Open Stack

From: Matthew Booth [mailto:mbooth at redhat.com]
Sent: Saturday, March 3, 2018 4:15 PM
To: OpenStack Development Mailing List (not for usage questions) <openstack-dev at lists.openstack.org>
Subject: Re: [openstack-dev] [Nova] [Cyborg] Tracking multiple functions

On 2 March 2018 at 14:31, Jay Pipes <jaypipes at gmail.com<mailto:jaypipes at gmail.com>> wrote:
On 03/02/2018 02:00 PM, Nadathur, Sundar wrote:
Hello Nova team,

     During the Cyborg discussion at Rocky PTG, we proposed a flow for FPGAs wherein the request spec asks for a device type as a resource class, and optionally a function (such as encryption) in the extra specs. This does not seem to work well for the usage model that I’ll describe below.

An FPGA device may implement more than one function. For example, it may implement both compression and encryption. Say a cluster has 10 devices of device type X, and each of them is programmed to offer 2 instances of function A and 4 instances of function B. More specifically, the device may implement 6 PCI functions, with 2 of them tied to function A, and the other 4 tied to function B. So, we could have 6 separate instances accessing functions on the same device.

Does this imply that Cyborg can't reprogram the FPGA at all?
[Mooney, Sean K] cyborg is intended to support fixed function acclerators also so it will not always be able to program the accelerator. In this case where an fpga is preprogramed with a multi function bitstream that is statically provisioned cyborge will not be able to reprogram the slot if any of the fuctions from that slot are already allocated to an instance. In this case it will have to treat it like a fixed function device and simply allocate a unused  vf  of the corret type if available.



In the current flow, the device type X is modeled as a resource class, so Placement will count how many of them are in use. A flavor for ‘RC device-type-X + function A’ will consume one instance of the RC device-type-X.  But this is not right because this precludes other functions on the same device instance from getting used.

One way to solve this is to declare functions A and B as resource classes themselves and have the flavor request the function RC. Placement will then correctly count the function instances. However, there is still a problem: if the requested function A is not available, Placement will return an empty list of RPs, but we need some way to reprogram some device to create an instance of function A.

Clearly, nova is not going to be reprogramming devices with an instance of a particular function.

Cyborg might need to have a separate agent that listens to the nova notifications queue and upon seeing an event that indicates a failed build due to lack of resources, then Cyborg can try and reprogram a device and then try rebuilding the original request.

It was my understanding from that discussion that we intend to insert Cyborg into the spawn workflow for device configuration in the same way that we currently insert resources provided by Cinder and Neutron. So while Nova won't be reprogramming a device, it will be calling out to Cyborg to reprogram a device, and waiting while that happens.
My understanding is (and I concede some areas are a little hazy):
* The flavors says device type X with function Y
* Placement tells us everywhere with device type X
* A weigher orders these by devices which already have an available function Y (where is this metadata stored?)
* Nova schedules to host Z
* Nova host Z asks cyborg for a local function Y and blocks
  * Cyborg hopefully returns function Y which is already available
  * If not, Cyborg reprograms a function Y, then returns it
Can anybody correct me/fill in the gaps?
[Mooney, Sean K] that correlates closely to my recollection also. As for the metadata I think the weigher may need to call to cyborg to retrieve this as it will not be available in the host state object.
Matt


--
Matthew Booth
Red Hat OpenStack Engineer, Compute DFG

Phone: +442070094448 (UK)

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