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

[Seth Jennings <sjenning@linux.vnet.ibm.com>]

On Wed, May 29, 2013 at 11:29:29AM -0700, Andrew Morton wrote:
> 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 :(

zswap_max_pool_ratio is fine with me.  I'm not entirely clear on the change
though.  Would that just be a name change or a change in meaning?

Also, we can keep the tunable as I imagine there will always be some use for a
manual override of the (future) dynamic policy.  When the dynamic policy is
available, we can just say that zswap_max_pool_ratio = 0 means "use dynamic
policy" and change the default to 0.  Does that sounds reasonable?

Seth

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