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

[Alexander Graf]

On 04.08.2010, at 17:25, Gleb Natapov wrote:

> 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.

What exactly is the reason for the slowdown? It can't be only boundary and 
permission checks, right?


Alex