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[Xen-bugs] [Bug 1075] New: Dom0 crashes with 40Mbps Iperf traffic eithou

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Subject: [Xen-bugs] [Bug 1075] New: Dom0 crashes with 40Mbps Iperf traffic eithout saturating the CPU
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Date: Thu, 4 Oct 2007 09:31:20 -0700
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           Summary: Dom0 crashes with 40Mbps Iperf traffic eithout
                    saturating the CPU
           Product: Xen
           Version: 3.0.0
          Platform: x86
        OS/Version: Linux
            Status: NEW
          Severity: critical
          Priority: P2
         Component: Hypervisor
        AssignedTo: xen-bugs@xxxxxxxxxxxxxxxxxxx
        ReportedBy: kaushikb@xxxxxxxxxxxxx


I have a 3 node experiment as detailed below to estimate the bridged networking
performance in para-virtualized Xen 3.0 on Emulab (https://www.emulab.net/)

Here is how the 3-node topology looks (This topology is specified by means of
an NS-2 file)


                |                               |
Node0:eth0 -----|Node1:eth0           Node1:eth1|------ Node2:eth0

Node0 and Node2 run some standard 2.6.* kernel whereas Node1 runs
para-virtualized Xen 3.0 using LVM-created root & swap partitions for DomU

Traffic flow:

           |                Node1               |
           |   ______________________________   |
           |  |            DomU              |  |
           |  |                              |  |
           |  |    eth0 ---------> eth1      |  |
           |  |_____|_______________|________|  |
           |        |               |           |
           |      vif1.0          vif1.1        |
           |        |               |           |
           |        |               |           |
           |      xenbr0          xenbr1        |
           |        |      Dom0     |           |
           |        |               |           |
Node0:eth0-|----> eth0            eth1 ---------|-----> Node2:eth0

Node0:eth0, Node1:DomU:eth0 are in 10.1.1/24 subnet whereas Node1:DomU:eth1 and
Node2:eth0 are in 10.1.2/24 subnet (Since Node1:eth0 and Node1:eth1 are
attached to bridges, they do not have IP addresses on them and are not

At Node0:eth0, I set the next hop of all traffic as Node1:DomU:eth0 and at
Node2, I set the next hop of all traffic as Node1:DomU:eth1.

Now, I try to run an Iperf server at Node2 for UDP traffic (const 64-byte
packets size):
iperf -s -i 2 -l 64 -u

and an Iperf client at Node0 for UDP traffic to node2:eth0 (const 64-bit packet
size, vary the Bandwidth from 10Mbps till possibly 80Mbps)
iperf -c <IP address of Node2:eth0> -u -l 64 -t 90 -i 2 -w 256k -b 10M

Where 10M is the bandwidth of iperf traffic between Node0 and Node2 (in Mbps).

I also monitor the CPU usage at Node1 Dom0 using "vmstat 1" at Dom0.

When B/W is 36.6M (Mbps), the CPU utilization at Dom0 is about 70-80% but when
the B/W is increased to 39.4M (Mbps), the Dom0 crashes ('hangs') in typically
5-15 seconds  but before it crashes, the CPU utiliation is till 75-85%. At this
point, I have to again boot Dom0 and the DomU on top of it.

Does anybody know why this happens ?? and how can I debug this situation (if at
all) ? I am new to the Xen source code so please be as detailed.


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