Subject: Re: [GIT PULL] AlacrityVM guest drivers for 2.6.33

From: Avi Kivity <[email-masked]>
Date: Mon, 21 Dec 2009 17:43:09 +0200

On 12/21/2009 05:34 PM, Gregory Haskins wrote:
>
>> I think it would be fair to point out that these patches have been objected to
>> by the KVM folks quite extensively,
>>      
> Actually, these patches have nothing to do with the KVM folks.  You are
> perhaps confusing this with the hypervisor-side discussion, of which
> there is indeed much disagreement.
>    

This is true, though these drivers are fairly pointless for 
virtualization without the host side support.

I did have a few issues with the guest drivers:
- the duplication of effort wrt virtio.  These drivers don't cover 
exactly the same problem space, but nearly so.
- no effort at scalability - all interrupts are taken on one cpu
- the patches introduce a new virtual interrupt controller for dubious 
(IMO) benefits

>  From my research, the reason why virt in general, and KVM in particular
> suffers on the IO performance front is as follows: IOs
> (traps+interrupts) are more expensive than bare-metal, and real hardware
> is naturally concurrent (your hbas and nics are effectively parallel
> execution engines, etc).
>
> Assuming my observations are correct, in order to squeeze maximum
> performance from a given guest, you need to do three things:  A)
> eliminate as many IOs as you possibly can, B) reduce the cost of the
> ones you can't avoid, and C) run your algorithms in parallel to emulate
> concurrent silicon.
>    

All these are addressed by vhost-net without introducing new drivers.

-- 
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