VM and performance (was Re: [Beowulf] best Linux distribution)

[Bogdan Costescu]

On Tue, 9 Oct 2007, Jeffrey B. Layton wrote:

> The last word I heard is that performance takes a substantial hit if 
> you are running a code in a VM.

I attended a talk some years ago by the Bochs author and I remember 
him saying that one possibility of speeding up the virtual code is to 
cache the code after "processing" it the first time - this could work 
very well for a computational code if the performance sensitive inner 
loop is treated this way. This was especially important for cases 
where the virtual and native hardware were not the same architecture.

> Some of the reasons are that the code has only virtualized access to 
> the hardware (particularly the NICs)

It's the same old problem like user space drivers...

You can have a domU Xen guest with direct access to a specific 
hardware device. Sure, you have to restrict the hardware device to a 
specific Xen guest, so that neither the host nor any other Xen guests 
can use that device anymore, but this allows for the "lucky" Xen guest 
the fast access to the hardware.

-- 
Bogdan Costescu

IWR - Interdisziplinaeres Zentrum fuer Wissenschaftliches Rechnen
Universitaet Heidelberg, INF 368, D-69120 Heidelberg, GERMANY
Telephone: +49 6221 54 8869, Telefax: +49 6221 54 8868
E-mail: Bogdan.Costescu at IWR.Uni-Heidelberg.De