Re: performance of virtual functions compared to virtio

[David Ahern <dsahern@gmail.com>]

On 04/25/11 15:02, Alex Williamson wrote:
> On Mon, 2011-04-25 at 14:40 -0600, David Ahern wrote:
>>
>> On 04/25/11 14:27, Alex Williamson wrote:
>>> On Mon, 2011-04-25 at 13:49 -0600, David Ahern wrote:
>>>>
>>>> On 04/25/11 13:29, Alex Williamson wrote:
>>>>> So we're effectively getting host-host latency/throughput for the VF,
>>>>> it's just that in the 82576 implementation of SR-IOV, the VF takes a
>>>>> latency hit that puts it pretty close to virtio.  Unfortunate.  I think
>>>>
>>>> For host-to-VM using VFs is worse than virtio which is counterintuitive.
>>>
>>> On the same host, just think about the data path of one versus the
>>> other.  On the guest side, there's virtio vs a physical NIC.  virtio is
>>> designed to be virtualization friendly, so hopefully has less context
>>> switches in setting up and processing transactions.  Once the packet
>>> leaves the assigned physical NIC, it has to come back up the entire host
>>> I/O stack, while the virtio device is connected to an internal bridge
>>> and bypasses all but the upper level network routing.
>>
>> I get the virtio path, but you lost me on the physical NIC. I thought
>> the point of VFs is to bypass the host from having to touch the packet,
>> so the processing path with a VM using a VF would be the same as a non-VM.
> 
> In the VF case, the host is only involved in processing the packet on
> it's end of the connection, but the packet still has to go all the way
> out to the physical device and all the way back.  Handled on one end by
> the VM and the other end by the host.
> 
> An analogy might be sending a letter to an office coworker in a
> neighboring cube.  You could just pass the letter over the wall (virtio)
> or you could go put it in the mailbox, signal the mail carrier, who
> comes and moves it to your neighbor's mailbox, who then gets signaled
> that they have a letter (device assignment).
> 
> Since the networks stacks are completely separate from one another,
> there's very little difference in data path whether you're talking to
> the host, a remote system, or a remote VM, which is reflected in your
> performance data.  Hope that helps,

Got you. I was thinking host-VM as VM on separate host; I didn't make
that clear. Thanks for clarifying - I like the letter example.

David

> 
> Alex
>