Re: [PATCHv12 3/4] zswap: add to mm/

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 29 May 2013 09:57:20 -0500 Seth Jennings <sjenning@linux.vnet.ibm.com> wrote:

> > > +/*********************************
> > > +* helpers
> > > +**********************************/
> > > +static inline bool zswap_is_full(void)
> > > +{
> > > +	return (totalram_pages * zswap_max_pool_percent / 100 <
> > > +		zswap_pool_pages);
> > > +}
> > 
> > We have had issues in the past where percentage-based tunables were too
> > coarse on very large machines.  For example, a terabyte machine where 0
> > bytes is too small and 10GB is too large.
> 
> Yes, this is known limitation of the code right now and it is a high priority
> to come up with something better.  It isn't clear what dynamic sizing policy
> should be used so, until such time as that policy can be determined, this is a
> simple stop-gap that works well enough for simple setups.

It's a module parameter and hence is part of the userspace interface. 
It's undesirable that the interface be changed, and it would be rather
dumb to merge it as-is when we *know* that it will be changed.

I don't think we can remove the parameter altogether (or can we?), so I
suggest we finalise it ASAP.  Perhaps rename it to
zswap_max_pool_ratio, with a range 1..999999.  Better ideas needed :(


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