Re: Paravirtualised drivers for fully virtualised domains, rev9

[Steven Smith <sos22-xen@srcf.ucam.org>]

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> Here is what I have found so far in trying to chase down the cause of the
> slowdown.
> The qemu-dm process is running 99.9% of the CPU on dom0.
That seems very wrong.  When I try this, the device model is almost
completely idle.  Could you see what strace says, please, or if there
are any strange messages in the /var/log/qemu-dm. file?

> It appears that a lot of time is spent running timers and getting the
> current time.  Not being familiar with the code, I am now crawling through
> it to see how timers are handled and how the xen-vnif PV driver uses them.
Timer handling isn't really changed by any of these patches.  Patch
02.ioemu_xen_evtchns.diff is in vaguely the same area, but I can't see
how it could cause the problems you're seeing, assuming your
hypervisor and libxc are up to date.

What changeset of xen-unstable did you apply the patches to?

> P.S.  This just in from a test running while I typed the above.  I noticed
> that qemu will start a "gui_timer" when VNC is not used.  I normally run
> without graphics (nographic=1 in the domain config file).  I changed the
> config file to use VNC. The qemu-dm CPU utilization in dom0 dropped to
> below 10%.   The network performance improved from 0.19 Mb/s to 9.75 Mb/s
> (still less than the 23.07 Mb/s for a fully virtualized domain).
When I try this, I see about 1600Mb/s between dom0 and a
paravirtualised domU, about 30Mb/s between dom0 and an ioemu domU, and
about 1200Mb/s between dom0 and an HVM domU running these drivers, all
collected using netpipe-tcp.  That is a regression, but much smaller
than you're seeing.

There are a couple of obvious things to check:

1) Do the statistics reported by ifconfig show any errors?
2) How often is the event channel interrupt firing according to
	/proc/interrupts?  I see about 50k-150k/second.
3) Is there any packet loss when you ping a domain?  Start your test
	and run a ping in parallel.

The other thing is that these drivers seem to be very sensitive to
kernel debugging options in the domU.  If you've got anything enabled
in the kernel hacking menu it might be worth trying again with that
switched off.

> It appears there is some interaction between using the xen-vnif
> driver and the qemu timer code.  I'm still exploring.
I'd be happier if I could reproduce this problem here.  Are you
running SMP?  PAE?  64 bit?  What kernel are you running in the domU?

Steven.

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