.. This work is licensed under a Creative Commons Attribution 3.0 Unported License. http://creativecommons.org/licenses/by/3.0/legalcode =========================================================== Resource tracker allows virt driver to update provider tree =========================================================== https://blueprints.launchpad.net/nova/+spec/update-provider-tree In the movement towards using placement for scheduling and resource management, the virt driver method ``get_available_resource`` was initially superseded by ``get_inventory``, whereby the driver could specify its inventory in terms understood by placement. In Queens, a `get_traits`_ driver method was added. But ``get_inventory`` is limited to expressing only inventory (not traits or aggregates). And both of these methods are limited to the resource provider corresponding to the compute node. Recent developments such as `Nested Resource Providers`_ necessitate the ability for the virt driver to have deeper control over what the resource tracker configures in placement on behalf of the compute node. This blueprint proposes a new virt driver method, ``update_provider_tree``, and its method of consumption by the resource tracker, allowing full control over the placement representation of the compute node and its associated providers and metadata. Problem description =================== Existing virt driver methods are limited in their ability to express resource provider information. Use Cases --------- As a virt driver developer, I wish to be able to model my compute node and associated entities as any combination of provider trees and sharing providers, along with inventories, traits, and aggregate associations for those providers. Proposed change =============== ComputeDriver.update_provider_tree ---------------------------------- ``ComputeDriver.update_provider_tree`` is introduced. It accepts two parameters: * A ``nova.compute.provider_tree.ProviderTree`` object representing all the providers in the tree associated with the compute node, and any sharing providers (those with the ``MISC_SHARES_VIA_AGGREGATE`` trait) associated via aggregate with any of those providers (but not *their* tree- or aggregate-associated providers), as currently known by placement. This object is fully owned by the ``update_provider_tree`` method, and can therefore be modified without locking/concurrency considerations. Note, however, that it may contain providers not directly owned/controlled by the compute host. Care must be taken not to remove or modify such providers inadvertently. * String name of the compute node (i.e. ``ComputeNode.hypervisor_hostname``) for which the caller is updating providers and inventory. Drivers may use this to help identify the compute node provider in the ProviderTree. Drivers managing more than one node (e.g. ironic) may also use it as a cue to indicate which node is being updated. The virt driver is expected to update the ProviderTree object with current resource provider and inventory information. When the method returns, the ProviderTree should represent the correct hierarchy of nested resource providers associated with this compute node, as well as the inventory, aggregates, and traits associated with those resource providers. .. note:: Despite the name, a ProviderTree instance may in fact contain more than one tree. For purposes of this specification, the ProviderTree passed to ``update_provider_tree`` will contain: * the entire tree associated with the compute node; and * any sharing providers (those with the ``MISC_SHARES_VIA_AGGREGATE`` trait) which are associated via aggregate with any of the providers in the compute node's tree. The sharing providers will be presented as lone roots in the ProviderTree, even if they happen to be part of a tree themselves. Consider the example below. ``SSP`` is a shared storage provider and ``BW1`` and ``BW2`` are shared bandwidth providers; all three have the ``MISC_SHARES_VIA_AGGREGATE`` trait:: CN1 SHR_ROOT CN2 / \ agg1 / /\ agg1 / \ NUMA1 NUMA2--------SSP--/--\-----------NUMA1 NUMA2 / \ / \ / \ / \ / \ PF1 PF2 PF3 PF4--------BW1 BW2------PF1 PF2 PF3 PF4 agg2 agg3 When ``update_provider_tree`` is invoked for ``CN1``, it is passed a ProviderTree containing:: CN1 (root) / \ agg1 NUMA1 NUMA2-------SSP (root) / \ / \ PF1 PF2 PF3 PF4------BW1 (root) agg2 This method supersedes ``get_inventory`` and ``get_traits``: if this method is implemented, neither ``get_inventory`` nor ``get_traits`` is used. Driver implementations of ``update_provider_tree`` are expected to use public ``ProviderTree`` methods to effect changes to the provider tree passed in. Some of the methods which may be useful are as follows: * ``new_root``: Add a new root provider to the tree. * ``new_child``: Add a new child under an existing provider. * ``data``: Access information (name, UUID, parent, inventory, traits, aggregates) about a provider in the tree. * ``remove``: Remove a provider **and its descendants** from the tree. Use caution in multiple-ownership scenarios. * ``update_inventory``: Set the inventory for a provider. * ``update_traits``: Set the traits for a provider. * ``update_aggregates``: Set the aggregates for a provider. .. note:: There is no supported mechanism for ``update_provider_tree`` to effect changes to allocations. This is intentional: in Nova, allocations are managed exclusively outside of virt. (Usually by the scheduler; sometimes - e.g. for migrations - by the conductor.) Porting from get_inventory ~~~~~~~~~~~~~~~~~~~~~~~~~~ Virt driver developers wishing to move from ``get_inventory`` to ``update_provider_tree`` should use the ``ProviderTree.update_inventory`` method, specifying the compute node as the provider and the same inventory as returned by ``get_inventory``. For example: .. code:: def get_inventory(self, nodename): inv_data = { 'VCPU': { ... }, 'MEMORY_MB': { ... }, 'DISK_GB': { ... }, } return inv_data would become: .. code:: def update_provider_tree(self, provider_tree, nodename): inv_data = { 'VCPU': { ... }, 'MEMORY_MB': { ... }, 'DISK_GB': { ... }, } provider_tree.update_inventory(nodename, inv_data) Porting from get_traits ~~~~~~~~~~~~~~~~~~~~~~~ To replace ``get_traits``, developers should use the ``ProviderTree.update_traits`` method, specifying the compute node as the provider and the same traits as returned by ``get_traits``. For example: .. code:: def get_traits(self, nodename): traits = ['HW_CPU_X86_AVX', 'HW_CPU_X86_AVX2', 'CUSTOM_GOLD'] return traits would become: .. code:: def update_provider_tree(self, provider_tree, nodename): traits = ['HW_CPU_X86_AVX', 'HW_CPU_X86_AVX2', 'CUSTOM_GOLD'] provider_tree.update_traits(nodename, traits) SchedulerReportClient.update_from_provider_tree ----------------------------------------------- This is the report client method responsible for accepting the ProviderTree as modified by the virt driver via ``update_provider_tree`` and making the necessary placement API calls to ensure that the representation in the placement service matches it. In particular: * Providers removed by ``update_provider_tree`` are removed from placement. * Providers added by ``update_provider_tree`` are created in placement. * If inventories, traits, or aggregates were changed for any providers by ``update_provider_tree``, those changes are flushed back to placement. .. note:: In multiple-ownership scenarios, virt drivers should be careful not to remove or modify providers not owned by the compute host. ResourceTracker._update ----------------------- This is where the virt driver is asked to report on compute resources. It is where, for example, the call to ``get_inventory`` was added to supersede the data returned by ``get_available_resource`` if ``get_inventory`` is implemented. Here we add another level to allow ``update_provider_tree`` to supersede ``get_inventory``. The logic changes from: .. code:: try: ComputeDriver.get_inventory() except NotImplementedError: SchedulerReportClient.update_compute_node() try: ComputeDriver.get_traits() except NotImplementedError: pass to: .. code:: try: ComputeDriver.update_provider_tree() SchedulerReportClient.update_from_provider_tree() except NotImplementedError: try: ComputeDriver.get_inventory() except NotImplementedError: SchedulerReportClient.update_compute_node() try: ComputeDriver.get_traits() except NotImplementedError: pass Alternatives ------------ * Continue to provide piecemeal methods in the spirit of ``get_inventory`` and ``get_traits``. The proposed solution can subsume the functionality of both of those methods and more, but it can also grow along with placement and Nova's use thereof. * Allow virt drivers direct control over placement. While we can't stop out-of-tree drivers from doing this, it has been discussed and decided that in-tree drivers should be funneled through the choke point of the SchedulerReportClient for actual placement API communication. Data model impact ----------------- None REST API impact --------------- None Security impact --------------- None Notifications impact -------------------- None Other end user impact --------------------- No direct impact. This change, followed by virt drivers implementing ``update_provider_tree``, followed by virt drivers extending their resource provider models, will ultimately allow operators to exert more power over scheduling operations. Performance Impact ------------------ This change increases the amount of traffic to the placement service, which has the potential to affect performance. However, there is as yet no evidence that doing lots of placement calls is "expensive" relative to the other processing occurring in these code paths. The intent is to mitigate such impact if and when it is demonstrated to be problematic. One mitigation strategy, already largely implemented, is caching the placement representation locally via a separate ProviderTree instance maintained in the SchedulerReportClient. The specifics are outside the scope of this document. However, the existing code in this area is inconsistent and needs to be codified in a separate specification so we can work towards consistency. Other deployer impact --------------------- None Developer impact ---------------- See above. Upgrade impact -------------- None Implementation ============== Assignee(s) ----------- Primary assignee: efried Work Items ---------- The code for this has been completed. Some of it merged in Queens, including: * https://review.openstack.org/#/c/521187/ introduces the ``update_provider_tree`` method in the ``ComputeDriver`` base class. * https://review.openstack.org/#/c/533821/ implements the ``update_from_provider_tree`` method in the report client. * https://review.openstack.org/#/c/520246/ implements the changes in the resource tracker to use the above. These changes were developed under the `Nested Resource Providers`_ blueprint. Dependencies ============ None (all dependencies have merged in Queens). Continuing development of such features as `Nested Resource Providers`_, `Granular Resource Requests`_, and shared resource providers will expand the range of things driver developers can do through their implementation of ``update_provider_tree``. Testing ======= Extensive functional testing is included in addition to unit tests. Documentation Impact ==================== None References ========== * `Nested Resource Providers`_ spec * Support `Traits`_ in Allocation Candidates spec * Support traits in the Ironic driver spec (`get_traits`_) * `Granular Resource Requests`_ spec .. _`Nested Resource Providers`: https://specs.openstack.org/openstack/nova-specs/specs/queens/approved/nested-resource-providers.html .. _`Traits`: https://specs.openstack.org/openstack/nova-specs/specs/queens/approved/add-trait-support-in-allocation-candidates.html .. _`get_traits`: https://specs.openstack.org/openstack/nova-specs/specs/queens/approved/ironic-driver-traits.html .. _`Granular Resource Requests`: https://specs.openstack.org/openstack/nova-specs/specs/queens/approved/granular-resource-requests.html History ======= .. list-table:: Revisions :header-rows: 1 * - Release Name - Description * - Queens - Code finished and mostly merged. * - Rocky - Figured we really ought to have something written down, so proposed an actual blueprint and this spec.