[libvirt] Qemu/KVM is 3x slower under libvirt

Chris Wright chrisw at sous-sol.org
Thu Sep 29 00:39:23 UTC 2011

* Reeted (reeted at shiftmail.org) wrote:
> On 09/28/11 11:28, Daniel P. Berrange wrote:
> >On Wed, Sep 28, 2011 at 11:19:43AM +0200, Reeted wrote:
> >>On 09/28/11 09:51, Daniel P. Berrange wrote:
> >>>You could have equivalently used
> >>>
> >>>  -netdev tap,ifname=tap0,script=no,downscript=no,id=hostnet0,vhost=on
> >>>  -device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:05:36:60,bus=pci.0,addr=0x3
> >>It's this! It's this!! (thanks for the line)
> >>
> >>It raises boot time by 10-13 seconds
> >Ok, that is truely bizarre and I don't really have any explanation
> >for why that is. I guess you could try 'vhost=off' too and see if that
> >makes the difference.
> YES!
> It's the vhost. With vhost=on it takes about 12 seconds more time to boot.

Can you help narrow down what is happening during the additional 12
seconds in the guest?  For example, does a quick simple boot to single
user mode happen at the same boot speed w/ and w/out vhost_net?

I'm guessing (hoping) that it's the network bring-up that is slow.
Are you using dhcp to get an IP address?  Does static IP have the same
slow down?

If it's just dhcp, can you recompile qemu with this patch and see if it
causes the same slowdown you saw w/ vhost?

diff --git a/hw/virtio-net.c b/hw/virtio-net.c
index 0b03b57..0c864f7 100644
--- a/hw/virtio-net.c
+++ b/hw/virtio-net.c
@@ -496,7 +496,7 @@ static int receive_header(VirtIONet *n, struct iovec *iov, int iovcnt,
     if (n->has_vnet_hdr) {
         memcpy(hdr, buf, sizeof(*hdr));
         offset = sizeof(*hdr);
-        work_around_broken_dhclient(hdr, buf + offset, size - offset);
+//        work_around_broken_dhclient(hdr, buf + offset, size - offset);
     /* We only ever receive a struct virtio_net_hdr from the tapfd,

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