Xen users - Jul 2006 - Performance of xen hosts with and without Virtualization Technology (VT) or AMD Virtualization

All,

I''m currious how the performance chart looks like what using xen  with
a
xen aware kernel. On a CPU with and without Virtualization Technology (VT)
or AMD Virtualization.

I have afeeling that it doesn''t effect the speed at all. Am i wrong?

Does Virtualization Technology (VT) or AMD Virtualization have any
advantages for xen aware oses?

Regards,

Joop Boonen.
IP-Services.


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> -----Original Message-----
> From: xen-users-bounces@lists.xensource.com 
> [mailto:xen-users-bounces@lists.xensource.com] On Behalf Of 
> Joop Boonen
> Sent: 21 July 2006 13:36
> To: xen-users@lists.xensource.com
> Subject: [Xen-users] Performance of xen hosts with and 
> without Virtualization Technology (VT) or AMD Virtualization
> 
> All,
> 
> I''m currious how the performance chart looks like what using 
> xen  with a
> xen aware kernel. On a CPU with and without Virtualization 
> Technology (VT)
> or AMD Virtualization.
At the moment, para-virtualized guests do not use AMDV/VT technology, so
there shouldn''t be any difference other than what differences there are
in the processor itself [for example, DDR vs. DDR2 in AMD processors may
make some particular operation faster or slower - in AMD processors
there shouldn''t be any performance difference in regular instructions -
some very rare operations may run one cycle or so slower, but I can
certainly say that for average code, the cycle-count is exactly
identical for older and new processors - I haven''t tried to figure out
which, if any, of the obscure/system instructions have changed). 
> 
> I have afeeling that it doesn''t effect the speed at all. Am i
wrong?
> 
> Does Virtualization Technology (VT) or AMD Virtualization have any
> advantages for xen aware oses?
It is theoretically possible to cherry-pick functions in AMDV/VMX that
will improve performance for para-virtualized guests, but in general,
it''s more beneficial to use the para-virtualization approach - since
most commonly the desired effect of a intercepted operation is more
complex than the single intercept, and often requires several
intercepts... As an example: To flush the page-table TLB, the programmer
will do:

	mov	%cr4, %rax
	mov	%rax, %rbx
	and	$~CR4_PGE, %rax    // Turn off PGE
	mov   %rax, %cr4
	mov	%rbx, %cr4

This will incur 3 intercepts (one for each "touch" of %cr4). In a
para-virtual OS, it can be reduced to a single function call that
perform the relevant TLB-flush code as a set. 

Likewise, if the guest is updating a range of page-table entries (say
the guest allocated a 4MB area of memory, this needs to be mapped from
guest-physical to virtual). This means (4MB/4KB pages) 1024 page-table
writes. In para-virtualization, you could batch these in a single cal to
the hypervisor, whilst the full virtualization is only seeing one
operation at a time, and doesn''t get the "whole picture". 

--
Mats
	
> 
> Regards,
> 
> Joop Boonen.
> IP-Services.
> 
> 
> _______________________________________________
> Xen-users mailing list
> Xen-users@lists.xensource.com
> http://lists.xensource.com/xen-users
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> 


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