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RE: [Xen-users] Xen Performance

        Does KVM have similar options that allow for communication directly 
between VMs?  I only used KVM once, with one VM, to determine fully-virtualized 
performance was too poor (the same as in Xen, but with less driver support).  
If so, I don't know why it doesn't suffer from the same issue, and I should 
hope Xen could be fixed.  Regardless, I am disappointed by RedHat's decision to 
jump ship and go to KVM.  I think xen makes more sense, and pvops would be 
ideal.  The primary reason I use Xen, though, is because I use Windows and KVM 
doesn't have the drivers yet, so I get better performance in Xen.

From: Grant McWilliams [mailto:grantmasterflash@xxxxxxxxx] 
Sent: Friday, October 16, 2009 10:37
To: Dustin.Henning@xxxxxxxxxxx
Cc: Fajar A. Nugraha; Fasiha Ashraf; xen-users@xxxxxxxxxxxxxxxxxxx
Subject: Re: [Xen-users] Xen Performance

On Fri, Oct 16, 2009 at 5:43 AM, Dustin Henning <Dustin.Henning@xxxxxxxxxxx> 
<snip> DomU to DomU performance is 1/3 to 2/3 that of DomU to Dom0 even though 
that the DomU to DomU traffic is probably traversing the same path as DomU to 
Dom0. <snip>

Grant McWilliams

Seeing as how each domU nic exists in dom0, as does the bridge, I would argue 
that traffic between domU and domU take 50% more steps than traffic between 
domU and dom0.  Looking at it the other way, that would be 33% less steps 
between domU and dom0 than domU and domU.  Here is why:

Between domU A and domU B, the traffic has to traverse domU A's vif in dom0, 
the bridge in dom0, and domU B's vif in dom0.  That is three virtual devices.

Between domU A and dom0, the traffic only has to traverse domU A's vif and the 
bridge, it has then arrived in dom0, that is only two virtual devices.

I could be wrong, and would love to be given a more detailed and technical 
answer as to how I am if that is the case.  Regarding the performance 
difference, my opinion is that we could always use more performance in all 
aspects, period.  Sure, eventually costs are prohibiting and we have to settle, 
but more performance wouldn't hurt no matter how many people think performance 
is good enough, and only seeing notably better performance can convince some 
people that any given performance isn't good enough.

   This is how it works... :-) This entire thread exists somewhere else since I 
went through the exact same process the first time I was doing my testing. I 
understand how Xen works and it's unfortunate that this limitation will never 
be replaced. Maybe it will but by then everyone will have migrated to KVM which 
doens't seem to suffer from it.  It's very inefficient to pass all traffic to 
Dom0 then pass it back to another domU. It's like you always have a router to 
go through even if the DomUs are on the same physical network. 

It also seems that it's the return trip that's slow (even without the broken 
pipe) and my testing before (and the resulting message thread before) showed 
that data going to the Dom0 was fast but data back wasn't. If we throw enough 
hardware at it we get wire speed or we assign a nic to each DomU but then we 
can only get wire speed.

Grant McWilliams

Some people, when confronted with a problem, think "I know, I'll use Windows." 
Now they have two problems.

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