[PATCH 25/35] btrfs: lower the dirty balacing rate limit

[Wu Fengguang <fengguang.wu@intel.com>]

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Call balance_dirty_pages_ratelimit_nr() on every 16 pages dirtied.

Experiments show that larger intervals (in the original code) can
easily make the bdi dirty limit exceeded on 100 concurrent dd.

CC: Chris Mason <chris.mason@oracle.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 fs/btrfs/file.c |    5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

--- linux-next.orig/fs/btrfs/file.c	2010-12-13 21:46:19.000000000 +0800
+++ linux-next/fs/btrfs/file.c	2010-12-13 21:46:20.000000000 +0800
@@ -924,9 +924,8 @@ static ssize_t btrfs_file_aio_write(stru
 	}
 
 	iov_iter_init(&i, iov, nr_segs, count, num_written);
-	nrptrs = min((iov_iter_count(&i) + PAGE_CACHE_SIZE - 1) /
-		     PAGE_CACHE_SIZE, PAGE_CACHE_SIZE /
-		     (sizeof(struct page *)));
+	nrptrs = min(DIV_ROUND_UP(iov_iter_count(&i), PAGE_CACHE_SIZE),
+		     min(16UL, PAGE_CACHE_SIZE / (sizeof(struct page *))));
 	pages = kmalloc(nrptrs * sizeof(struct page *), GFP_KERNEL);
 
 	/* generic_write_checks can change our pos */


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