Re: [Qemu-devel] Anyone seeing huge slowdown launching qemu with Linux 2.6.35?

[Gleb Natapov]

On Wed, Aug 04, 2010 at 07:17:30PM -0400, Kevin O'Connor wrote:
> On Wed, Aug 04, 2010 at 06:25:52PM +0300, Gleb Natapov wrote:
> > On Wed, Aug 04, 2010 at 09:57:17AM -0500, Anthony Liguori wrote:
> > > There are better ways like using string I/O and optimizing the PIO
> > > path in the kernel.  That should cut down the 1s slow down with a
> > > 100MB initrd by a bit.  But honestly, shaving a couple hundred ms
> > > further off the initrd load is just not worth it using the current
> > > model.
> > > 
> > The slow down is not 1s any more. String PIO emulation had many bugs
> > that were fixed in 2.6.35. I verified how much time it took to load 100M
> > via fw_cfg interface on older kernel and on 2.6.35. On older kernels on
> > my machine it took ~2-3 second on 2.6.35 it took 26s. Some optimizations
> > that was already committed make it 20s. I have some code prototype that
> > makes it 11s. I don't see how we can get below that, surely not back to
> > ~2-3sec.
> 
> I guess this slowness is primarily for kvm.  I just ran some tests on
> the latest qemu (with TCG).  I pulled in a 400Meg file over fw_cfg
> using the SeaBIOS interface - it takes 9.8 seconds (pretty
> consistently).  Oddly, if I change SeaBIOS to use insb (string pio) it
> takes 11.5 seconds (again, pretty consistently).  These times were
> measured on the host - they don't include the extra time it takes qemu
> to start up (during which it reads the file into its memory).
> 
Yes only KVM is affected, nothing has changed in qemu itself.

--
                        Gleb.