Commits
Jan Kara committed 9c12a831d73
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" <zheng.z.yan@intel.com> Signed-off-by: Jan Kara <jack@suse.cz> Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>