[Xen-users] NFS performance in domU

[Harald Kubota <hkubota@xxxxxxx>]

Hi,

I've got 2 completly different machines serving as my dom0 machines:
one VIA C3 machine (0.5GB RAM, 1GHz C3) and one P3 machine (1GB RAM, 
800MHz, dual CPU).
Each is connected to a shared firewire disk, the C3 via 1394a, the P3 
via 1394b.
On the dom0 in each machine I get about 20 resp. 45MB/s read performance 
which is
what I expect from 1394a/b.
In a domU which directly mounts the exported partitions I get the same 
values.
So far so good.

The problem starts when I have 2 domU running, one exporting a 
filesystem and the other
one mounting it. dom0 is not involved in any way here, at least not 
directly.
In the C3 case, I get about 10MB/s read performance. Which is a bit on 
the low side, but
that's not unexpected.
However on the dual P3, where each domU can use one CPU for itself, I 
get 11MB/s.
Now that is unexplainably slow.

I get higher performance when I split both domU on one computer each 
(12MB/s resp. 16MB/s,
interconnected via GBit Ethernet).

Why is it that slow when running on one physical computer, even when 
that one has 2 CPUs
and is capable of moving 45MB/s easily?

I'm not using any iptables rules, no JumboFrames. Any idea why it's that 
slow?
Harald


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