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

[Gleb Natapov <gleb@redhat.com>]

On Wed, Aug 04, 2010 at 09:57:17AM -0500, Anthony Liguori wrote:
> On 08/04/2010 09:51 AM, David S. Ahern wrote:
> >
> >On 08/03/10 12:43, Avi Kivity wrote:
> >>libguestfs does not depend on an x86 architectural feature.
> >>qemu-system-x86_64 emulates a PC, and PCs don't have -kernel.  We should
> >>discourage people from depending on this interface for production use.
> >That is a feature of qemu - and an important one to me as well. Why
> >should it be discouraged? You end up at the same place -- a running
> >kernel and in-ram filesystem; why require going through a bootloader
> >just because the hardware case needs it?
> 
> It's smoke and mirrors.  We're still providing a boot loader it's
> just a little tiny one that we've written soley for this purpose.
> 
> And it works fine for production use.  The question is whether we
> ought to be aggressively optimizing it for large initrd sizes.  To
> be honest, after a lot of discussion of possibilities, I've come to
> the conclusion that it's just not worth it.
> 
> 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.

> If this is important to someone, we ought to look at refactoring the
> loader completely to be disk based which is a higher performance
> interface.
> 
> Regards,
> 
> Anthony Liguori
> 
> >David
> 
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--
			Gleb.