[libvirt] [RFC]Add new mdev interface for QoS

Gao, Ping A ping.a.gao at intel.com
Wed Aug 2 12:59:59 UTC 2017

On 2017/8/2 18:19, Kirti Wankhede wrote:
> On 8/2/2017 3:56 AM, Alex Williamson wrote:
>> On Tue, 1 Aug 2017 13:54:27 +0800
>> "Gao, Ping A" <ping.a.gao at intel.com> wrote:
>>> On 2017/7/28 0:00, Gao, Ping A wrote:
>>>> On 2017/7/27 0:43, Alex Williamson wrote:  
>>>>> [cc +libvir-list]
>>>>> On Wed, 26 Jul 2017 21:16:59 +0800
>>>>> "Gao, Ping A" <ping.a.gao at intel.com> wrote:
>>>>>> The vfio-mdev provide the capability to let different guest share the
>>>>>> same physical device through mediate sharing, as result it bring a
>>>>>> requirement about how to control the device sharing, we need a QoS
>>>>>> related interface for mdev to management virtual device resource.
>>>>>> E.g. In practical use, vGPUs assigned to different quests almost has
>>>>>> different performance requirements, some guests may need higher priority
>>>>>> for real time usage, some other may need more portion of the GPU
>>>>>> resource to get higher 3D performance, corresponding we can define some
>>>>>> interfaces like weight/cap for overall budget control, priority for
>>>>>> single submission control.
>>>>>> So I suggest to add some common attributes which are vendor agnostic in
>>>>>> mdev core sysfs for QoS purpose.  
>>>>> I think what you're asking for is just some standardization of a QoS
>>>>> attribute_group which a vendor can optionally include within the
>>>>> existing mdev_parent_ops.mdev_attr_groups.  The mdev core will
>>>>> transparently enable this, but it really only provides the standard,
>>>>> all of the support code is left for the vendor.  I'm fine with that,
>>>>> but of course the trouble with and sort of standardization is arriving
>>>>> at an agreed upon standard.  Are there QoS knobs that are generic
>>>>> across any mdev device type?  Are there others that are more specific
>>>>> to vGPU?  Are there existing examples of this that we can steal their
>>>>> specification?  
>>>> Yes, you are right, standardization QoS knobs are exactly what I wanted.
>>>> Only when it become a part of the mdev framework and libvirt, then QoS
>>>> such critical feature can be leveraged by cloud usage. HW vendor only
>>>> need to focus on the implementation of the corresponding QoS algorithm
>>>> in their back-end driver.
>>>> Vfio-mdev framework provide the capability to share the device that lack
>>>> of HW virtualization support to guests, no matter the device type,
>>>> mediated sharing actually is a time sharing multiplex method, from this
>>>> point of view, QoS can be take as a generic way about how to control the
>>>> time assignment for virtual mdev device that occupy HW. As result we can
>>>> define QoS knob generic across any device type by this way. Even if HW
>>>> has build in with some kind of QoS support, I think it's not a problem
>>>> for back-end driver to convert mdev standard QoS definition to their
>>>> specification to reach the same performance expectation. Seems there are
>>>> no examples for us to follow, we need define it from scratch.
>>>> I proposal universal QoS control interfaces like below:
>>>> Cap: The cap limits the maximum percentage of time a mdev device can own
>>>> physical device. e.g. cap=60, means mdev device cannot take over 60% of
>>>> total physical resource.
>>>> Weight: The weight define proportional control of the mdev device
>>>> resource between guests, it’s orthogonal with Cap, to target load
>>>> balancing. E.g. if guest 1 should take double mdev device resource
>>>> compare with guest 2, need set weight ratio to 2:1.
>>>> Priority: The guest who has higher priority will get execution first,
>>>> target to some real time usage and speeding interactive response.
>>>> Above QoS interfaces cover both overall budget control and single
>>>> submission control. I will sent out detail design later once get aligned.  
>>> Hi Alex,
>>> Any comments about the interface mentioned above?
>> Not really.
>> Kirti, are there any QoS knobs that would be interesting
>> for NVIDIA devices?
> We have different types of vGPU for different QoS factors.
> When mdev devices are created, its resources are allocated irrespective
> of which VM/userspace app is going to use that mdev device. Any
> parameter we add here should be tied to particular mdev device and not
> to the guest/app that are going to use it. 'Cap' and 'Priority' are
> along that line. All mdev device might not need/use these parameters,
> these can be made optional interfaces.

We also define some QoS parameters in Intel vGPU types, but it only
provided a default fool-style way. We still need a flexible approach
that give user the ability to change QoS parameters freely and
dynamically according to their requirement , not restrict to the current
limited and static vGPU types.

> In the above proposal, I'm not sure how 'Weight' would work for mdev
> devices on same physical device.
> In the above example, "if guest 1 should take double mdev device
> resource compare with guest 2" but what if guest 2 never booted, how
> will you calculate resources?

Cap is try to limit the max physical GPU resource for vGPU, it's a
vertical limitation, but weight is a horizontal limitation that define
the GPU resource consumption ratio between vGPUs. Cap is easy to
understand as it's just a percentage. For weight. for example, if we
define the max weight is 16, the vGPU_1 who get weight 8 should been
assigned double GPU resources compared to the vGPU_2 whose weight is 4,
we can translate it to this formula:  resource_of_vGPU_1 = 8 / (8+4) *

If there is only one guest exist, then there is no target to compare, 
weight become meaningless and the single guest enjoy the whole physical GPU.

> If libvirt/other toolstack decides to do smart allocation based on type
> name without taking physical host device as input, guest 1 and guest 2
> might get mdev devices created on different physical device. Then would
> weightage matter here?

What your mean if it's the case that there are two discrete GPU cards
exist and the vGPU types can be freely allocated on them, IMO the
back-end driver should handle such case, as the number of physical
device is transparent to tool stack. e.g. present multi-physical device
as a logic one to mdev.

> Thanks,
> Kirti
>> Implementing libvirt support at the same time might be an interesting
>> exercise if we don't have a second user in the kernel to validate
>> against.  We could at least have two communities reviewing the feature
>> then.  Thanks,
>> Alex

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