From 546a1924224078c6f582e68f890b05b387b42653 Mon Sep 17 00:00:00 2001 From: Dave Chinner Date: Tue, 24 Aug 2010 11:44:34 +1000 Subject: [PATCH] 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: Signed-off-by: Dave Chinner Reviewed-by: Wu Fengguang Reviewed-by: Christoph Hellwig --- mm/page-writeback.c | 26 ++++++++++---------------- 1 file changed, 10 insertions(+), 16 deletions(-) diff --git a/mm/page-writeback.c b/mm/page-writeback.c index c09ef5219cbe..a803f5e33471 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -985,22 +985,16 @@ 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); -- 2.20.1