Commit 546a1924 authored Aug 24, 2010 by Dave Chinner Committed by Dave Chinner Aug 24, 2010

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

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.

Cc:<stable@kernel.org>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

parent 4536f2ad

mm/page-writeback.c

+10 −16

Original line number	Diff line number	Diff line
		@@ -985,24 +985,18 @@ 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.
		* 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);
		cond_resched();
		}

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