[Devel] Re: [PATCH -mm] mm: fine-grained dirty_ratio_pcm and dirty_background_ratio_pcm (v2)

[David Rientjes]

On Mon, 10 Nov 2008, Andrea Righi wrote:

> The KB limit is a static value, the other depends on the dirtyable
> memory. If we want to preserve the same behaviour we should do the
> following:
> 
> - when dirty_ratio changes to x:
>   dirty_amount_in_bytes = x * dirtyable_memory / 100.
> 
> - when dirty_amount_in_bytes changes to x:
>   dirty_ratio = x / dirtyable_memory * 100
> 

I think the idea is for a dynamic dirty_ratio based on a static value 
dirty_amount_in_bytes:

	dirtyable_memory = determine_dirtyable_memory() * PAGE_SIZE;
	dirty_ratio = dirty_amount_in_bytes / dirtyable_memory;

> But anytime the dirtyable memory changes (as well as the total memory in
> the system) we should update both values accordingly to preserve the
> coherency between them.
> 

Only dirty_ratio is actually updated if dirty_amount_in_bytes is static.

This allows you to control how many pages are NR_FILE_DIRTY or 
NR_UNSTABLE_NFS and gives you the granularity that you want with 
dirty_ratio_pcm, but on a byte scale instead of percent.

It's also a clean interface:

	echo 200M > /proc/sys/vm/dirty_ratio_bytes
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