Re: [PATCH -mm] page-writeback: fine-grained dirty_ratio and dirty_background_ratio

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Thu, 09 Oct 2008 17:29:46 +0200
Andrea Righi <righi.andrea@gmail.com> wrote:

> The current granularity of 5% of dirtyable memory for dirty pages writeback is
> too coarse for large memory machines and this will get worse as
> memory-size/disk-speed ratio continues to increase.
> 
> These large writebacks can be unpleasant for desktop or latency-sensitive
> environments, where the time to complete a writeback can be perceived as a
> lack of responsiveness by the whole system.
> 
> So, something to define fine grained settings is needed.
> 
> Following there's a similar solution as discussed in [1], but I tried to
> simplify the things a little bit, in order to provide the same functionality
> (in particular try to avoid backward compatibility problems) and reduce the
> amount of code needed to implement an in-kernel parser to handle percentages
> with decimals digits.
> 
> The kernel provides the following parameters:
>  - dirty_ratio, dirty_background_ratio in percentage
>    (1 ... 100)
>  - dirty_ratio_pcm, dirty_background_ratio_pcm in units of percent mille
>    (1 ... 100,000)
> 
> Both dirty_ratio and dirty_ratio_pcm refer to the same vm_dirty_ratio variable,
> only the interface to read/write this value is different. The same is valid for
> dirty_background_ratio and dirty_background_ratio_pcm.
> 
> In this way it's possible to provide a fine grained interface to configure the
> writeback policy and at the same time preserve the compatibility with the old
> coarse grained dirty_ratio / dirty_background_ratio users.
> 
> Examples:
>  # echo 5 > /proc/sys/vm/dirty_ratio
>  # cat /proc/sys/vm/dirty_ratio
>  5
>  # cat /proc/sys/vm/dirty_ratio_pcm
>  5000
> 
>  # echo 500 > /proc/sys/vm/dirty_ratio_pcm
>  # cat /proc/sys/vm/dirty_ratio
>  0
>  # cat /proc/sys/vm/dirty_ratio_pcm
>  500
> 
>  # echo 5500 > /proc/sys/vm/dirty_ratio_pcm
>  # cat /proc/sys/vm/dirty_ratio
>  5
>  # cat /proc/sys/vm/dirty_ratio_pcm
>  5500
> 
I like this. thanks.

<snip>

> -int dirty_background_ratio = 5;
> +int dirty_background_ratio = 5 * PERCENT_PCM;
>  
>  /*
>   * free highmem will not be subtracted from the total free memory
> @@ -77,7 +77,7 @@ int vm_highmem_is_dirtyable;
>  /*
>   * The generator of dirty data starts writeback at this percentage
>   */
> -int vm_dirty_ratio = 10;
> +int vm_dirty_ratio = 10 * PERCENT_PCM;
>  
>  /*
>   * The interval between `kupdate'-style writebacks, in jiffies
> @@ -135,7 +135,8 @@ static int calc_period_shift(void)
>  {
>  	unsigned long dirty_total;
>  
> -	dirty_total = (vm_dirty_ratio * determine_dirtyable_memory()) / 100;
> +	dirty_total = (vm_dirty_ratio * determine_dirtyable_memory())
> +			/ ONE_HUNDRED_PCM;
>  	return 2 + ilog2(dirty_total - 1);
>  }
>  
I wonder...isn't this overflow in 32bit system ?

Thanks,
-Kame


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