Re: swapping and the value of /proc/sys/vm/swappiness

[Marcelo Tosatti <marcelo.tosatti@cyclades.com>]

On Wed, Sep 08, 2004 at 02:35:03PM -0500, Ray Bryant wrote:
> 
> 
> Marcelo Tosatti wrote:
> 
> >
> >
> >Huh, that changes the meaning of the dirty limits. Dont think its suitable
> >for mainline.
> >
> >
> 
> The change is, in fact, not much different from what is already actually 
> there.  The code in get_dirty_limits() adjusts the value of the user 
> supplied parameters in /proc/sys/vm depending on how much mapped memory 
> there is.  If you undo the convoluted arithmetic that is in there, one 
> finds that if you are using the default dirty_ratio of 40%, then if the 
> unmapped_ratio is between 80% and 10%, then
> 
>    dirty_ratio = unmapped_ratio / 2;
> 
> and, a little bit of algebra later:
> 
>    dirty = (total_pages - wbs->nr_mapped)/2
> 
> and
> 
>    background = dirty_background_ratio/vm_background_ratio * (total_pages
> 	- wbs->nr_mapped)
> 
> That is, for a wide range of memory usage, you are really running with an
> dirty ratio of 50% stated in terms of the number of unmapped pages, and 
> there is no direct way to override this.

OK I see, yes. 

> Of course, at the edges, the code changes these calculations.  It just 
> seems to me that rather than continue the convoluted calculation that is in
> get_dirty_limits(), we just make the outcome more explicit and tell the user
> what is really going on.
> 
> We'd still have to figure out how to encourage a minimum page cache size of
> some kind, which is what I understand the 5% min value for dirty_ratio is 
> in there for.
 
For the user "dirty_ratio" and "dirty_background_ratio" means "percentage
of total memory" (thats how it has been traditionally in Linux). And right now,
 as you noted, we dont do that way.

There's probably a good reason for the "no more than half of unmapped memory".
Andrew ?