Re: [PATCH 02/35] writeback: safety margin for bdi stat error

[Jan Kara]

On Mon 13-12-10 22:46:48, Wu Fengguang wrote:
> In a simple dd test on a 8p system with "mem=256M", I find all light
> dirtier tasks on the root fs are get heavily throttled. That happens
> because the global limit is exceeded. It's unbelievable at first sight,
> because the test fs doing the heavy dd is under its bdi limit.  After
> doing some tracing, it's discovered that
> 
>         bdi_dirty < bdi_dirty_limit() < global_dirty_limit() < nr_dirty
          ^^ bdi_dirty is the number of pages dirtied on BDI? I.e.
bdi_nr_reclaimable + bdi_nr_writeback?

> So the root cause is, the bdi_dirty is well under the global nr_dirty
> due to accounting errors. This can be fixed by using bdi_stat_sum(),
  So which statistic had the big error? I'd just like to understand
this (and how come your patch improves the situation)...

> however that's costly on large NUMA machines. So do a less costly fix
> of lowering the bdi limit, so that the accounting errors won't lead to
> the absurd situation "global limit exceeded but bdi limit not exceeded".
> 
> This provides guarantee when there is only 1 heavily dirtied bdi, and
> works by opportunity for 2+ heavy dirtied bdi's (hopefully they won't
> reach big error _and_ exceed their bdi limit at the same time).
> 
...
> @@ -458,6 +464,14 @@ unsigned long bdi_dirty_limit(struct bac
>  	long numerator, denominator;
>  
>  	/*
> +	 * try to prevent "global limit exceeded but bdi limit not exceeded"
> +	 */
> +	if (likely(dirty > bdi_stat_error(bdi)))
> +		dirty -= bdi_stat_error(bdi);
> +	else
> +		return 0;
> +
  Ugh, so if by any chance global_dirty_limit() <= bdi_stat_error(bdi), you
will limit number of unreclaimable pages for that bdi 0? Why?

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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