A background flush work may run for ever. So it's reasonable for it to mimic the kupdate behavior of syncing old/expired inodes first. At each queue_io() time, first try enqueuing only newly expired inodes. If there are zero expired inodes to work with, then relax the rule and enqueue all dirty inodes. This will help reduce the number of dirty pages encountered by page reclaim, eg. the pageout() calls. Normally older inodes contain older dirty pages, which are more close to the end of the LRU lists. So syncing older inodes first helps reducing the dirty pages reached by the page reclaim code. More background: as Mel put it, "it makes sense to write old pages first to reduce the chances page reclaim is initiating IO." Rik also presented the situation with a graph: LRU head [*] dirty page [ * * * * * * * * * * *] Ideally, most dirty pages should lie close to the LRU tail instead of LRU head. That requires the flusher thread to sync old/expired inodes first (as there are obvious correlations between inode age and page age), and to give fair opportunities to newly expired inodes rather than sticking with some large eldest inodes (as larger inodes have weaker correlations in the inode<=>page ages). This patch helps the flusher to meet both the above requirements. Side effects: it might reduce the batch size and hence reduce inode_wb_list_lock hold time, but in turn make the cluster-by-partition logic in the same function less effective on reducing disk seeks. v2: keep policy changes inside wb_writeback() and keep the wbc.older_than_this visibility as suggested by Dave. CC: Dave Chinner CC: Jan Kara CC: Rik van Riel Acked-by: Mel Gorman Signed-off-by: Wu Fengguang --- fs/fs-writeback.c | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-) --- linux-next.orig/fs/fs-writeback.c 2011-05-05 23:30:25.000000000 +0800 +++ linux-next/fs/fs-writeback.c 2011-05-05 23:30:26.000000000 +0800 @@ -718,7 +718,7 @@ static long wb_writeback(struct bdi_writ if (work->for_background && !over_bground_thresh()) break; - if (work->for_kupdate) { + if (work->for_kupdate || work->for_background) { oldest_jif = jiffies - msecs_to_jiffies(dirty_expire_interval * 10); wbc.older_than_this = &oldest_jif; @@ -729,6 +729,7 @@ static long wb_writeback(struct bdi_writ wbc.pages_skipped = 0; wbc.inodes_cleaned = 0; +retry: trace_wbc_writeback_start(&wbc, wb->bdi); if (work->sb) __writeback_inodes_sb(work->sb, wb, &wbc); @@ -752,6 +753,19 @@ static long wb_writeback(struct bdi_writ if (wbc.inodes_cleaned) continue; /* + * background writeback will start with expired inodes, and + * if none is found, fallback to all inodes. This order helps + * reduce the number of dirty pages reaching the end of LRU + * lists and cause trouble to the page reclaim. + */ + if (work->for_background && + wbc.older_than_this && + list_empty(&wb->b_io) && + list_empty(&wb->b_more_io)) { + wbc.older_than_this = NULL; + goto retry; + } + /* * No more inodes for IO, bail */ if (!wbc.more_io) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/