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

[Wu Fengguang]

> 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;

This one looks better:
---
vmscan: raise the bar to PAGEOUT_IO_SYNC stalls

Fix "system goes totally unresponsive with many dirty/writeback pages"
problem:

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

The root cause 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().

In Andreas' case, 512MB/1024 = 512KB, this is way too low comparing to
the 22MB writeback and 190MB dirty pages. There can easily be a
continuous range of 512KB dirty/writeback pages in the LRU, which will
trigger the wait logic.

To make it worse, when there are 50MB writeback pages and USB 1.1 is
writing them in 1MB/s, wait_on_page_writeback() may stuck for up to 50
seconds.

So only enter sync write&wait when priority goes below DEF_PRIORITY/3,
or 6.25% LRU. As the default dirty throttle ratio is 20%, sync write&wait
will hardly be triggered by pure dirty pages.

Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 mm/vmscan.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- linux-next.orig/mm/vmscan.c	2010-07-22 16:36:58.000000000 +0800
+++ linux-next/mm/vmscan.c	2010-07-22 17:03:47.000000000 +0800
@@ -1206,7 +1206,7 @@ static unsigned long shrink_inactive_lis
 		 * but that should be acceptable to the caller
 		 */
 		if (nr_freed < nr_taken && !current_is_kswapd() &&
-		    sc->lumpy_reclaim_mode) {
+		    sc->lumpy_reclaim_mode && priority < DEF_PRIORITY / 3) {
 			congestion_wait(BLK_RW_ASYNC, HZ/10);
 
 			/*
--
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