RE: [Xen-devel] Shadow page tables?

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> If the shadow pagetable is enabled, Xen will maintain pagetables for
every guest 
> process. If 1000 processes are running in the guest, xen will at least
allocate 1000 
> pages as shadow pagetables. Will it consume too many memory frames?

That's the downside of shadow pagetables, and hence why we prefer to run
guests in 'direct mode' unless we're doing something 'weird' to them -
e.g. when we migrate a domain we switch to shadow page tables for the
duration of the migration.

Apache 1.3 is a good example of an application that requires large
amounts of memory for pagetables (many thousands of pages), and with
shadow pagetables this requiment effectively doubles. The shadow
pagetables are just a cache, so its simple to evict pages if you run low
on memory, but then performance will suck big time.

Ian

On Fri, 08 Oct 2004 10:11:18 +0100, Keir Fraser
<keir.fraser@xxxxxxxxxxxx> wrote:
> 
> Currently, shadow p.t.'s are only enabled when migrating OSes between 
> machines. This allows us to copy the memory image while the OS is 
> still running, and track which pages are updated after being copied 
> (and so need to be recopied).
> 
> We intend to flesh out the shadow p.t. code a little mroe to support 
> full memory virtualisation. It's not there yet, but it won't require 
> an enormous amount of code to get it going.
> 
> -- Keir
> 
> 
> 
> > Hi,
> >
> > I'm using Xen 2.0 (base kernel is 2.6.8.1) and found that it doesn't

> > enable the shadow page tables in Xen though there is code there.
> >
> > If the shadow page tables are enabled, does it mean that the guest 
> > Linux' memory management module could be fully virtualized?
> >
> > Thanks,
> > Chengyuan
> >
> > 
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