Re: [PATCH 1/4] vmscan: delegate pageout io to flusher thread if current is kswapd

[Suleiman Souhlal <ssouhlal@freebsd.org>]

On Apr 14, 2010, at 9:11 PM, KOSAKI Motohiro wrote:

> Now, vmscan pageout() is one of IO throuput degression source.
> Some IO workload makes very much order-0 allocation and reclaim
> and pageout's 4K IOs are making annoying lots seeks.
>
> At least, kswapd can avoid such pageout() because kswapd don't
> need to consider OOM-Killer situation. that's no risk.
>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

What's your opinion on trying to cluster the writes done by pageout,  
instead of not doing any paging out in kswapd?
Something along these lines:

     Cluster writes to disk due to memory pressure.

     Write out logically adjacent pages to the one we're paging out
     so that we may get better IOs in these situations:
     These pages are likely to be contiguous on disk to the one we're
     writing out, so they should get merged into a single disk IO.

     Signed-off-by: Suleiman Souhlal <suleiman@google.com>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index c26986c..4e5a613 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -48,6 +48,8 @@

  #include "internal.h"

+#define PAGEOUT_CLUSTER_PAGES	16
+
  struct scan_control {
  	/* Incremented by the number of inactive pages that were scanned */
  	unsigned long nr_scanned;
@@ -350,6 +352,8 @@ typedef enum {
  static pageout_t pageout(struct page *page, struct address_space  
*mapping,
  						enum pageout_io sync_writeback)
  {
+	int i;
+
  	/*
  	 * If the page is dirty, only perform writeback if that write
  	 * will be non-blocking.  To prevent this allocation from being
@@ -408,6 +412,37 @@ static pageout_t pageout(struct page *page,  
struct address_space *mapping,
  		}

  		/*
+		 * Try to write out logically adjacent dirty pages too, if
+		 * possible, to get better IOs, as the IO scheduler should
+		 * merge them with the original one, if the file is not too
+		 * fragmented.
+		 */
+		for (i = 1; i < PAGEOUT_CLUSTER_PAGES; i++) {
+			struct page *p2;
+			int err;
+
+			p2 = find_get_page(mapping, page->index + i);
+			if (p2) {
+				if (trylock_page(p2) == 0) {
+					page_cache_release(p2);
+					break;
+				}
+				if (page_mapped(p2))
+					try_to_unmap(p2, 0);
+				if (PageDirty(p2)) {
+					err = write_one_page(p2, 0);
+					page_cache_release(p2);
+					if (err)
+						break;
+				} else {
+					unlock_page(p2);
+					page_cache_release(p2);
+					break;
+				}
+			}
+		}
+
+		/*
  		 * Wait on writeback if requested to. This happens when
  		 * direct reclaiming a large contiguous area and the
  		 * first attempt to free a range of pages fails.

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