Re: VM Requirement Document - v0.0

From: Daniel Phillips (
Date: Wed Jul 04 2001 - 09:44:24 EST

On Wednesday 04 July 2001 10:32, Marco Colombo wrote:
> On Tue, 3 Jul 2001, Daniel Phillips wrote:
> > On Monday 02 July 2001 20:42, Rik van Riel wrote:
> > > On Thu, 28 Jun 2001, Marco Colombo wrote:
> > > > I'm not sure that, in general, recent pages with only one access are
> > > > still better eviction candidates compared to 8 hours old pages. Here
> > > > we need either another way to detect one-shot activity (like the one
> > > > performed by updatedb),
> > >
> > > Fully agreed, but there is one problem with this idea.
> > > Suppose you have a maximum of 20% of your RAM for these
> > > "one-shot" things, now how are you going to be able to
> > > page in an application with a working set of, say, 25%
> > > the size of RAM ?
> >
> > Easy. What's the definition of working set? Those pages that are
> > frequently referenced. So as the application starts up some of its pages
> > will get promoted from used-once to used-often. (On the other hand, the
> > target behavior here conflicts with the goal of grouping together several
> > temporally-related accesses to the same page together as one access, so
> > there's a subtle distinction to be made here, see below.)
> [...]
> In Rik example, the ws is larger than available memory. Part of it
> (the "hottest" one) will get double-accesses, but other pages will keep
> condending the few available (physical) pages with no chance of being
> accessed twice. But see my previous posting...

But that's exactly what we want. Note that the idea of reserving a fixed
amount of memory for "one-shot" pages wasn't mine. I see no reason to set a
limit. There's only one critereon: does a page get referenced between the
time it's created and when its probation period expires?

Once a page makes it into the active (level 2) set it's on an equal footing
with lots of others and it's up to our intrepid one-hand clock to warm it up
or cool it down as appropriate. On the other hand, if the page gets sent to
death row it still has a few chances to prove its worth before being cleaned
up and sent to the aba^H^H^H^H^H^H^H^H reclaimed. (Apologies for the
multiplying metaphors ;-)

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