[PATCH] writeback: write_cache_pages doesn't terminate at nr_to_write <= 0

Dave Chinner
2010-08-23T04:43:22Z

From: Dave Chinner <dchinner@redhat.com>

I noticed XFS writeback in 2.6.36-rc1 was much slower than it should have
been. Enabling writeback tracing showed:

flush-253:16-8516 [007] 1342952.351608: wbc_writepage: bdi 253:16: towrt=1024 skip=0 mode=0 kupd=0 bgrd=1 reclm=0 cyclic=1 more=0 older=0x0 start=0x0 end=0x0
flush-253:16-8516 [007] 1342952.351654: wbc_writepage: bdi 253:16: towrt=1023 skip=0 mode=0 kupd=0 bgrd=1 reclm=0 cyclic=1 more=0 older=0x0 start=0x0 end=0x0
flush-253:16-8516 [000] 1342952.369520: wbc_writepage: bdi 253:16: towrt=0 skip=0 mode=0 kupd=0 bgrd=1 reclm=0 cyclic=1 more=0 older=0x0 start=0x0 end=0x0
flush-253:16-8516 [000] 1342952.369542: wbc_writepage: bdi 253:16: towrt=-1 skip=0 mode=0 kupd=0 bgrd=1 reclm=0 cyclic=1 more=0 older=0x0 start=0x0 end=0x0
flush-253:16-8516 [000] 1342952.369549: wbc_writepage: bdi 253:16: towrt=-2 skip=0 mode=0 kupd=0 bgrd=1 reclm=0 cyclic=1 more=0 older=0x0 start=0x0 end=0x0

Writeback is not terminating in background writeback if ->writepage is
returning with wbc->nr_to_write == 0, resulting in sub-optimal single page
writeback on XFS.

Fix the write_cache_pages loop to terminate correctly when this situation
occurs and so prevent this sub-optimal background writeback pattern. This
improves sustained sequential buffered write performance from around
250MB/s to 750MB/s for a 100GB file on an XFS filesystem on my 8p test VM.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
mm/page-writeback.c | 26 ++++++++++----------------
1 files changed, 10 insertions(+), 16 deletions(-)

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 7262aac..f069782 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -984,22 +984,16 @@ continue_unlock:
}
}

- if (wbc->nr_to_write > 0) {
- if (--wbc->nr_to_write == 0 &&
- wbc->sync_mode == WB_SYNC_NONE) {
- /*
- * We stop writing back only if we are
- * not doing integrity sync. In case of
- * integrity sync we have to keep going
- * because someone may be concurrently
- * dirtying pages, and we might have
- * synced a lot of newly appeared dirty
- * pages, but have not synced all of the
- * old dirty pages.
- */
- done = 1;
- break;
- }
+ /*
+ * We stop writing back only if we are not doing
+ * integrity sync. In case of integrity sync we have to
+ * keep going until we have written all the pages
+ * we tagged for writeback prior to entering this loop.
+ */
+ if (--wbc->nr_to_write <= 0 &&
+ wbc->sync_mode == WB_SYNC_NONE) {
+ done = 1;
+ break;
}
}
pagevec_release(&pvec);
--
1.7.1