[PATCH 42/47] nfs: heuristics to avoid commit

[Wu Fengguang]

The heuristics introduced by commit 420e3646 ("NFS: Reduce the number of
unnecessary COMMIT calls") do not work well for large inodes being
actively written to.

Refine the criterion to
- it has gone quiet (all data transfered to server)
- has accumulated >= 4MB data to commit (so it will be large IO)
- too few active commits (hence active IO) in the server

Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 fs/nfs/write.c |   31 ++++++++++++++++++++++++++-----
 1 file changed, 26 insertions(+), 5 deletions(-)

--- linux-next.orig/fs/nfs/write.c	2010-12-09 12:19:24.000000000 +0800
+++ linux-next/fs/nfs/write.c	2010-12-09 12:20:20.000000000 +0800
@@ -1518,17 +1518,38 @@ out_mark_dirty:
 	return res;
 }
 
-static int nfs_commit_unstable_pages(struct inode *inode, struct writeback_control *wbc)
+static bool nfs_should_commit(struct inode *inode,
+			      struct writeback_control *wbc)
 {
+	struct nfs_server *nfss = NFS_SERVER(inode);
 	struct nfs_inode *nfsi = NFS_I(inode);
+	unsigned long npages = nfsi->npages;
+	unsigned long to_commit = nfsi->ncommit;
+	unsigned long in_commit = atomic_long_read(&nfss->in_commit);
+
+	/* no more active writes */
+	if (to_commit == npages)
+		return true;
+
+	/* big enough */
+	if (to_commit >= MIN_WRITEBACK_PAGES)
+		return true;
+
+	/* active commits drop low: kick more IO for the server disk */
+	if (to_commit > in_commit / 2)
+		return true;
+
+	return false;
+}
+
+static int nfs_commit_unstable_pages(struct inode *inode,
+				     struct writeback_control *wbc)
+{
 	int flags = FLUSH_SYNC;
 	int ret = 0;
 
 	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.
-		 */
-		if (nfsi->ncommit <= (nfsi->npages >> 1))
+		if (!nfs_should_commit(inode, wbc))
 			goto out_mark_dirty;
 
 		/* don't wait for the COMMIT response */


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