Enable CPU Sharing

About CPU Sharing

CPU sharing allows the virtual CPUs (vCPUs) of different VMs to run on the same physical CPU, just like how multiple processes run concurrently on a single CPU. Internally the hypervisor adopts time slicing scheduling and periodically switches among those vCPUs.

This feature can help improve overall CPU utilization when the VMs are not fully loaded. However, sharing a physical CPU among multiple vCPUs increases the worst-case response latency of them, and thus is not suitable for vCPUs running latency-sensitive workloads.

Dependencies and Constraints

Consider the following dependencies and constraints:

• CPU sharing is a hypervisor feature that is hardware and OS neutral.

• CPU sharing is not available for real-time VMs or for VMs with local APIC passthrough (via the LAPIC passthrough option in the ACRN Configurator or via the Device Model --lapic_pt option).

• You can choose the scheduler the hypervisor uses. A scheduler is an algorithm for determining the priority of VMs running on a shared virtual CPU. ACRN supports the following schedulers:

  • Borrowed Virtual Time (BVT), which fairly allocates time slices to multiple vCPUs pinned to the same physical CPU. The BVT scheduler is the default and is sufficient for most use cases.

  • No-Operation (NOOP), which runs at most one vCPU on each physical CPU.

  • Priority based, which supports vCPU scheduling based on their static priorities defined in the scenario configuration. A vCPU can be running only if there is no higher-priority vCPU running on the same physical CPU.

Configuration Overview

You use the ACRN Configurator Tool to enable CPU sharing by assigning the same set of physical CPUs to multiple VMs and selecting a scheduler. The following documentation is a general overview of the configuration process.

To assign the same set of physical CPUs to multiple VMs, set the following parameters in each VM’s Basic Parameters:

• VM type: Standard (Real-time VMs don’t support CPU sharing)

• Physical CPU affinity > pCPU ID: Select a physical CPU by its core ID.

• To add another physical CPU, click + on the right side of an existing CPU. Or click - to delete a CPU.

• Repeat the process to assign the same physical CPUs to another VM.

To select a scheduler, go to Hypervisor Global Settings > Advanced Parameters > Virtual CPU scheduler and select a scheduler from the list.

Example Configuration

The following steps show how to enable and verify CPU sharing between two User VMs. The example extends the information provided in the Getting Started Guide.

1. In the ACRN Configurator, create a shared scenario with a Service VM and two post-launched User VMs.

2. For the first User VM, set the following parameters in the VM’s Basic Parameters:

   • VM name: This example uses POST_STD_VM1.

   • VM type: Standard

   • Physical CPU affinity: Select pCPU ID 1, then click + and select pCPU ID 2 to assign the VM to CPU cores 1 and 2.

   

3. For the second User VM, set the following parameters in the VM’s Basic Parameters:

   • VM name: This example uses POST_STD_VM2.

   • VM type: Standard

   • Physical CPU affinity: Select pCPU ID 1 and 2. The pCPU IDs must be the same as those of POST_STD_VM1 to use the CPU sharing function.

4. In Hypervisor Global Settings > Advanced Parameters > Virtual CPU scheduler, confirm that the default scheduler, Borrowed Virtual Time, is selected.

5. Save the scenario and launch script.

6. Build ACRN, copy all the necessary files from the development computer to the target system, and launch the Service VM and post-launched User VMs.

7. In the ACRN hypervisor shell, check the CPU sharing via the vcpu_list command. For example:

   ACRN:\>vcpu_list
   
   VM ID    PCPU ID    VCPU ID    VCPU ROLE    VCPU STATE    THREAD STATE
   =====    =======    =======    =========    ==========    ==========
     0         0          0        PRIMARY      Running       RUNNABLE
     0         1          1        SECONDARY    Running       BLOCKED
     0         2          2        SECONDARY    Running       BLOCKED
     0         3          3        SECONDARY    Running       BLOCKED
     1         1          0        PRIMARY      Running       RUNNING
     1         2          1        SECONDARY    Running       BLOCKED
     2         1          0        PRIMARY      Running       BLOCKED
     2         2          1        SECONDARY    Running       RUNNING
   

   The VM ID, PCPU ID, VCPU ID, and THREAD STATE columns provide information to help you check CPU sharing. In the VM ID column, VM 0 is the Service VM, VM 1 is POST_STD_VM1, and VM 2 is POST_STD_VM2. The output shows that ACRN assigned all physical CPUs (pCPUs) to VM 0 as expected. It also confirms that you assigned pCPUs 1 and 2 to VMs 1 and 2 (via the ACRN Configurator). vCPU 1 of VM 0 and vCPU 0 of VM 1 and VM 2 are running on the same physical CPU; they are sharing the physical CPU execution time. The thread state column shows the current states of the vCPUs. The BLOCKED state can occur for different reasons, most likely the vCPU is waiting for an I/O operation to be completed. Once it is done, the state will change to RUNNABLE. When this vCPU gets its pCPU execution time, its state will change to RUNNING, then the vCPU is actually running on the pCPU.

Learn More

For details on the ACRN CPU virtualization high-level design, For the CPU Virtualization.