Re: 64 bit guest much faster ?

[Avi Kivity <avi@redhat.com>]

On 10/26/2009 10:58 AM, Gerd Hoffmann wrote:
> On 10/23/09 17:54, Stefan wrote:
>>
>> Hello,
>>
>> I have a simple question (sorry I'm a kvm beginner):
>> Is it right that a 64bit guest (8 CPUs, 16GB) is
>> much faster than a 32bit guest (8 CPUs, 16GB PAE).
>                                           ^^^^
> Yes.  With *that* much memory the 32bit guest struggles with address 
> space limitations (32bit -> 4G), whereas the 64bit guest doesn't.
>
> With up to 1G you shouldn't see a noticable difference.  But the more 
> highmem the 32bit guest uses the higher is the penalty.  Especially 
> without ept/npt as every kmap() of a high page is a roundtrip to the 
> hypervisor then.
>

Oh yes, without ept/npt the slowdown should indeed be significant with 
this much memory.

-- 
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