Re: [BUG]Writeback Cgroup/Dirty Throttle: very small buffered write thoughput caused by writeback cgroup and dirty thottle

[Miao Xie]

on 2016/5/12 at 23:32, Tejun Heo wrote:
> On Thu, May 12, 2016 at 09:11:33AM +0800, Miao Xie wrote:
> > > My box has 48 cores and 188GB memory, but I set
> > > vm.dirty_background_bytes = 268435456
> > > vm.dirty_bytes = 536870912
> > >
> > > if I set vm.dirty_background_bytes and vm.dirty_bytes to be a large number(vm.dirty_background_bytes = 3GB,
> > > vm.dirty_bytes = 4GB), then fio thoughput would be more than 1500MB/s. and then if I reset them to the original
> > > value(the above ones), the thoughout would be down to 500MB/s.
> > >
> > > And according my debug, I found fio sleeped for 1ms every time we dirty a page(balance dirty pages) when
> > > the thoughput was down to 4MB/s, it might be a bug of dirty throttle when we open write back cgroup, I think.
>
> Heh, so, for cgroups, the absolute byte limits can't applied directly
> and converted to percentage value before being applied.  You're
> specifying 0.27% for threshold.  Unfortunately, the ratio is
> translated into a percentage number and 0.27% becomes 0, so your
> cgroups are always over limit and being throttled.
>
> Can you please see whether the following patch fixes the issue?

Better than the kernel without patch. Now the benchmark could reach the device bandwidth after 5-8 seconds.
But at the beginning, it was still very slow, and its thoughput was only 4MB/s for ~4 seconds, then it
could go up in 1~3 seconds.

Thanks
Miao

> Thanks.
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 999792d..a455a21 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -369,8 +369,9 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc)
>   	struct dirty_throttle_control *gdtc = mdtc_gdtc(dtc);
>   	unsigned long bytes = vm_dirty_bytes;
>   	unsigned long bg_bytes = dirty_background_bytes;
> -	unsigned long ratio = vm_dirty_ratio;
> -	unsigned long bg_ratio = dirty_background_ratio;
> +	/* convert ratios to per-PAGE_SIZE for higher precision */
> +	unsigned long ratio = (vm_dirty_ratio * PAGE_SIZE) / 100;
> +	unsigned long bg_ratio = (dirty_background_ratio * PAGE_SIZE) / 100;
>   	unsigned long thresh;
>   	unsigned long bg_thresh;
>   	struct task_struct *tsk;
> @@ -382,26 +383,28 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc)
>   		/*
>   		 * The byte settings can't be applied directly to memcg
>   		 * domains.  Convert them to ratios by scaling against
> -		 * globally available memory.
> +		 * globally available memory.  As the ratios are in
> +		 * per-PAGE_SIZE, they can be obtained by dividing bytes by
> +		 * pages.
>   		 */
>   		if (bytes)
> -			ratio = min(DIV_ROUND_UP(bytes, PAGE_SIZE) * 100 /
> -				    global_avail, 100UL);
> +			ratio = min(DIV_ROUND_UP(bytes, global_avail),
> +				    PAGE_SIZE);
>   		if (bg_bytes)
> -			bg_ratio = min(DIV_ROUND_UP(bg_bytes, PAGE_SIZE) * 100 /
> -				       global_avail, 100UL);
> +			bg_ratio = min(DIV_ROUND_UP(bg_bytes, global_avail),
> +				       PAGE_SIZE);
>   		bytes = bg_bytes = 0;
>   	}
>
>   	if (bytes)
>   		thresh = DIV_ROUND_UP(bytes, PAGE_SIZE);
>   	else
> -		thresh = (ratio * available_memory) / 100;
> +		thresh = (ratio * available_memory) / PAGE_SIZE;
>
>   	if (bg_bytes)
>   		bg_thresh = DIV_ROUND_UP(bg_bytes, PAGE_SIZE);
>   	else
> -		bg_thresh = (bg_ratio * available_memory) / 100;
> +		bg_thresh = (bg_ratio * available_memory) / PAGE_SIZE;
>
>   	if (bg_thresh >= thresh)
>   		bg_thresh = thresh / 2;
>
> .