Re: virtio-net mq vq initialization

[Sasha Levin <sasha.levin@oracle.com>]

On 04/12/2013 07:36 AM, Rusty Russell wrote:
> Sasha Levin <sasha.levin@oracle.com> writes:
>> On 04/11/2013 12:36 PM, Will Deacon wrote:
>>> Hello folks,
>>>
>>> Here's the latest round of ARM fixes and updates for kvmtool. Most of
>>> this is confined to the arm/ subdirectory, with the exception of a fix
>>> to the virtio-mmio vq definitions due to the multi-queue work from
>>> Sasha. I'm not terribly happy about that code though, since it seriously
>>> increases the memory footprint of the guest.
>>>
>>> Without multi-queue, we can boot Debian Wheezy to a prompt in 38MB. With
>>> the new changes, that increases to 170MB! Any chance we can try and tackle
>>> this regression please? I keep getting bitten by the OOM killer :(
>>
>> (cc Rusty, MST)
>>
>> The spec defines the operation of a virtio-net device with regards to multiple
>> queues as follows:
>>
>> """
>> Device Initialization
>>
>> 	1. The initialization routine should identify the receive and transmission
>> virtqueues, up to N+1 of each kind. If VIRTIO_NET_F_MQ feature
>> bit is negotiated, N=max_virtqueue_pairs-1, otherwise identify N=0.
>>
>> 	[...]
>>
>> 	5. Only receiveq0, transmitq0 and controlq are used by default. To use more
>> queues driver must negotiate the VIRTIO_NET_F_MQ feature; initialize
>> up to max_virtqueue_pairs of each of transmit and receive queues; execute_
>> VIRTIO_NET_CTRL_MQ_VQ_PAIRS_SET command specifying
>> the number of the transmit and receive queues that is going to be
>> used and wait until the device consumes the controlq buffer and acks this
>> command.
>> """
>>
>> And kvmtool follows that to the letter: It will initialize the maximum amount of
>> queues it can support during initialization and will start using them only when
>> the device tells it it should use them.
>>
>> As Will has stated, this causes a memory issue since all the data structures that hold
>> all possible queues get initialized regardless of whether we actually need them or not,
>> which is quite troublesome for systems with small RAM.
>>
>>
>> Rusty, MST, would you be open to a spec and code change that would initialize the
>> RX/TX vqs on demand instead of on device initialization? Or is there an easier way
>> to work around this issue?
> 
> I'm confused.  kvmtool is using too much memory, or the guest?  If
> kvmtool, the Device Initialization section above applies to the driver,
> not the device.  If the guest, well, the language says "UP TO N+1".  You
> want a small guest, don't use them all.  Or any...
> 
> What am I missing?

It's in the guest - sorry. I was only trying to say that kvmtool doesn't do anything
odd with regards to initializing virtio-net.

The thing is that there should be a difference between just allowing a larger number
of queues and actually using them (i.e. enabling them with ethtool). Right now I see
the kernel lose 130MB just by having kvmtool offer 8 queue pairs, without actually
using those queues.

Yes, we can make it configurable in kvmtool (and I will make it so so the arm folks
could continue working with tiny guests) but does it make sense that you have to do
this configuration in *2* places? First in the hypervisor and then inside the guest?

Memory usage should ideally depend on whether you are actually using multiple queues,
not on whether you just allow using those queues.


Thanks,
Sasha