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

[Joe Landman]

Jeffrey B. Layton wrote:
> The recent emails from rgb and Doug lead me to a question. Has anyone
> tested codes running under a VM versus running them "natively" on
> the hardware (native isn't a good word and I hope everyone gets my
> meaning)?  The last word I heard is that performance takes a substantial

You have two major types, the heavyweight emulators (VMware, et al) and 
the lighter weight hypervisors.  I have seen studies of various codes 
that show not a huge hit ... the caveat being that these codes spent 
very little time in system calls (usually thunked to the host somehow) 
and most of the time computing.

> hit if you are running a code in a VM. Some of the reasons are that the
> code has only virtualized access to the hardware (particularly the NICs)
> and memory management is a bit more difficult (although Barcelona with
> nested page tables should help there). I do remember VirtualIron saying
> that they had IB drivers so that the VM had direct access to the hardware.

The more layers you have to pass through, the lower your overall 
performance (of course).

> 
> Thanks!
> 
> Jeff
> 
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