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

[Alexander Graf]

On 04.08.2010, at 19:14, Avi Kivity wrote:

> On 08/04/2010 08:01 PM, Alexander Graf wrote:
>> 
>>> 
>>>> 2) Using a different interface (that could also be DMA fw_cfg - remember, 
>>>> we're on a private interface anyways)
>>> A guest/host interface is not private.
>> fw_cfg is as private as it gets with host/guest interfaces. It's about as 
>> close as CPU specific MSRs or SMC chips.
>> 
> 
> Well, it isn't.  Two external projects already use it.  You can't change it 
> due to the needs to live migrate from older versions.

You can always extend it. You can even break it with a new -M.

> 
>>>> Admittedly 1 would also help in more cases than just booting with -kernel 
>>>> and -initrd, but if that won't get us to acceptable levels (and yes, 8 
>>>> seconds for 100MB is unacceptable) I don't see any way around 2.
>>> 3) don't use -kernel for 100MB or more.  It's not the right tool.
>> Why not? You're the one always ranting about caring about users. Now you get 
>> at least 3 users from the Qemu development community actually using a 
>> feature and you just claim it's wrong? Please, we've added way more useless 
>> features for worse reasons.
>> 
> 
> It's not wrong in itself, but using it with supersized initrds is wrong.  The 
> data is stored in qemu, host pagecache, and the guest, so three copies, it's 
> limited by guest RAM, has to be live migrated.  Sure we could optimize it, 
> but it's better to spend our efforts on more mainstream users.

It's only stored twice. The host pagecache copy is gone during the lifetime of 
the VM. Migration also doesn't make sense for most -kernel/-initrd use cases. 
And it's awesome for fast prototyping. Of course, once that fast becomes dog 
slow, it's not useful anymore.

I bet within the time everybody spent on this thread we would have a working 
and stable DMA fw_cfg interface plus extra spare time for supporting breakage 
already.


Alex