Re: self-paging

[Emmanuel Colbus]

Jonathan S. Shapiro a écrit :

> On Wed, 2005-12-07 at 11:30 +0100, Emmanuel Colbus wrote:
>  
>
> > Jonathan S. Shapiro a écrit :
> >
> >    
> >
> > > On Tue, 2005-12-06 at 00:05 +0100,
> > >      
> > Emmanuel Colbus wrote:
> >    
> >
> > > > On Sun, 04 Dec 2005 14:26:28 -0500, Jonathan S. Shapiro wrote:
> > > >
> > > >        
> > > >
> > > > > I provisionally believe that if we want to use any sort of
> > > > >          
> > LRU,
> >    
> >
> > > > > the gathering of age data must be performed by the kernel.
> > > > >          
> > Since
> >    
> >
> > > > > the kernel needs to collect this data anyway,
> > > > >          
> > > > Why does it?
> > > >        
> > > Because the kernel is in a position to use the translation hardware
> > > in order to measure this. Applications, generally speaking, are
> > >      
> > not.
> >
> > I meant : why does the kernel needs to collect the data *anyway*, that is,
> > even if we don't use any kind of LRU. Clearly, if this data has to be 
> > collected,
> > only the kernel can do it.
> >    
>
> Because if it doesn't, it cannot tell which pages are active.
What do you mean, an active page? A page which is actively used? I
though the kernel should not care about that, except for LRU-like memory 
management. A page which is used by an application? Well, for this 
purpose, the kernel just needs to recall which pages he hasn't given to 
anybody, doesn't he?

> Doing the
> "accessed" check is where the expense is. Once you have that, the LRU is
> just a counter.
>
>  
Agreed.


> > > Well, if the keeper doesn't know the order of page arrival, how
> > >      
> > does
> >    
> >
> > > it decide what to throw out? Suppose we wanted to implement LRU in
> > > the keeper, for example. How does it know?
> > >      
> > It uses the "accessed" bit in the page table entry : if it is 0, the page
> > hasn't been accessed since the last time the keeper set it to 0, so its
> > new age is "previous age + 1 keeper cycle time"; if it is 1, its new age
> > is 0. At page arrival, "previous age" is set to 0 (or was set to 0 for 
> > long,
> > because it is also the value for unused pages); but the keeper can't
> > differentiate such a page from a recently accessed one.
> >    
>
> The keeper is not in the kernel. It does not have the ability to examine
> the hardware page table state.
>  

Oh, it's not in the kernel... That's more difficult (for me! :-) ). But
I continue to think that, to implement a LRU, the keeper needs to know 
the age of last use of the pages, but not their order of arrival. He may 
just need to be notified when some new pages are coming (but maybe it 
was the point you were refering to?).

> shap
>
>  

Emmanuel