NetBSD/Xen samba performance low (compared to NetBSD/amd64)

[Matthias Petermann <mp%petermann-it.de@localhost>]

Hello everybody,

on a small Intel NUC with Realtek network chip I want to operate
a Xen host. My NetBSD Xen guests are supposed to host different web apps 
as well as an Asterisk PBX.

I also want to provide network drives via Samba as performant as 
possible. To keep the overhead of hard drive access low I thought it 
could be a good idea to let Samba - although not architecturally clean - 
operate from Dom0.

When trying to do this, I noticed a significant difference in network speed.

Constellation 1 (Pure NetBSD kernel):

* NetBSD/amd64 9.0 Kernel + Samba: throughput ~60 MByte/s

Constellation 2 (NetBSD/Xen Domain 0):

* Xen 4.11 + NetBSD/Xen Dom0 + Samba: throughput ~12 MByte/s

I measured this by copying an 8 GB ISO file from a Windows host.
In constellation 2, no guests had started and the full main memory of 
Dom0 was assigned. In my view, the only significant difference is that 
NetBSD can only use one of the two CPU cores under Xen. Since the CPU 
was idle on average at 20% during copying, that doesn't seem to be the 
bottleneck?

Are such differences in I/O performance to be expected?

Thank you & best regards
Matthias