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

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 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.

> Also, mdev devices are not necessarily the exclusive users of the
> hardware, we can have a native user such as a local X client.  They're
> not an mdev user, so we can't support them via the mdev_attr_group.
> Does there need to be a per mdev parent QoS attribute_group standard
> for somehow defining the QoS of all the child mdev devices, or perhaps
> representing the remaining host QoS attributes?

That's really an open, if we don't take host workload into consideration
for cloud usage, it's not a problem any more, however such assumption is
not reasonable. Any way if we take mdev devices as clients of host
driver, and host driver provide the capability to divide out a portion
HW resource to mdev devices, then it's only need to take care about the
resource that host assigned for mdev devices. Follow this way QoS for
mdev focus on the relationship between mdev devices no need to take care
the host workload.


> Ultimately libvirt and upper level management tools would be the
> consumer of these control knobs, so let's immediately get libvirt
> involved in the discussion.  Thanks,
> Alex

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