Re: [uml-devel] Performance tests (was: Using UML in libguestfs)

["Richard W.M. Jones" <rjones@redhat.com>]

On Thu, Aug 15, 2013 at 07:37:06PM +0200, richard -rw- weinberger wrote:
> On Wed, Aug 14, 2013 at 7:59 PM, Richard W.M. Jones <rjones@redhat.com> wrote:
> >
> > I performed some benchmarks against KVM:
> >
> > http://rwmj.wordpress.com/2013/08/14/performance-of-user-mode-linux-as-a-libguestfs-backend/#content
> >
> > Notes for those that don't want to read all that, there are at least
> > two bugs I found:
> >
> > (1) fsync /dev/ubda (in the guest) is not honoured by the user-mode
> > linux process.  It should do an fsync on the corresponding backing file.
> 
> What exactly does libguestfs do?

http://libguestfs.org/ ...

> > (2) The ssl<N>=... serial port is really slow, at least 10 times
> > slower than virtio-serial which is the equivalent KVM feature.
> > libguestfs is chatty over the serial port, and so this has a
> > significant knock-on impact all over the place.
> 
> So, libguestfs transfers all data over a serial line?!

Well, it requires an 8-bit clean, reliable, bi-directional connection
into the guest.  We used to use TCP/IP but it's pretty slow compared
to virtio-serial and has a bunch of other problems.

For the UML backend I used the serial port.  There's no problem with
it -- it works and it's reliable, it's just slow.

> We could hack a faster transport into UML.
> As UML is only a process we can very easily access
> the host.
> Have looked at hostfs?

Not sure how hostfs would help?

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine.  Supports Linux and Windows.
http://people.redhat.com/~rjones/virt-df/

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