Re: [Qemu-devel] KVM VM(rhel-5.5) %si is too high when TX/RX packets

[Paolo Bonzini]

Il 03/05/2013 06:05, Zhanghaoyu (A) ha scritto:
> I running a VM(RHEL-5.5) on KVM hypervisor(linux-3.8 + QEMU-1.4.1), and 
> direct-assign intel 82576 VF to the VM. When TX/RX packets on VM to the other 
> host via iperf tool, top tool result on VM shown that the %si is too high, 
> approximately 95% ~ 100%, but from the view of host, the VM's total CPU usage 
> is about 20% - 30%. And the throughput rate is approximately 200Mb/s, far 
> from the line rate 1Gb/s, 
> And, I found  the hardirq rate is lower than normal by running "watch -d -n 1 
> cat /proc/interrupts", I think it's caused by the too high %si, because the 
> NIC's hardirq was disabled during the softirq process.
> Then, I direct-assign the intel 82576 to the VM, the same case happened too. 
> I found the intel 82576 and intel 82576 VF's interrupt mode are both 
> PCI-MSI-X.
> 
> And,
> I rmmod the igb driver, and, re-insmod the igb driver(igb-4.1.2) with the 
> parameter IntMode=0/1(0:legacy, 1:MSI, 2:MSI-x), the problem then gone, the 
> %si is approximately 20% -30%, and the throughput rate came to the line rate, 
> about 940Mb/s.
> I update the VM to RHEL-6.1, the problem disappeared too.
> And, I found a very strange thing, the VM's 82576VF's irq routing is set one 
> time on Vf's one interrupt received, so frequently.

RHEL 5.5 is a very old update.  Can you try RHEL 5.9?

In any case, this looks a lot like a bug in the version of the driver
that was included in RHEL5.5; you should contact Red Hat support
services if you can still reproduce it with the latest RHEL5 update.

Paolo