[openstack-dev] [Nova] [Cyborg] Tracking multiple functions

Matthew Booth mbooth at redhat.com
Sat Mar 3 16:15:10 UTC 2018

On 2 March 2018 at 14:31, Jay Pipes <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?

>> 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?


Matthew Booth
Red Hat OpenStack Engineer, Compute DFG

Phone: +442070094448 (UK)
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