Virtual Topology and CPU Pinning

Overview

In this guide you’ll learn to set up OpenNebula to control how VM resources are mapped onto the hypervisor ones. These settings will help you to fine tune the performance of VMs. We will use the following concepts:

  • Cores, Threads and Sockets. A computer processor is connected to the motherboard through a socket. A processor can pack one or more cores, each one implements a separate processing unit that shares some cache levels, memory, and I/O ports. CPU cores’ performance can be improved by the use of hardware multi-threading (SMT) that permits multiple execution flows to run simultaneously on a single core.
  • NUMA. Multi-processor servers are usually arranged in nodes or cells. Each NUMA node holds a fraction of the overall system memory. In this configuration, a processor accesses memory and I/O ports local to its node faster than those of non-local ones.
  • Hugepages. Systems with big physical memory also use a large number of virtual memory pages. This big number makes the use of virtual-to-physical translation caches inefficient. Hugepages reduces the number of virtual pages in the system and optimizes the virtual memory subsystem.

In OpenNebula the virtual topology of a VM is defined by the number of sockets, cores and threads. We assume that a NUMA node or cell is equivalent to a socket and they will be used interchangeably in this guide.

Note

Different limitations might exist regarding virtual topology definition and CPU pinning depending on the hypervisor. In order to get hypervisor-specific information, please check the corresponding hypervisor driver guide.

Defining a Virtual Topology

Basic Configuration

The most basic configuration is to define just the number of vCPU (virtual CPU) and the amount of memory of the VM. In this case the guest OS will see VCPU sockets of one core and one thread each. The VM template in this case for four vCPUs VM is:

MEMORY = 1024
VCPU   = 4

A VM running with this configuration will see the following topology:

# lscpu
...
CPU(s):                4
On-line CPU(s) list:   0-3
Thread(s) per core:    1
Core(s) per socket:    1
Socket(s):             4
NUMA node(s):          1

# numactl -H
available: 1 nodes (0)
node 0 cpus: 0 1 2 3
node 0 size: 1023 MB
node 0 free: 607 MB
node distances:
node   0
  0:  10

CPU Topology

You can give more detail to the previous scenario by defining a custom number of sockets, cores, and threads for a given number of vCPUs. Usually, there is no significant difference between how you arrange the number of cores and sockets performance-wise. However, some software products may require a specific topology setup in order to work.

For example a VM with two sockets and two cores per sockets, and two threads per core is defined by the following template:

VCPU   = 8
MEMORY = 1024

TOPOLOGY = [ SOCKETS = 2, CORES = 2, THREADS = 2 ]

and the associated guest OS view:

# lscpu
...
CPU(s):                8
On-line CPU(s) list:   0-7
Thread(s) per core:    2
Core(s) per socket:    2
Socket(s):             2
NUMA node(s):          1
...

# numactl -H
available: 1 nodes (0)
node 0 cpus: 0 1 2 3 4 5 6 7
node 0 size: 1023 MB
node 0 free: 600 MB
node distances:
node   0
  0:  10

Important

When defining a custom CPU Topology you need to set the number of sockets, cores, and threads, and it must match the total number of vCPUS, e.g. VCPU = SOCKETS * CORES * THREAD.

NUMA Topology

You can provide further detail to the topology of your VM by defining the placement of the sockets (NUMA nodes) in the hypervisor NUMA nodes. In this scenario each VM SOCKET will be exposed to guest OS as a separated NUMA node with its own local memory.

The previous example can expose a two socket (NUMA node) by setting a PIN_POLICY (see below):

VCPU   = 8
MEMORY = 1024

TOPOLOGY = [ PIN_POLICY = thread, SOCKETS = 2, CORES = 2, THREADS = 2 ]

In this case OpenNebula will generate an entry for each NUMA node, extending the previous VM template with:

NUMA_NODE = [ MEMORY = 512, TOTAL_CPUS = 4 ]
NUMA_NODE = [ MEMORY = 512, TOTAL_CPUS = 4 ]

The in-guest OS view is for this example:

# lscpu
...
CPU(s):                8
On-line CPU(s) list:   0-7
Thread(s) per core:    2
Core(s) per socket:    2
Socket(s):             2
NUMA node(s):          2
...

# numactl -H
available: 2 nodes (0-1)
node 0 cpus: 0 1 2 3
node 0 size: 511 MB
node 0 free: 235 MB
node 1 cpus: 4 5 6 7
node 1 size: 511 MB
node 1 free: 359 MB
node distances:
node   0   1
  0:  10  20
  1:  20  10

Asymmetric topology

For some applications you may need an asymmetric NUMA configuration, i.e. not distributing the VM resources evenly across the nodes. You can define each node configuration by manually setting the NUMA_NODE attributes. For example:

MEMORY = 3072
VCPU = 6
CPU  = 6
TOPOLOGY = [ PIN_POLICY = CORE, SOCKETS = 2 ]

NUMA_NODE = [ MEMORY = 1024, TOTAL_CPUS = 2 ]
NUMA_NODE = [ MEMORY = 2048, TOTAL_CPUS = 4 ]

Important

OpenNebula will also check that the total MEMORY in all the nodes matches that set in the VM.

CPU and NUMA Pinning

When you need to expose the NUMA topology to the guest, you have to set a pinning policy to map each virtual NUMA node’s resources (memory and vCPUs) onto the hypervisor nodes. OpenNebula can work with four different policies:

  • CORE: each vCPU is assigned to a whole hypervisor core. No other threads in that core will be used. This policy can be useful to isolate the VM workload for security reasons.
  • THREAD: each vCPU is assigned to a hypervisor CPU thread.
  • SHARED: the VM is assigned to a set of the hypervisor CPUS shared by all the VM vCPUs.
  • NONE: the VM is not assigned to any hypervisor CPUs. Access to the resources (i.e CPU time) will be limited by the CPU attribute.

VM memory is assigned to the closet hypervisor NUMA node where the vCPUs are pinned, to prioritize local memory access.

When using a pinning policy it is recommended to fix only the number of vCPUs by letting the scheduler pick the number of cores and threads of the virtual topology. OpenNebula will try to optimize the VM performance by selecting the threads per core according to the following:

  • For the CORE pin policy the number of THREADS is set to 1.
  • A hardware configuration as close as possible to that of the Host is preferred.
  • The threads per core will not exceed those of the hypervisor.
  • The configuration with the highest number of threads/cores that fits in the Host is preferred.

Important

When THREADS is set, OpenNebula will look for a Host that can allocate that number of threads per core; if not found the VM will remain in PENDING state. This may be required if you want the VM to run with a fixed number of threads per core.

For example to run a two NUMA node VM with eight vCPUS and 4G of memory, using the THREAD policy you can use:

VCPU   = 8
MEMORY = 4096

TOPOLOGY = [ PIN_POLICY = thread, SOCKETS = 2 ]

Important

For pinned VMs the CPU (assigned hypervisor capacity) is automatically set to the vCPU number. No overcommitment is allowed for pinned workloads.

PCI Passthrough

The scheduling process is slightly modified when a pinned VM includes PCI passthrough devices. In this case, the NUMA nodes where the PCI devices are attached are prioritized to pin the VM vCPUs and memory to speed up I/O operations. No additional configuration is needed.

Using Hugepages

To enable the use of hugepages for the memory allocation of the VM just add the desired page size in the TOPOLOGY attribute. The size must be expressed in megabytes. For example to use 2M hugepages use:

TOPOLOGY = [ HUGEPAGE_SIZE = 2 ]

OpenNebula will look for a Host with enough free pages of the requested size to allocate the VM. The resources of each virtual node will be placed as close as possible to the node providing the hugepages.

Summary of Virtual Topology Attributes

TOPOLOGY attribute Meaning
PIN_POLICY vCPU pinning preference: CORE, THREAD, SHARED, NONE
SOCKETS Number of sockets or NUMA nodes
CORES Number of cores per node
THREADS Number of threads per core
HUGEPAGE_SIZE Size of the hugepages (MB). If not defined no hugepages will be used
MEMORY_ACCESS Control whether the memory is to be mapped shared or private

Configuring the Host

When running VMs with a specific topology it is important to map (pin) it as close as possible to that on the hypervisor, so vCPUs and memory are allocated in the same NUMA node. However, by default, a VM is assigned to all the resources in the system, making the running of pinned and non-pinned workloads incompatible in the same Host.

First, you need to define which Hosts are going to be used to run pinned workloads, and define the PIN_POLICY tag through Sunstone or by using the onehost update command. A Host can operate in two modes:

  • NONE. Default mode where no NUMA or hardware characteristics are considered. Resources are assigned and balanced by an external component, e.g. numad or kernel.
  • PINNED. VMs are allocated and pinned to specific nodes according to different policies.

Note

You can also create an OpenNebula Cluster including all the Hosts devoted to running pinned workloads, and set the PIN_POLICY at the cluster level.

The Host monitoring probes should also return the NUMA topology and usage status of the hypervisors. The following command shows a single node hypervisor with four cores and two threads running a 2 vCPU VM:

$ onehost show 0
...
MONITORING INFORMATION
PIN_POLICY="PINNED"
...

NUMA NODES

  ID CORES                                              USED FREE
   0 X- X- -- --                                        4    4

NUMA MEMORY

 NODE_ID TOTAL    USED_REAL            USED_ALLOCATED       FREE
       0 7.6G     6.8G                 1024M                845.1M

In this output, the string X- X- -- -- represents the NUMA allocation: each group is a core and when a thread is free it’s shown as -; x means the thread is in use and X means that the thread is used and the core has no free threads. In this case the VM is using the CORE pin policy.

Note

If you want to use hugepages of a given size you need to allocate them first. This can be done either at boot time or dynamically. Also you may need to mount the hugetlbfs filesystem. Please refer to your OS documentation to learn how to do this.

You can also isolate some hypervisor CPUS from the NUMA scheduler. Isolated CPUs will not be used to pin any VM. The isolated CPUs are defined by the ISOLCPUS attribute; the attribute is a comma-separated list of CPU IDs. For example ISOLCPUS="0,5" will isolate CPUs 0,5 and hence will not be used to pin any VM.

CPU Pinning and Overcommitment

When using a pinned policy, overcommitment is disabled by default (CPU = 1 in the VM template). However, some scenarios may require you to fix the CPU thread where a VM is running while letting more VMs run in the very same CPU thread.

You can configure the number of VMs per physical thread for each Host by setting the VMS_THREAD (defaults to 1) variable in the Host template. For example VMS_THREAD = 4 will pin up to four VMs per physical thread in each core.

Important

When using overcommitment and NUMA you need to set the Host overcommitment in the same way, so the total CPU number accounts for the new VMS_THREAD value. For example, a Host with eight CPUs (TOTAL_CPU=800) and VMS_THREAD=4 needs to overcommit the CPU number so the TOTAL_CPU is at most 3200 (8 * 4 = 32 CPUs, max.). You can do this with the RESERVED_CPU attribute for the Host, RESERVED_CPU = "-2400" in this case (3200 = 800 - (-2400).

A Complete Example

Let’s define a VM with two NUMA nodes using 2M hugepages, four vCPUs and 1G of memory. The template is as follows:

MEMORY = "1024"

CPU  = "4"
VCPU = "4"
CPU_MODEL = [ MODEL="host-passthrough" ]

TOPOLOGY = [
  HUGEPAGE_SIZE = "2",
  MEMORY_ACCESS = "shared",
  NUMA_NODES    = "2",
  PIN_POLICY    = "THREAD" ]

DISK = [ IMAGE="CentOS7" ]
NIC  = [ IP="10.4.4.11", NETWORK="Management" ]

CONTEXT = [ NETWORK="YES", SSH_PUBLIC_KEY="$USER[SSH_PUBLIC_KEY]" ]

The VM is deployed in a hypervisor with the following characteristics, one node, eight CPUs and four cores:

# numactl -H
available: 1 nodes (0)
node 0 cpus: 0 1 2 3 4 5 6 7
node 0 size: 7805 MB
node 0 free: 2975 MB
node distances:
node   0
  0:  10

and 8G of memory with a total of 2048 2M hugepages:

# numastat -m
                          Node 0           Total
                 --------------- ---------------
MemTotal                 7805.56         7805.56
MemFree                   862.80          862.80
MemUsed                  6942.76         6942.76
...
HugePages_Total          2048.00         2048.00
HugePages_Free           1536.00         1536.00
HugePages_Surp              0.00            0.00

These characteristics can be also queried through the OpenNebula CLI:

$ onehost show 0
...

NUMA NODES

  ID CORES                                              USED FREE
   0 XX XX -- --                                        4    4

NUMA MEMORY

 NODE_ID TOTAL    USED_REAL            USED_ALLOCATED       FREE
       0 7.6G     6.8G                 1024M                845.1M

NUMA HUGEPAGES

 NODE_ID SIZE     TOTAL    FREE     USED
       0 2M       2048     1536     512
       0 1024M    0        0        0
...

Note that in this case the previous VM has been pinned to four CPUS (0,4,1,5) and it is using 512 pages of 2M. You can verify that the VM is actually running in this resource through libvirt:

virsh # vcpuinfo 1
VCPU:           0
CPU:            0
State:          running
CPU time:       13.0s
CPU Affinity:   y-------

VCPU:           1
CPU:            4
State:          running
CPU time:       5.8s
CPU Affinity:   ----y---

VCPU:           2
CPU:            1
State:          running
CPU time:       39.1s
CPU Affinity:   -y------

VCPU:           3
CPU:            5
State:          running
CPU time:       25.4s
CPU Affinity:   -----y--

You can also check the Guest OS point of view by executing the previous commands in the VM. It should show two nodes with two CPUs (threads) per core and 512M each:

# numactl -H
available: 2 nodes (0-1)
node 0 cpus: 0 1
node 0 size: 511 MB
node 0 free: 401 MB
node 1 cpus: 2 3
node 1 size: 511 MB
node 1 free: 185 MB
node distances:
node   0   1
  0:  10  20
  1:  20  10

# numastat -m

Per-node system memory usage (in MBs):
                          Node 0          Node 1           Total
                 --------------- --------------- ---------------
MemTotal                  511.62          511.86         1023.48
MemFree                   401.13          186.23          587.36
MemUsed                   110.49          325.62          436.11
...

If you prefer, the OpenNebula CLI will show this information:

$ onevm show 0
...
NUMA NODES

  ID   CPUS     MEMORY TOTAL_CPUS
   0    0,4       512M          2
   0    1,5       512M          2

TOPOLOGY

NUMA_NODES  CORES  SOCKETS  THREADS
         2      2        1        2

Considerations and Limitations

Please consider the following limitations when using pinned VMs:

  • VM Migration. Pinned VMs cannot be VM live migrated; you need to migrate the VMs through a power off - power on cycle.
  • Re-sizing of asymmetric virtual topologies is not supported, as the NUMA nodes are re-generated with the new VCPU and MEMORY values. Also, note that the pinned CPUs may change.
  • Asymmetric configurations. As qemu 4.0 and libvirt 5.4 NUMA nodes cannot be defined with no memory or without any CPU, you’ll get the following errors:
error: Failed to create domain from deployment.0
error: internal error: process exited while connecting to monitor:  qemu-system-x86_64: -object memory-backend-ram,id=ram-node1,size=0,host-nodes=0,policy=bind: property 'size' of memory-backend-ram doesn't take value '0'

virsh create deployment.0
error: Failed to create domain from deployment.0
error: XML error: Missing 'cpus' attribute in NUMA cell