[PATCH] btrfs: lower metadata writeback threshold on low dirtythreshold

[Fengguang Wu]

This helps write performance when setting the dirty threshold to tiny numbers.

     3.4.0-rc2         3.4.0-rc2-btrfs4+
  ------------  ------------------------
         96.92        -0.4%        96.54  bay/thresh=1000M/btrfs-100dd-1-3.4.0-rc2
         98.47        +0.0%        98.50  bay/thresh=1000M/btrfs-10dd-1-3.4.0-rc2
         99.38        -0.3%        99.06  bay/thresh=1000M/btrfs-1dd-1-3.4.0-rc2
         98.04        -0.0%        98.02  bay/thresh=100M/btrfs-100dd-1-3.4.0-rc2
         98.68        +0.3%        98.98  bay/thresh=100M/btrfs-10dd-1-3.4.0-rc2
         99.34        -0.0%        99.31  bay/thresh=100M/btrfs-1dd-1-3.4.0-rc2
  ==>    88.98        +9.6%        97.53  bay/thresh=10M/btrfs-10dd-1-3.4.0-rc2
  ==>    86.99       +13.1%        98.39  bay/thresh=10M/btrfs-1dd-1-3.4.0-rc2
  ==>     2.75     +2442.4%        69.88  bay/thresh=1M/btrfs-10dd-1-3.4.0-rc2
  ==>     3.31     +2634.1%        90.54  bay/thresh=1M/btrfs-1dd-1-3.4.0-rc2

Signed-off-by: Fengguang Wu <fengguang.wu@xxxxxxxxx>
---
 fs/btrfs/disk-io.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- linux-next.orig/fs/btrfs/disk-io.c	2012-05-02 14:04:00.989262395 +0800
+++ linux-next/fs/btrfs/disk-io.c	2012-05-02 14:04:01.773262414 +0800
@@ -930,7 +930,8 @@ static int btree_writepages(struct addre
 
 		/* this is a bit racy, but that's ok */
 		num_dirty = root->fs_info->dirty_metadata_bytes;
-		if (num_dirty < thresh)
+		if (num_dirty < min(thresh,
+				    global_dirty_limit << (PAGE_CACHE_SHIFT-2)))
 			return 0;
 	}
 	return btree_write_cache_pages(mapping, wbc);
--
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