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@xxxxxxxxx>
> Tested-by: Li Shaohua <shaohua.li@xxxxxxxxx>
> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> ---
> 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@xxxxxxx>
SUSE Labs, CR
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