Avi Kivity wrote:
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.
How it is with 4Gb guest/mem without PAE (I mean, with CONFIG_HIGHMEM_4G=y)?
Or even 2Gb? In case of npt or without.
Can we construct a sort of a table of expected slowdowns (not in numbers
but just in terms "significant", "minor" etc) of running <4Gb or >4Gb
(and <1Gb and >1Gb if that makes significant diffencece) 32bit guests
with and without npt and 64bit guests, please? I guess it's quite
interesting to many users.
From the above it looks like it's better to run 64bit kernel in the 32bit
guest in these situations too.
I haven't measured it, just because it never occured to me that there
MAY be any difference. But I've only non-npt hardware here at the
moment.
Thanks!
/mjt
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