>>> On 8/6/2009 at 11:50 AM, in message <4A7AFBE3.5080200@xxxxxxxxxx>, Avi Kivity <avi@xxxxxxxxxx> wrote: > On 08/06/2009 06:40 PM, Arnd Bergmann wrote: >> 3. The ioq method seems to be the real core of your work that makes >> venet perform better than virtio-net with its virtqueues. I don't see >> any reason to doubt that your claim is correct. My conclusion from >> this would be to add support for ioq to virtio devices, alongside >> virtqueues, but to leave out the extra bus_type and probing method. >> > > The current conjecture is that ioq outperforms virtio because the host > side of ioq is implemented in the host kernel, while the host side of > virtio is implemented in userspace. AFAIK, no one pointed out > differences in the protocol which explain the differences in performance. There *are* protocol difference that matter, though I think they are slowly being addressed. For an example: Earlier versions of virtio-pci had a single interrupt for all ring events, and you had to do an extra MMIO cycle to learn the proper context. That will hurt...a _lot_ especially for latency. I think recent versions of KVM switched to MSI-X per queue which fixed this particular ugly. However, generally I think Avi is right. The main reason why it outperforms virtio-pci by such a large margin has more to do with all the various inefficiencies in the backend (such as requiring multiple hops U->K, K->U per packet), coarse locking, lack of parallel processing, etc. I went through and streamlined all the bottlenecks (such as putting the code in the kernel, reducing locking/context switches, etc). I have every reason to believe that someone will skills/time equal to myself could develop a virtio-based backend that does not use vbus and achieve similar numbers. However, as stated in my last reply, I am interested in this backend supporting more than KVM, and I designed vbus to fill that role. Therefore, it does not interest me to endeavor such an effort if it doesn't involve a backend that is independent of KVM. Based on this, I will continue my efforts surrounding to use of vbus including its use to accelerate KVM for AlacrityVM. If I can find a way to do this in such a way that KVM upstream finds acceptable, I would be very happy and will work towards whatever that compromise might be. OTOH, if the KVM community is set against the concept of a generalized/shared backend, and thus wants to use some other approach that does not involve vbus, that is fine too. Choice is one of the great assets of open source, eh? :) Kind Regards, -Greg -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
