Re: VM Requirement Document - v0.0

[Daniel Phillips]

On Thursday 28 June 2001 17:21, Jonathan Morton wrote:
> >There is a simple change in strategy that will fix up the updatedb case
> > quite nicely, it goes something like this: a single access to a page
> > (e.g., reading it) isn't enough to bring it to the front of the LRU
> > queue, but accessing it twice or more is.  This is being looked at.
>
> Say, when a page is created due to a page fault, page->age is set to
> zero instead of whatever it is now.

This isn't quite enough.  We do want to be able to assign a ranking to 
members of the accessed-once set, and we do want to distinguish between newly 
created pages and pages that have aged all the way to zero.

> Then, on the first access, it is
> incremented to one.  All accesses where page->age was previously zero
> cause it to be incremented to one, and subsequent accesses where
> page->age is non-zero cause a doubling rather than an increment.
> This gives a nice heavy priority boost to frequently-accessed pages...

While on that topic, could somebody please explain to me why exponential 
aging is better than linear aging by a suitably chosen increment?  It's clear 
what's wrong with it: after 32 hits you lose all further information.  I 
suspect there are more problems with it than that.

--
Daniel