Re: [Xen-devel] Poor HVM performance with 8 vcpus

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

At 09:08 +0100 on 07 Oct (1254906487), James Harper wrote:
> > At the very least it would be good to have a predictor which figured
> out which
> > of the several heuristics should actually be used for a given VM. A
> simple
> > "try whichever one worked last time first" should work fine.
> > 
> > Even smarter would be two just have heuristics for the two general
> classes of
> > mapping (1:1 and recursive), and have the code automatically figure
> out the
> > starting virtual address being used for a given guest.
> > 
> 
> Are there any other of these heuristics tucked away in xen? Would there
> be any benefit to specifying the OS being virtualised in the config? Eg
> "os=windows"?

It would be better to allow the specific heuristic to be specified in
the Xen interface (e.g. that it's a recursive pagetable at a particular
address, or a one-to-one mapping).  Which isn't to say the python layer
couldn't put some syntactic sugar on it. 

But the bulk of the win will be had from adding BS2000 to the list of
heuristics.  There's probably some benefit in making the heuristic list
pull-to-front, too.

Automatically detecting 1:1 mappings and linear pagetable schemes would
be fun and is probably the Right Thing[tm], but making sure it works
with all the OSes that currently work (e.g. all HALs of all Windows
versions) will be a significant investment in time. :)

Also, before getting too stuck into this it'd be worth running once more
with performance counters enabled and checking that this is actually
your problem!  You should see a much higher number for "shadow writeable
brute-force" running BS2000 than running Windows.

Cheers,

Tim.

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Citrix Systems (R&D) Ltd.
[Company #02300071, SL9 0DZ, UK.]

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