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

Mel Gorman
2010-07-22T09:42:27Z

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