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

[Kevin O'Connor]

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).

-Kevin