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