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

[Anthony Liguori]

On 08/03/2010 12:01 PM, Avi Kivity wrote:
> > You mean, only one class of users cares about the performance of 
> > loading an initrd.  However, you've also argued in other threads how 
> > important it is not to break libvirt even if it means we have to do 
> > silly things (like change help text).
> >
> > So... why is it that libguestfs has to change itself and yet we 
> > should bend over backwards so libvirt doesn't have to change itself?
>
>
> libvirt is a major user that is widely deployed, and would be 
> completely broken if we change -help.  Changing -help is of no 
> consequence to us.
> libguestfs is a (pardon me) minor user that is not widely used, and 
> would suffer a performance regression, not total breakage, unless we 
> add a fw-dma interface.  Adding the interface is of consequence to us: 
> we have to implement live migration and backwards compatibility, and 
> support this new interface for a long while.

I certainly buy the argument about making changes of little consequence 
to us vs. ones that we have to be concerned about long term.

However, I don't think we can objectively differentiate between a 
"major" and "minor" user.  Generally speaking, I would rather that we 
not take the position of "you are a minor user therefore we're not going 
to accommodate you".

Regards,

Anthony Liguori

> In an ideal world we wouldn't tolerate any regression.  The world is 
> not ideal, so we prioritize.
>
> the -help change scores very high on benfit/cost.  fw-dma, much lower.
>
> Note in both cases the long term solution is for the user to move to 
> another interface (cap reporting, virtio), so adding an interface 
> which would only be abandoned later by its only user drops the 
> benfit/cost ratio even further.