CPU topologies

The NUMA topology and CPU pinning features in OpenStack provide high-level control over how instances run on hypervisor CPUs and the topology of virtual CPUs available to instances. These features help minimize latency and maximize performance.

Important

In deployments older than Train, or in mixed Stein/Train deployments with a rolling upgrade in progress, unless specifically enabled, live migration is not possible for instances with a NUMA topology when using the libvirt driver. A NUMA topology may be specified explicitly or can be added implicitly due to the use of CPU pinning or huge pages. Refer to bug #1289064 for more information. As of Train, live migration of instances with a NUMA topology when using the libvirt driver is fully supported.

SMP, NUMA, and SMT

Symmetric multiprocessing (SMP)

SMP is a design found in many modern multi-core systems. In an SMP system, there are two or more CPUs and these CPUs are connected by some interconnect. This provides CPUs with equal access to system resources like memory and input/output ports.

Non-uniform memory access (NUMA)

NUMA is a derivative of the SMP design that is found in many multi-socket systems. In a NUMA system, system memory is divided into cells or nodes that are associated with particular CPUs. Requests for memory on other nodes are possible through an interconnect bus. However, bandwidth across this shared bus is limited. As a result, competition for this resource can incur performance penalties.

Simultaneous Multi-Threading (SMT)

SMT is a design complementary to SMP. Whereas CPUs in SMP systems share a bus and some memory, CPUs in SMT systems share many more components. CPUs that share components are known as thread siblings. All CPUs appear as usable CPUs on the system and can execute workloads in parallel. However, as with NUMA, threads compete for shared resources.

Non-Uniform I/O Access (NUMA I/O)

In a NUMA system, I/O to a device mapped to a local memory region is more efficient than I/O to a remote device. A device connected to the same socket providing the CPU and memory offers lower latencies for I/O operations due to its physical proximity. This generally manifests itself in devices connected to the PCIe bus, such as NICs or vGPUs, but applies to any device support memory-mapped I/O.

In OpenStack, SMP CPUs are known as cores, NUMA cells or nodes are known as sockets, and SMT CPUs are known as threads. For example, a quad-socket, eight core system with Hyper-Threading would have four sockets, eight cores per socket and two threads per core, for a total of 64 CPUs.

PCPU and VCPU

PCPU

Resource class representing an amount of dedicated CPUs for a single guest.

VCPU

Resource class representing a unit of CPU resources for a single guest approximating the processing power of a single physical processor.

Customizing instance NUMA placement policies

Important

The functionality described below is currently only supported by the libvirt/KVM and Hyper-V driver. The Hyper-V driver may require some host configuration for this to work.

When running workloads on NUMA hosts, it is important that the vCPUs executing processes are on the same NUMA node as the memory used by these processes. This ensures all memory accesses are local to the node and thus do not consume the limited cross-node memory bandwidth, adding latency to memory accesses. Similarly, large pages are assigned from memory and benefit from the same performance improvements as memory allocated using standard pages. Thus, they also should be local. Finally, PCI devices are directly associated with specific NUMA nodes for the purposes of DMA. Instances that use PCI or SR-IOV devices should be placed on the NUMA node associated with these devices.

NUMA topology can exist on both the physical hardware of the host and the virtual hardware of the instance. In OpenStack, when booting a process, the hypervisor driver looks at the NUMA topology field of both the instance and the host it is being booted on, and uses that information to generate an appropriate configuration.

By default, an instance floats across all NUMA nodes on a host. NUMA awareness can be enabled implicitly through the use of huge pages or pinned CPUs or explicitly through the use of flavor extra specs or image metadata. If the instance has requested a specific NUMA topology, compute will try to pin the vCPUs of different NUMA cells on the instance to the corresponding NUMA cells on the host. It will also expose the NUMA topology of the instance to the guest OS.

In all cases where NUMA awareness is used, the NUMATopologyFilter filter must be enabled. Details on this filter are provided in Compute schedulers.

Caution

The NUMA node(s) used are normally chosen at random. However, if a PCI passthrough or SR-IOV device is attached to the instance, then the NUMA node that the device is associated with will be used. This can provide important performance improvements. However, booting a large number of similar instances can result in unbalanced NUMA node usage. Care should be taken to mitigate this issue. See this discussion for more details.

Caution

Inadequate per-node resources will result in scheduling failures. Resources that are specific to a node include not only CPUs and memory, but also PCI and SR-IOV resources. It is not possible to use multiple resources from different nodes without requesting a multi-node layout. As such, it may be necessary to ensure PCI or SR-IOV resources are associated with the same NUMA node or force a multi-node layout.

When used, NUMA awareness allows the operating system of the instance to intelligently schedule the workloads that it runs and minimize cross-node memory bandwidth. To restrict an instance’s vCPUs to a single host NUMA node, run:

$ openstack flavor set [FLAVOR_ID] --property hw:numa_nodes=1

Some workloads have very demanding requirements for memory access latency or bandwidth that exceed the memory bandwidth available from a single NUMA node. For such workloads, it is beneficial to spread the instance across multiple host NUMA nodes, even if the instance’s RAM/vCPUs could theoretically fit on a single NUMA node. To force an instance’s vCPUs to spread across two host NUMA nodes, run:

$ openstack flavor set [FLAVOR_ID] --property hw:numa_nodes=2

The allocation of instances vCPUs and memory from different host NUMA nodes can be configured. This allows for asymmetric allocation of vCPUs and memory, which can be important for some workloads. To spread the 6 vCPUs and 6 GB of memory of an instance across two NUMA nodes and create an asymmetric 1:2 vCPU and memory mapping between the two nodes, run:

$ openstack flavor set [FLAVOR_ID] --property hw:numa_nodes=2
# configure guest node 0
$ openstack flavor set [FLAVOR_ID] \
  --property hw:numa_cpus.0=0,1 \
  --property hw:numa_mem.0=2048
# configure guest node 1
$ openstack flavor set [FLAVOR_ID] \
  --property hw:numa_cpus.1=2,3,4,5 \
  --property hw:numa_mem.1=4096

Note

Hyper-V does not support asymmetric NUMA topologies, and the Hyper-V driver will not spawn instances with such topologies.

For more information about the syntax for hw:numa_nodes, hw:numa_cpus.N and hw:num_mem.N, refer to the NUMA topology guide.

Customizing instance CPU pinning policies

Important

The functionality described below is currently only supported by the libvirt/KVM driver and requires some host configuration for this to work. Hyper-V does not support CPU pinning.

Note

There is no correlation required between the NUMA topology exposed in the instance and how the instance is actually pinned on the host. This is by design. See this invalid bug for more information.

By default, instance vCPU processes are not assigned to any particular host CPU, instead, they float across host CPUs like any other process. This allows for features like overcommitting of CPUs. In heavily contended systems, this provides optimal system performance at the expense of performance and latency for individual instances.

Some workloads require real-time or near real-time behavior, which is not possible with the latency introduced by the default CPU policy. For such workloads, it is beneficial to control which host CPUs are bound to an instance’s vCPUs. This process is known as pinning. No instance with pinned CPUs can use the CPUs of another pinned instance, thus preventing resource contention between instances.

CPU pinning policies can be used to determine whether an instance should be pinned or not. There are three policies: dedicated, mixed and shared (the default). The dedicated CPU policy is used to specify that all CPUs of an instance should use pinned CPUs. To configure a flavor to use the dedicated CPU policy, run:

$ openstack flavor set [FLAVOR_ID] --property hw:cpu_policy=dedicated

This works by ensuring PCPU allocations are used instead of VCPU allocations. As such, it is also possible to request this resource type explicitly. To configure this, run:

$ openstack flavor set [FLAVOR_ID] --property resources:PCPU=N

where N is the number of vCPUs defined in the flavor.

Note

It is not currently possible to request PCPU and VCPU resources in the same instance.

The shared CPU policy is used to specify that an instance should not use pinned CPUs. To configure a flavor to use the shared CPU policy, run:

$ openstack flavor set [FLAVOR_ID] --property hw:cpu_policy=shared

The mixed CPU policy is used to specify that an instance use pinned CPUs along with unpinned CPUs. The instance pinned CPU could be specified in the hw:cpu_dedicated_mask or, if real-time is enabled (hw:cpu_realtime= yes), in the hw:cpu_realtime_mask extra spec. For example, to configure a flavor to use the mixed CPU policy with 4 vCPUs in total and the first 2 vCPUs as pinned CPUs, with the hw:cpu_realtime_mask extra spec, run:

$ openstack flavor set [FLAVOR_ID] \
  --vcpus=4 \
  --property hw:cpu_policy=mixed \
  --property hw:cpu_dedicated_mask=0-1

To create the mixed instance with the real-time extra specs, run:

$ openstack flavor set [FLAVOR_ID] \
  --vcpus=4 \
  --property hw:cpu_policy=mixed \
  --property hw:cpu_realtime=yes \
  --property hw:cpu_realtime_mask=0-1

Note

For more information about the syntax for hw:cpu_policy, hw:cpu_dedicated_mask, hw:realtime_cpu and hw:cpu_realtime_mask, refer to the Flavors guide.

It is also possible to configure the CPU policy via image metadata. This can be useful when packaging applications that require real-time or near real-time behavior by ensuring instances created with a given image are always pinned regardless of flavor. To configure an image to use the dedicated CPU policy, run:

$ openstack image set [IMAGE_ID] --property hw_cpu_policy=dedicated

Likewise, to configure an image to use the shared CPU policy, run:

$ openstack image set [IMAGE_ID] --property hw_cpu_policy=shared

Note

For more information about image metadata, refer to the Image metadata guide.

Important

Flavor-based policies take precedence over image-based policies. For example, if a flavor specifies a CPU policy of dedicated then that policy will be used. If the flavor specifies a CPU policy of shared and the image specifies no policy or a policy of shared then the shared policy will be used. However, the flavor specifies a CPU policy of shared and the image specifies a policy of dedicated, or vice versa, an exception will be raised. This is by design. Image metadata is often configurable by non-admin users, while flavors are only configurable by admins. By setting a shared policy through flavor extra-specs, administrators can prevent users configuring CPU policies in images and impacting resource utilization.

Customizing instance CPU thread pinning policies

Important

The functionality described below requires the use of pinned instances and is therefore currently only supported by the libvirt/KVM driver and requires some host configuration for this to work. Hyper-V does not support CPU pinning.

When running pinned instances on SMT hosts, it may also be necessary to consider the impact that thread siblings can have on the instance workload. The presence of an SMT implementation like Intel Hyper-Threading can boost performance by up to 30% for some workloads. However, thread siblings share a number of components and contention on these components can diminish performance for other workloads. For this reason, it is also possible to explicitly request hosts with or without SMT.

To configure whether an instance should be placed on a host with SMT or not, a CPU thread policy may be specified. For workloads where sharing benefits performance, you can request hosts with SMT. To configure this, run:

$ openstack flavor set [FLAVOR_ID] \
  --property hw:cpu_policy=dedicated \
  --property hw:cpu_thread_policy=require

This will ensure the instance gets scheduled to a host with SMT by requesting hosts that report the HW_CPU_HYPERTHREADING trait. It is also possible to request this trait explicitly. To configure this, run:

$ openstack flavor set [FLAVOR_ID] \
  --property resources:PCPU=N \
  --property trait:HW_CPU_HYPERTHREADING=required

For other workloads where performance is impacted by contention for resources, you can request hosts without SMT. To configure this, run:

$ openstack flavor set [FLAVOR_ID] \
  --property hw:cpu_policy=dedicated \
  --property hw:cpu_thread_policy=isolate

This will ensure the instance gets scheduled to a host with SMT by requesting hosts that do not report the HW_CPU_HYPERTHREADING trait. It is also possible to request this trait explicitly. To configure this, run:

$ openstack flavor set [FLAVOR_ID] \
  --property resources:PCPU=N \
  --property trait:HW_CPU_HYPERTHREADING=forbidden

Finally, for workloads where performance is minimally impacted, you may use thread siblings if available and fallback to not using them if necessary. This is the default, but it can be set explicitly:

$ openstack flavor set [FLAVOR_ID] \
  --property hw:cpu_policy=dedicated \
  --property hw:cpu_thread_policy=prefer

This does not utilize traits and, as such, there is no trait-based equivalent.

Note

For more information about the syntax for hw:cpu_thread_policy, refer to the Manage Flavors guide.

As with CPU policies, it also possible to configure the CPU thread policy via image metadata. This can be useful when packaging applications that require real-time or near real-time behavior by ensuring instances created with a given image are always pinned regardless of flavor. To configure an image to use the require CPU policy, run:

$ openstack image set [IMAGE_ID] \
  --property hw_cpu_policy=dedicated \
  --property hw_cpu_thread_policy=require

Likewise, to configure an image to use the isolate CPU thread policy, run:

$ openstack image set [IMAGE_ID] \
  --property hw_cpu_policy=dedicated \
  --property hw_cpu_thread_policy=isolate

Finally, to configure an image to use the prefer CPU thread policy, run:

$ openstack image set [IMAGE_ID] \
  --property hw_cpu_policy=dedicated \
  --property hw_cpu_thread_policy=prefer

If the flavor does not specify a CPU thread policy then the CPU thread policy specified by the image (if any) will be used. If both the flavor and image specify a CPU thread policy then they must specify the same policy, otherwise an exception will be raised.

Note

For more information about image metadata, refer to the Image metadata guide.

Customizing instance emulator thread pinning policies

Important

The functionality described below requires the use of pinned instances and is therefore currently only supported by the libvirt/KVM driver and requires some host configuration for this to work. Hyper-V does not support CPU pinning.

In addition to the work of the guest OS and applications running in an instance, there is a small amount of overhead associated with the underlying hypervisor. By default, these overhead tasks - known collectively as emulator threads - run on the same host CPUs as the instance itself and will result in a minor performance penalty for the instance. This is not usually an issue, however, for things like real-time instances, it may not be acceptable for emulator thread to steal time from instance CPUs.

Emulator thread policies can be used to ensure emulator threads are run on cores separate from those used by the instance. There are two policies: isolate and share. The default is to run the emulator threads on the same core. The isolate emulator thread policy is used to specify that emulator threads for a given instance should be run on their own unique core, chosen from one of the host cores listed in compute.cpu_dedicated_set. To configure a flavor to use the isolate emulator thread policy, run:

$ openstack flavor set [FLAVOR_ID] \
  --property hw:cpu_policy=dedicated \
  --property hw:emulator_threads_policy=isolate

The share policy is used to specify that emulator threads from a given instance should be run on the pool of host cores listed in compute.cpu_shared_set. To configure a flavor to use the share emulator thread policy, run:

$ openstack flavor set [FLAVOR_ID] \
  --property hw:cpu_policy=dedicated \
  --property hw:emulator_threads_policy=share

Note

For more information about the syntax for hw:emulator_threads_policy, refer to the Manage Flavors guide.

Customizing instance CPU topologies

Important

The functionality described below is currently only supported by the libvirt/KVM driver.

Note

Currently it also works with libvirt/QEMU driver but we don’t recommend it in production use cases. This is because vCPUs are actually running in one thread on host in qemu TCG (Tiny Code Generator), which is the backend for libvirt/QEMU driver. Work to enable full multi-threading support for TCG (a.k.a. MTTCG) is on going in QEMU community. Please see this MTTCG project page for detail.

In addition to configuring how an instance is scheduled on host CPUs, it is possible to configure how CPUs are represented in the instance itself. By default, when instance NUMA placement is not specified, a topology of N sockets, each with one core and one thread, is used for an instance, where N corresponds to the number of instance vCPUs requested. When instance NUMA placement is specified, the number of sockets is fixed to the number of host NUMA nodes to use and the total number of instance CPUs is split over these sockets.

Some workloads benefit from a custom topology. For example, in some operating systems, a different license may be needed depending on the number of CPU sockets. To configure a flavor to use a maximum of two sockets, run:

$ openstack flavor set [FLAVOR_ID] --property hw:cpu_sockets=2

Similarly, to configure a flavor to use one core and one thread, run:

$ openstack flavor set [FLAVOR_ID] \
  --property hw:cpu_cores=1 \
  --property hw:cpu_threads=1

Caution

If specifying all values, the product of sockets multiplied by cores multiplied by threads must equal the number of instance vCPUs. If specifying any one of these values or the multiple of two values, the values must be a factor of the number of instance vCPUs to prevent an exception. For example, specifying hw:cpu_sockets=2 on a host with an odd number of cores fails. Similarly, specifying hw:cpu_cores=2 and hw:cpu_threads=4 on a host with ten cores fails.

For more information about the syntax for hw:cpu_sockets, hw:cpu_cores and hw:cpu_threads, refer to the Manage Flavors guide.

It is also possible to set upper limits on the number of sockets, cores, and threads used. Unlike the hard values above, it is not necessary for this exact number to used because it only provides a limit. This can be used to provide some flexibility in scheduling, while ensuring certain limits are not exceeded. For example, to ensure no more than two sockets are defined in the instance topology, run:

$ openstack flavor set [FLAVOR_ID] --property hw:cpu_max_sockets=2

For more information about the syntax for hw:cpu_max_sockets, hw:cpu_max_cores, and hw:cpu_max_threads, refer to the Manage Flavors guide.

Applications are frequently packaged as images. For applications that prefer certain CPU topologies, configure image metadata to hint that created instances should have a given topology regardless of flavor. To configure an image to request a two-socket, four-core per socket topology, run:

$ openstack image set [IMAGE_ID] \
  --property hw_cpu_sockets=2 \
  --property hw_cpu_cores=4

To constrain instances to a given limit of sockets, cores or threads, use the max_ variants. To configure an image to have a maximum of two sockets and a maximum of one thread, run:

$ openstack image set [IMAGE_ID] \
  --property hw_cpu_max_sockets=2 \
  --property hw_cpu_max_threads=1

The value specified in the flavor is treated as the absolute limit. The image limits are not permitted to exceed the flavor limits, they can only be equal to or lower than what the flavor defines. By setting a max value for sockets, cores, or threads, administrators can prevent users configuring topologies that might, for example, incur an additional licensing fees.

For more information about image metadata, refer to the Image metadata guide.

Configuring libvirt compute nodes for CPU pinning

Changed in version 20.0.0: Prior to 20.0.0 (Train), it was not necessary to explicitly configure hosts for pinned instances. However, it was not possible to place pinned instances on the same host as unpinned CPUs, which typically meant hosts had to be grouped into host aggregates. If this was not done, unpinned instances would continue floating across all enabled host CPUs, even those that some instance CPUs were pinned to. Starting in 20.0.0, it is necessary to explicitly identify the host cores that should be used for pinned instances.

Nova treats host CPUs used for unpinned instances differently from those used by pinned instances. The former are tracked in placement using the VCPU resource type and can be overallocated, while the latter are tracked using the PCPU resource type. By default, nova will report all host CPUs as VCPU inventory, however, this can be configured using the compute.cpu_shared_set config option, to specify which host CPUs should be used for VCPU inventory, and the compute.cpu_dedicated_set config option, to specify which host CPUs should be used for PCPU inventory.

Consider a compute node with a total of 24 host physical CPU cores with hyperthreading enabled. The operator wishes to reserve 1 physical CPU core and its thread sibling for host processing (not for guest instance use). Furthermore, the operator wishes to use 8 host physical CPU cores and their thread siblings for dedicated guest CPU resources. The remaining 15 host physical CPU cores and their thread siblings will be used for shared guest vCPU usage, with an 8:1 allocation ratio for those physical processors used for shared guest CPU resources.

The operator could configure nova.conf like so:

[DEFAULT]
cpu_allocation_ratio=8.0

[compute]
cpu_dedicated_set=2-17
cpu_shared_set=18-47

The virt driver will construct a provider tree containing a single resource provider representing the compute node and report inventory of PCPU and VCPU for this single provider accordingly:

COMPUTE NODE provider
    PCPU:
        total: 16
        reserved: 0
        min_unit: 1
        max_unit: 16
        step_size: 1
        allocation_ratio: 1.0
    VCPU:
        total: 30
        reserved: 0
        min_unit: 1
        max_unit: 30
        step_size: 1
        allocation_ratio: 8.0

Instances using the dedicated CPU policy or an explicit PCPU resource request, PCPU inventory will be consumed. Instances using the shared CPU policy, meanwhile, will consume VCPU inventory.

Note

PCPU and VCPU allocations are currently combined to calculate the value for the cores quota class.

Configuring Hyper-V compute nodes for instance NUMA policies

Hyper-V is configured by default to allow instances to span multiple NUMA nodes, regardless if the instances have been configured to only span N NUMA nodes. This behaviour allows Hyper-V instances to have up to 64 vCPUs and 1 TB of memory.

Checking NUMA spanning can easily be done by running this following PowerShell command:

(Get-VMHost).NumaSpanningEnabled

In order to disable this behaviour, the host will have to be configured to disable NUMA spanning. This can be done by executing these following PowerShell commands:

Set-VMHost -NumaSpanningEnabled $false
Restart-Service vmms

In order to restore this behaviour, execute these PowerShell commands:

Set-VMHost -NumaSpanningEnabled $true
Restart-Service vmms

The Virtual Machine Management Service (vmms) is responsible for managing the Hyper-V VMs. The VMs will still run while the service is down or restarting, but they will not be manageable by the nova-compute service. In order for the effects of the host NUMA spanning configuration to take effect, the VMs will have to be restarted.

Hyper-V does not allow instances with a NUMA topology to have dynamic memory allocation turned on. The Hyper-V driver will ignore the configured dynamic_memory_ratio from the given nova.conf file when spawning instances with a NUMA topology.