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

[Mel Gorman]

On Thu, Jul 22, 2010 at 04:52:10PM +0800, Wu Fengguang wrote:
> > 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().
> 

Lumpy reclaim is for high-order allocations. A simple dd should not be
triggering it regularly unless there was a lot of forking going on at the
same time. Also, how would a random or unrelated process get blocked on
writeback unless they were also doing high-order allocations?  What was the
source of the high-order allocations?

> 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);
>  		}
>  

I'm not seeing how this helps. It delays when lumpy reclaim waits on IO
to clean contiguous ranges of pages.

I'll read that full thread as I wasn't aware of it before.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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