On Sun, 5 Dec 2010 14:44:30 +0800
Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> I noticed that my NFSROOT test system goes slow responding when there
> is heavy dd to a local disk. Traces show that the NFSROOT's bdi_limit
> is near 0 and many tasks in the system are repeatedly stuck in
> balance_dirty_pages().
>
> There are two related problems:
>
> - light dirtiers at one device (more often than not the rootfs) get
> heavily impacted by heavy dirtiers on another independent device
>
> - the light dirtied device does heavy throttling because bdi_limit=0,
> and the heavy throttling may in turn withhold its bdi_limit in 0 as
> it cannot dirty fast enough to grow up the bdi's proportional weight.
>
> Fix it by introducing some "low pass" gate, which is a small (<=8MB)
> value reserved by others and can be safely "stole" from the current
> global dirty margin. It does not need to be big to help the bdi gain
> its initial weight.
>
The changelog refers to something called "bdi_limit". But there is no
such thing. It occurs nowhere in the Linux tree and it has never
before been used in a changelog.
Can we please use carefully-chosen terminology and make sure that
everyone can easily understand what the terms are referring to?
I'm assuming from context that you've created a new term to refer to
the bdi_dirty_limit() return value for this bdi.
And ... oh geeze, you made me look at the code. Grumbles forthcoming.
>
> Peter, I suspect this will do good for 2.6.37. Please help review, thanks!
>
> include/linux/writeback.h | 3 ++-
> mm/backing-dev.c | 2 +-
> mm/page-writeback.c | 23 +++++++++++++++++++++--
> 3 files changed, 24 insertions(+), 4 deletions(-)
>
> --- linux-next.orig/mm/page-writeback.c 2010-12-05 14:29:24.000000000 +0800
> +++ linux-next/mm/page-writeback.c 2010-12-05 14:31:39.000000000 +0800
> @@ -444,7 +444,9 @@ void global_dirty_limits(unsigned long *
> * The bdi's share of dirty limit will be adapting to its throughput and
> * bounded by the bdi->min_ratio and/or bdi->max_ratio parameters, if set.
> */
> -unsigned long bdi_dirty_limit(struct backing_dev_info *bdi, unsigned long dirty)
> +unsigned long bdi_dirty_limit(struct backing_dev_info *bdi,
> + unsigned long dirty,
> + unsigned long dirty_pages)
Forgot to update the bdi_dirty_limit() kerneldoc.
While you're there, please document the bdi_dirty_limit() return value.
<looks>
It mentions "100" a lot. ah-hah! It returns a 0..99 percentage!
<looks further>
No, ratelimit_pages() compares it with a variable called dirty_pages,
so it returns an absolute number of pages!
But maybe ratelimit_pages() is buggy.
<looks further>
balance_dirty_pages() passes the bdi_dirty_limit() return value to
task_dirty_limit() which secretly takes a number-of-pages arg and
secretly returns a number-of-pages return value.
So I will pronounce with moderate confidence that bdi_dirty_limit()
returns a page count!
See what I mean? It shouldn't be that hard!
> {
> u64 bdi_dirty;
> long numerator, denominator;
> @@ -459,6 +461,22 @@ unsigned long bdi_dirty_limit(struct bac
> do_div(bdi_dirty, denominator);
>
> bdi_dirty += (dirty * bdi->min_ratio) / 100;
> +
> + /*
> + * There is a chicken and egg problem: when bdi A (eg. /pub) is heavy
> + * dirtied and bdi B (eg. /) is light dirtied hence has 0 dirty limit,
> + * tasks writing to B always get heavily throttled and bdi B's dirty
> + * limit may never be able to grow up from 0.
> + *
> + * So if we can dirty N more pages globally, honour N/2 to the bdi that
> + * runs low. To provide such a global margin, we slightly decrease all
> + * heavy dirtied bdi's limit.
> + */
> + if (bdi_dirty < (dirty - dirty_pages) / 2 && dirty > dirty_pages)
> + bdi_dirty = (dirty - dirty_pages) / 2;
> + else
> + bdi_dirty -= min(bdi_dirty / 128, 8192ULL >> (PAGE_SHIFT-10));
Good lord, what have we done.
Ho hum.
This problem isn't specific to NFS, is it? All backing-devices start
out with a bdi_limit (which doesn't actually exist) of zero, yes? And
this "bdi_limit" is a per-bdi state which is stored via some
undescribed means in some or all of `completions',
`write_bandwidth_update_time', `write_bandwidth', `dirty_exceeded',
`min_ratio', `max_ratio' and `max_prop_frac'. All of which are
undocumented, naturally.
I admire your ability to work on this code, I really do. I haven't
looked at it in detail for a year or so and I am aghast at its opacity.
This makes it extremely hard to review any changes to it. This is a
problem. And I don't think I can (or will) review this patch for these
reasons. My dummy is thoroughly spat out.
And what's up with that 8192? I assume it refers to pages? 32MB? So
if we're working on eight devices concurrently on a 256MB machine, what
happens?
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