Re: Page host virtual assist patches.

[Martin Schwidefsky <schwidefsky@de.ibm.com>]

On Mon, 2006-04-24 at 18:01 -0700, Andrew Morton wrote:
> Martin Schwidefsky <schwidefsky@de.ibm.com> wrote:
> >
> >  The basic idea of host virtual assist (hva) is to give a host system
> >  which virtualizes the memory of its guest systems on a per page basis
> >  usage information for the guest pages. The host can then use this
> >  information to optimize the management of guest pages, in particular
> >  the paging. This optimizations can be used for unused (free) guest
> >  pages, for clean page cache pages, and for clean swap cache pages.
> 
> This is pretty significant stuff.  It sounds like something which needs to
> be worked through with other possible users - UML, Xen, vware, etc.
> 
> How come the reclaim has to be done in the host?  I'd have thought that a
> much simpler approach would be to perform a host->guest upcall saying
> either "try to free up this many pages" or "free this page" or "free this
> vector of pages"?

Because calling into the guest is too slow. You need to schedule a cpu,
the code that does the allocation needs to run, which might need other
pages, etc. The beauty of the scheme is that the host can immediately
remove a page that is mark as volatile or unused. No i/o, no scheduling,
nothing. Consider what that does to the latency of the hosts memory
allocation. Even if the percentage of discardable pages is small, lets
say 25% of the guests memory, the host will quickly find reusable
memory. If the vmscan of the host attempts to evict 100 pages, on
average it will start i/o for 75 of them, the other 25 are immediately
free for reuse.

-- 
blue skies,
  Martin.

Martin Schwidefsky
Linux for zSeries Development & Services
IBM Deutschland Entwicklung GmbH

"Reality continues to ruin my life." - Calvin.


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