Re: [PATCH 7/8] writeback: sync old inodes first in background writeback

[Wu Fengguang <fengguang.wu@intel.com>]

> Some insight on how the other writeback changes that are being floated
> around might affect the number of dirty pages reclaim encounters would also
> be helpful.

Here is an interesting related problem about the wait_on_page_writeback() call
inside shrink_page_list():

        http://lkml.org/lkml/2010/4/4/86

The problem is, wait_on_page_writeback() is called too early in the
direct reclaim path, which blocks many random/unrelated processes when
some slow (USB stick) writeback is on the way.

A simple dd can easily create a big range of dirty pages in the LRU
list. Therefore priority can easily go below (DEF_PRIORITY - 2) in a
typical desktop, which triggers the lumpy reclaim mode and hence
wait_on_page_writeback().

I proposed this patch at the time, which was confirmed to solve the problem:

--- linux-next.orig/mm/vmscan.c	2010-06-24 14:32:03.000000000 +0800
+++ linux-next/mm/vmscan.c	2010-07-22 16:12:34.000000000 +0800
@@ -1650,7 +1650,7 @@ static void set_lumpy_reclaim_mode(int p
 	 */
 	if (sc->order > PAGE_ALLOC_COSTLY_ORDER)
 		sc->lumpy_reclaim_mode = 1;
-	else if (sc->order && priority < DEF_PRIORITY - 2)
+	else if (sc->order && priority < DEF_PRIORITY / 2)
 		sc->lumpy_reclaim_mode = 1;
 	else
 		sc->lumpy_reclaim_mode = 0;


However KOSAKI and Minchan raised concerns about raising the bar.
I guess this new patch is more problem oriented and acceptable:

--- linux-next.orig/mm/vmscan.c	2010-07-22 16:36:58.000000000 +0800
+++ linux-next/mm/vmscan.c	2010-07-22 16:39:57.000000000 +0800
@@ -1217,7 +1217,8 @@ static unsigned long shrink_inactive_lis
 			count_vm_events(PGDEACTIVATE, nr_active);
 
 			nr_freed += shrink_page_list(&page_list, sc,
-							PAGEOUT_IO_SYNC);
+					priority < DEF_PRIORITY / 3 ?
+					PAGEOUT_IO_SYNC : PAGEOUT_IO_ASYNC);
 		}
 
 		nr_reclaimed += nr_freed;

Thanks,
Fengguang