Re: [PATCH v9 12/13] memcg: create support routines for page writeback

[Jan Kara <jack@suse.cz>]

On Thu 18-08-11 10:36:10, Wu Fengguang wrote:
> Subject: squeeze max-pause area and drop pass-good area
> Date: Tue Aug 16 13:37:14 CST 2011
> 
> Remove the pass-good area introduced in ffd1f609ab10 ("writeback:
> introduce max-pause and pass-good dirty limits") and make the
> max-pause area smaller and safe.
> 
> This fixes ~30% performance regression in the ext3 data=writeback
> fio_mmap_randwrite_64k/fio_mmap_randrw_64k test cases, where there are
> 12 JBOD disks, on each disk runs 8 concurrent tasks doing reads+writes.
> 
> Using deadline scheduler also has a regression, but not that big as
> CFQ, so this suggests we have some write starvation.
> 
> The test logs show that
> 
> - the disks are sometimes under utilized
> 
> - global dirty pages sometimes rush high to the pass-good area for
>   several hundred seconds, while in the mean time some bdi dirty pages
>   drop to very low value (bdi_dirty << bdi_thresh).
>   Then suddenly the global dirty pages dropped under global dirty
>   threshold and bdi_dirty rush very high (for example, 2 times higher
>   than bdi_thresh). During which time balance_dirty_pages() is not
>   called at all.
> 
> So the problems are
> 
> 1) The random writes progress so slow that they break the assumption of
> the max-pause logic that "8 pages per 200ms is typically more than
> enough to curb heavy dirtiers".
> 
> 2) The max-pause logic ignored task_bdi_thresh and thus opens the
>    possibility for some bdi's to over dirty pages, leading to
>    (bdi_dirty >> bdi_thresh) and then (bdi_thresh >> bdi_dirty) for others.
> 
> 3) The higher max-pause/pass-good thresholds somehow leads to some bad
>    swing of dirty pages.
> 
> The fix is to allow the task to slightly dirty over task_bdi_thresh, but
> no way to exceed bdi_dirty and/or global dirty_thresh.
> 
> Tests show that it fixed the JBOD regression completely (both behavior
> and performance), while still being able to cut down large pause times
> in balance_dirty_pages() for single-disk cases.
> 
> Reported-by: Li Shaohua <shaohua.li@intel.com>
> Tested-by: Li Shaohua <shaohua.li@intel.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
>  include/linux/writeback.h |   11 -----------
>  mm/page-writeback.c       |   15 ++-------------
>  2 files changed, 2 insertions(+), 24 deletions(-)
> 
> --- linux.orig/mm/page-writeback.c	2011-08-18 09:52:59.000000000 +0800
> +++ linux/mm/page-writeback.c	2011-08-18 10:28:57.000000000 +0800
> @@ -786,21 +786,10 @@ static void balance_dirty_pages(struct a
>  		 * 200ms is typically more than enough to curb heavy dirtiers;
>  		 * (b) the pause time limit makes the dirtiers more responsive.
>  		 */
> -		if (nr_dirty < dirty_thresh +
> -			       dirty_thresh / DIRTY_MAXPAUSE_AREA &&
> +		if (nr_dirty < dirty_thresh &&
> +		    bdi_dirty < (task_bdi_thresh + bdi_thresh) / 2 &&
>  		    time_after(jiffies, start_time + MAX_PAUSE))
>  			break;
  This looks definitely much safer than the original patch since we now
always observe global dirty limit. I just wonder: We have throttled the
task because bdi_nr_reclaimable > task_bdi_thresh. Now in practice there
should be some pages under writeback and this task should have submitted
even more just a while ago. So the condition
  bdi_dirty < (task_bdi_thresh + bdi_thresh) / 2
looks still relatively weak. Shouldn't there be
  bdi_nr_reclaimable < (task_bdi_thresh + bdi_thresh) / 2?
Since bdi_nr_reclaimable is really the number we want to limit...
Alternatively, I could see also a reason for
  bdi_dirty < task_bdi_thresh
which leaves the task pages under writeback as the pausing area. But since
these are not really well limited, I'd prefer my first suggestion.

								Honza
> -		/*
> -		 * pass-good area. When some bdi gets blocked (eg. NFS server
> -		 * not responding), or write bandwidth dropped dramatically due
> -		 * to concurrent reads, or dirty threshold suddenly dropped and
> -		 * the dirty pages cannot be brought down anytime soon (eg. on
> -		 * slow USB stick), at least let go of the good bdi's.
> -		 */
> -		if (nr_dirty < dirty_thresh +
> -			       dirty_thresh / DIRTY_PASSGOOD_AREA &&
> -		    bdi_dirty < bdi_thresh)
> -			break;
>  
>  		/*
>  		 * Increase the delay for each loop, up to our previous
> --- linux.orig/include/linux/writeback.h	2011-08-16 23:34:27.000000000 +0800
> +++ linux/include/linux/writeback.h	2011-08-18 09:53:03.000000000 +0800
> @@ -12,15 +12,6 @@
>   *
>   *	(thresh - thresh/DIRTY_FULL_SCOPE, thresh)
>   *
> - * The 1/16 region above the global dirty limit will be put to maximum pauses:
> - *
> - *	(limit, limit + limit/DIRTY_MAXPAUSE_AREA)
> - *
> - * The 1/16 region above the max-pause region, dirty exceeded bdi's will be put
> - * to loops:
> - *
> - *	(limit + limit/DIRTY_MAXPAUSE_AREA, limit + limit/DIRTY_PASSGOOD_AREA)
> - *
>   * Further beyond, all dirtier tasks will enter a loop waiting (possibly long
>   * time) for the dirty pages to drop, unless written enough pages.
>   *
> @@ -31,8 +22,6 @@
>   */
>  #define DIRTY_SCOPE		8
>  #define DIRTY_FULL_SCOPE	(DIRTY_SCOPE / 2)
> -#define DIRTY_MAXPAUSE_AREA		16
> -#define DIRTY_PASSGOOD_AREA		8
>  
>  /*
>   * 4MB minimal write chunk size
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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