When background_writeout() (mm/page-writeback.c) finds any pages_skipped in writeback_inodes() and it didn't meet any congestion, it exits even when it hasn't written enough pages yet.
Performing 2 ( or more) concurrent copies of a large file, often creates lots of skipped pages (1000+) making background_writeout exit and so pages don't get written out until we reach dirty_ratio. I added some instrumentation to fs/buffer.c in __block_write_full_page(..) and all the skipped pages come from here :- done: if (nr_underway == 0) { /* * The page was marked dirty, but the buffers were * clean. Someone wrote them back by hand with * ll_rw_block/submit_bh. A rare case. */ end_page_writeback(page); /* * The page and buffer_heads can be released at any time from * here on. */ wbc->pages_skipped++; /* We didn't write this page */ maybe not such a rare case! :) I've been testing 2.6.23 on an AMD64x2. Here's a quick patch for background_writeout to ignore pages_skipped. It helps keep nr_dirty between dirty_background_ratio & dirty_ratio, and once the copies have finish nr_dirty quickly drops back to dirty_background_ratio. Without the patch during the copy nr_dirty stays around dirty_ratio and takes a long time to drop after it finishes. It seems that this patch tackles the problem, but is there a better way to fix it? And is there a good reason to abandon this writeout loop if a page gets skipped for any other reason? thanks richard diff --git a/mm/page-writeback.c b/mm/page-writeback.c index 4472036..5a6747b 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -371,6 +371,7 @@ static void background_writeout(unsigned long _min_pages) .nr_to_write = 0, .nonblocking = 1, .range_cyclic = 1, + .encountered_congestion = 0, }; for ( ; ; ) { @@ -382,17 +383,16 @@ static void background_writeout(unsigned long _min_pages) global_page_state(NR_UNSTABLE_NFS) < background_thresh && min_pages <= 0) break; - wbc.encountered_congestion = 0; + if (wbc.encountered_congestion) { + congestion_wait(WRITE, HZ/10); + wbc.encountered_congestion = 0; + } wbc.nr_to_write = MAX_WRITEBACK_PAGES; wbc.pages_skipped = 0; writeback_inodes(&wbc); min_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write; - if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) { - /* Wrote less than expected */ - congestion_wait(WRITE, HZ/10); - if (!wbc.encountered_congestion) - break; - } + if (wbc.nr_to_write > 0 && !wbc.encountered_congestion) + break; } } - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/