linux-ext4 - [PATCH] ext4: fix 50% disk write performance regression

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Date:	Mon, 30 Aug 2010 22:43:46 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Bill Fink <billfink@...dspring.com>
CC:	tytso@....edu, adilger@....com, linux-ext4@...r.kernel.org,
	bill.fink@...a.gov
Subject: [PATCH] ext4: fix 50% disk write performance regression

Can you give this a shot?

The first hunk is, I think, the biggest problem.  Even if
we get the max number of pages we need, we keep scanning forward
until "done" without doing any more actual, useful work.

The 2nd hunk is an oddity, some places assign nr_to_write
to LONG_MAX, and we get here and multiply -that- by 8... giving
us "-8" for nr_to_write, that can't help things when we
do later comparisons on that number...

I also see us asking to find pages starting at "idx" and
the first dirty page we find is well ahead of that,
I'm not sure if that's indicative of a problem or not.

Anyway, want to give this a shot, in place of the patch you sent,
and see how it fares compared to stock and/or with your patch?

It's build-and-sanity tested but not really performance tested here.

Thanks,
-Eric

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 4b8debe..33c2167 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -1207,8 +1207,10 @@ static pgoff_t ext4_num_dirty_pages(struct inode *inode, pgoff_t idx,
 				break;
 			idx++;
 			num++;
-			if (num >= max_pages)
-				break;
+			if (num >= max_pages) {
+				pagevec_release(&pvec);
+				return num;
+			}
 		}
 		pagevec_release(&pvec);
 	}
@@ -3002,7 +3004,7 @@ static int ext4_da_writepages(struct address_space *mapping,
 	 * sbi->max_writeback_mb_bump whichever is smaller.
 	 */
 	max_pages = sbi->s_max_writeback_mb_bump << (20 - PAGE_CACHE_SHIFT);
-	if (!range_cyclic && range_whole)
+	if (!range_cyclic && range_whole && wbc->nr_to_write != LONG_MAX)
 		desired_nr_to_write = wbc->nr_to_write * 8;
 	else
 		desired_nr_to_write = ext4_num_dirty_pages(inode, index,

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