This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
Home Products Support Community News


Re: [Xen-devel] Paravirtualised drivers for fully virtualised domains, r

On Fri, 2006-08-11 at 11:17 +0100, Steven Smith wrote:
> > 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?

I haven't tried the patches being discussed in this thread but I'm
seeing similar problems with qemu-dm anyway...

I've been looking into bugzilla 725 and I'm also seeing 100% cpu usage
by qemu-dm.  xm-test uses the nographic flag and I find that if this is
not set then the cpu usage drops to normal levels and the test passes.

> > It appears that a lot of time is spent running timers and getting the
> > current time.

Yes, this is what I was seeing with the nographic flag set.

> 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?

I've been seeing the problem on recent unstable changesets without the
patches.  Changesets 10992, 10949 for example.

> > 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%.

Yep, that's what I see without the patches.

> 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.
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-devel

Xen-devel mailing list

<Prev in Thread] Current Thread [Next in Thread>