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

[Avi Kivity <avi@redhat.com>]

  On 08/03/2010 07:53 PM, Anthony Liguori wrote:
> On 08/03/2010 11:50 AM, Avi Kivity wrote:
>>  On 08/03/2010 07:46 PM, Anthony Liguori wrote:
>>>> It doesn't appear to support live migration, or hiding the feature 
>>>> for -M older.
>>>>
>>>> It's not a good path to follow.  Tomorrow we'll need to load 300MB 
>>>> initrds and we'll have to rework this yet again.  Meanwhile the 
>>>> kernel and virtio support demand loading of any image size you'd 
>>>> want to use.
>>>
>>>
>>> firmware is totally broken with respect to -M older FWIW.
>>>
>>
>> Well, then this is adding to the brokenness.
>>
>> fwcfg dma is going to have exactly one user, libguestfs.  Much better 
>> to have libguestfs move to some other interface and improve are 
>> users-to-interfaces ratio.
>
> 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.

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.

-- 
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