From 9c12a831d73dd938a22418d70b39aed4feb4bdf2 Mon Sep 17 00:00:00 2001 From: Jan Kara Date: Mon, 16 Sep 2013 08:24:26 -0400 Subject: ext4: fix performance regression in writeback of random writes The Linux Kernel Performance project guys have reported that commit 4e7ea81db5 introduces a performance regression for the following fio workload: [global] direct=0 ioengine=mmap size=1500M bs=4k pre_read=1 numjobs=1 overwrite=1 loops=5 runtime=300 group_reporting invalidate=0 directory=/mnt/ file_service_type=random:36 file_service_type=random:36 [job0] startdelay=0 rw=randrw filename=data0/f1:data0/f2 [job1] startdelay=0 rw=randrw filename=data0/f2:data0/f1 ... [job7] startdelay=0 rw=randrw filename=data0/f2:data0/f1 The culprit of the problem is that after the commit ext4_writepages() are more aggressive in writing back pages. Thus we have less consecutive dirty pages resulting in more seeking. This increased aggressivity is caused by a bug in the condition terminating ext4_writepages(). We start writing from the beginning of the file even if we should have terminated ext4_writepages() because wbc->nr_to_write <= 0. After fixing the condition the throughput of the fio workload is about 20% better than before writeback reorganization. Reported-by: "Yan, Zheng" Signed-off-by: Jan Kara Signed-off-by: "Theodore Ts'o" --- fs/ext4/inode.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c index 9115f2807515..4cf2619f007c 100644 --- a/fs/ext4/inode.c +++ b/fs/ext4/inode.c @@ -2559,7 +2559,7 @@ retry: break; } blk_finish_plug(&plug); - if (!ret && !cycled) { + if (!ret && !cycled && wbc->nr_to_write > 0) { cycled = 1; mpd.last_page = writeback_index - 1; mpd.first_page = 0; -- cgit v1.2.2