RE: [Xen-users] big latency, packet losses with HVM guests

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Tomasz Chmielewski
> Sent: 30 November 2006 14:08
> To: Xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-users] big latency, packet losses with HVM guests
> 
> Networking in Windows (2003 R2) looks a bit problematic:
> 
> 1. Latency is very big - this is made from domU, there is 
> almost no load
> on dom0 and domU (Windows):
> 
> # ping 192.168.111.186
> PING 192.168.111.186 (192.168.111.186) 56(84) bytes of data.
> 64 bytes from 192.168.111.186: icmp_seq=1 ttl=128 time=4.06 ms
> 64 bytes from 192.168.111.186: icmp_seq=2 ttl=128 time=4.82 ms
> 64 bytes from 192.168.111.186: icmp_seq=3 ttl=128 time=6.25 ms
> 64 bytes from 192.168.111.186: icmp_seq=4 ttl=128 time=7.70 ms
> 64 bytes from 192.168.111.186: icmp_seq=5 ttl=128 time=9.14 ms
> 64 bytes from 192.168.111.186: icmp_seq=6 ttl=128 time=0.580 ms
> 64 bytes from 192.168.111.186: icmp_seq=7 ttl=128 time=2.01 ms
> 64 bytes from 192.168.111.186: icmp_seq=8 ttl=128 time=3.49 ms
> 64 bytes from 192.168.111.186: icmp_seq=9 ttl=128 time=4.89 ms
> 64 bytes from 192.168.111.186: icmp_seq=10 ttl=128 time=6.33 ms
> 
> --- 192.168.111.186 ping statistics ---
> 10 packets transmitted, 10 received, 0% packet loss, time 9000ms
> rtt min/avg/max/mdev = 0.580/4.931/9.144/2.437 ms
> 
> 
> For non-HVM guests I have latency of about 0.074 ms.
> 
> 
> 2. There are slight packet losses:
> 
> # ping -c 10000 -f 192.168.111.186
> PING 192.168.111.186 (192.168.111.186) 56(84) bytes of data.
> ................
> --- 192.168.111.186 ping statistics ---
> 10000 packets transmitted, 9984 received, 0% packet loss, time 4708ms
> rtt min/avg/max/mdev = 0.295/0.400/9.534/0.345 ms, ipg/ewma 
> 0.470/0.438 ms
> 
> 
> For non-HVM guestes I have no packet losses.
> 
> 
> Is it because of the qemu/realtek network driver?

Not directly. But the latency has something to do with the fact that
each part of a transaction for the network driver goes from the guest,
into the HVM code in Xen, then to qemu via Dom0. If Dom0 is "busy" doing
something else, then it may also be a delay before QEMU is started.
Since one network packet consists of more than one transaction to/from
QEMU, the latency _WILL_ be noticable. 

Packet losses are probably also related to the fact that you have high
latency, at least indirectly - essentially that you haven't got enough
time to process all the packets that get sent from one place to the next
before the buffers are full and packets are dropped. 

You may find that if you can run one CPU for Dom0 and another for DomU
(say in a dual core system), you may get better performance than if you
run both CPU's for both Dom0 and DomU. 

--
Mats
> 
> 
> -- 
> Tomasz Chmielewski
> http://wpkg.org
> 
> 
> 
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> 



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