot of outstanding writes for this mapping, until
+		 * collected enough pages to commit.
 		 */
-		if (nfsi->ncommit <= (nfsi->npages >> 1))
+		if (nfsi->ncommit <= nfsi->npages / 32 /* DIRTY_MARGIN */)
 			goto out_mark_dirty;
 
 		/* don't wait for the COMMIT response */


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