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?
> Looks like I jumped the gun in relating the 99.9% CPU usage for qemu-dm and
> the network.  I start up the HVM domain and without running any tests
> qemu-dm is chewing up 99.9% of the CPU in dom0.  So it appears that the
> 100% CPU qemu usage is a problem by itself.  Looks like the same problem
> Harry Butterworth is seeing.
qemu-dm misbehaving could certainly lead to the netif going very
slowly.

> > 2) How often is the event channel interrupt firing according to
> >    /proc/interrupts?  I see about 50k-150k/second.
> I'm seeing ~ 500/s when netpipe-tcp reports decent throughput at smaller
> buffer sizes and then ~50/s when the throughput drops at larger buffer
> sizes.
How large do they have to be to cause problems?

> > 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.
> Kernel debugging is on.  I also have Oprofile enabled.  I'll build a kernel
> without those and see if it helps.
Worth a shot.  It shouldn't cause the problems with qemu, though.

> > > 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?
> UP kernels in both the domU and dom0 (although the scheduler likes to move
> the 1 vcpu in dom0 around to different physical CPUs).  64-bit kernels on
> both.
I've mostly been testing with 32 bit PAE.  I'll have a go with a 64
bit system on Monday.

Thanks,

Steven.

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