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On 3/23/2018 12:44 PM, Eric Fried wrote:<br>
<blockquote type="cite"
cite="mid:11e51bc9-cc4a-27e1-29f1-3a4c04ce733d@fried.cc">
<pre wrap="">Sundar-
First thought is to simplify by NOT keeping inventory information in
the cyborg db at all. The provider record in the placement service
already knows the device (the provider ID, which you can look up in the
cyborg db) the host (the root_provider_uuid of the provider representing
the device) and the inventory, and (I hope) you'll be augmenting it with
traits indicating what functions it's capable of. That way, you'll
always get allocation candidates with devices that *can* load the
desired function; now you just have to engage your weigher to prioritize
the ones that already have it loaded so you can prefer those.</pre>
</blockquote>
Eric, <br>
Thanks for the response. <br>
<br>
Traits only indicate whether a qualitative capability exists. To
check if a free instance of the requested function exists in the
host, we have to count the total count and free count of the needed
function. Otherwise, we may pick a host because it *can* host a
function, though it doesn't have a free instance of the function.<br>
<br>
IIUC, your reply seems to expect that we can always reprogram a
function as needed. The specific case we are looking at here is one
where no reprogramming is involved. In the terminology of <a
moz-do-not-send="true"
href="https://review.openstack.org/#/c/554717/">Cyborg/Nova
rescheduling spec</a>, this is the pre-programmed scenario
(reasons why an operator may want this are stated in the spec).
However, even if reprogramming is allowed, to prioritize hosts with
free instances of the needed function, we will need to count how
many free instances there are.<br>
<br>
Since we said that only device types will be tracked as resource
classes, and not functions, the scheduler will count available
instances of device types, and Cyborg would have to count the
functions separately. <br>
<br>
Please let me know if I missed something.<br>
<br>
Thanks & Regards,<br>
Sundar <br>
<blockquote type="cite"
cite="mid:11e51bc9-cc4a-27e1-29f1-3a4c04ce733d@fried.cc">
<pre wrap="">
Am I missing something?
efried
On 03/22/2018 11:27 PM, Nadathur, Sundar wrote:
</pre>
<blockquote type="cite">
<pre wrap="">Hi all,
There seems to be a possibility of a race condition in the
Cyborg/Nova flow. Apologies for missing this earlier. (You can refer to
the proposed Cyborg/Nova spec
<a class="moz-txt-link-rfc2396E" href="https://review.openstack.org/#/c/554717/1/doc/specs/rocky/cyborg-nova-sched.rst"><https://review.openstack.org/#/c/554717/1/doc/specs/rocky/cyborg-nova-sched.rst></a>
for details.)
Consider the scenario where the flavor specifies a resource class for a
device type, and also specifies a function (e.g. encrypt) in the extra
specs. The Nova scheduler would only track the device type as a
resource, and Cyborg needs to track the availability of functions.
Further, to keep it simple, say all the functions exist all the time (no
reprogramming involved).
To recap, here is the scheduler flow for this case:
* A request spec with a flavor comes to Nova conductor/scheduler. The
flavor has a device type as a resource class, and a function in the
extra specs.
* Placement API returns the list of RPs (compute nodes) which contain
the requested device types (but not necessarily the function).
* Cyborg will provide a custom filter which queries Cyborg DB. This
needs to check which hosts contain the needed function, and filter
out the rest.
* The scheduler selects one node from the filtered list, and the
request goes to the compute node.
For the filter to work, the Cyborg DB needs to maintain a table with
triples of (host, function type, #free units). The filter checks if a
given host has one or more free units of the requested function type.
But, to keep the # free units up to date, Cyborg on the selected compute
node needs to notify the Cyborg API to decrement the #free units when an
instance is spawned, and to increment them when resources are released.
Therein lies the catch: this loop from the compute node to controller is
susceptible to race conditions. For example, if two simultaneous
requests each ask for function A, and there is only one unit of that
available, the Cyborg filter will approve both, both may land on the
same host, and one will fail. This is because Cyborg on the controller
does not decrement resource usage due to one request before processing
the next request.
This is similar to this previous Nova scheduling issue
<a class="moz-txt-link-rfc2396E" href="https://specs.openstack.org/openstack/nova-specs/specs/pike/implemented/placement-claims.html"><https://specs.openstack.org/openstack/nova-specs/specs/pike/implemented/placement-claims.html></a>.
That was solved by having the scheduler claim a resource in Placement
for the selected node. I don't see an analog for Cyborg, since it would
not know which node is selected.
Thanks in advance for suggestions and solutions.
Regards,
Sundar
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