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1. Problem description
In Apache CloudStack (ACS), when a VM is deployed in a host with the KVM hypervisor, an XML file is created in the assigned host, which has a property shares that defines the weight of the VM to access the host CPU. The value of this property has no unit, and it is a relative measure to calculate how much CPU a given VM will have in the host. However, this value has a limit, which depends on the version of cgroup utilized by the host's kernel. The problem lies at the range value of shares that varies between both versions: [2, 264144] for cgroups version 1; and [1, 10000] for cgroups version 2. Currently, ACS calculates the value of shares using Equation 1, presented below, where CPU is the number of cores and speed is the CPU frequency; both specified in the VM's compute offering. Therefore, if a compute offering has, for example, 6 cores at 2 GHz, the shares value will be 12000 and an exception will be thrown by libvirt if the host utilizes cgroup v2. The second version is becoming the default one in current Linux distributions; thus, it is necessary to address this limitation.
Equation 1
shares = CPU * speed
Fixes: #6744
2. Proposed changes
To address the problem described, we propose to apply a scale conversion considering the max shares of the host. Using the same formula currently utilized by ACS, it is possible to calculate the maximum shares of a VM for a given host. In other words, using the number of cores and the nominal speed of the host's CPU as the upper limit of shares allowed to a VM. Then, this value will be scaled to the allowed interval of [1, 10000] of cgroup v2 by using a linear scale conversion.
The VM shares would be calculated as Equation 2, presented below, where VM requested shares is the requested shares value calculated using Equation 1, cgroup upper limit is fixed with a value of 10000 (cgroups v2 upper limit), and host max shares is the maximum shares value of the host, calculated using Equation 1. Using Equation 2, the only case where a VM passes the cgroup v2 limit is when the user requests more resources than the host has, which is not possible with the current implementation of ACS.
Equation 2
shares = (VM requested shares * cgroup upper limit)/host max shares
To implement the proposal, the following APIs will be updated: deployVirtualMachine, migrateVirtualMachine and scaleVirtualMachine. When a VM is being deployed, a new verification will be added to find a suitable host. The max shares of each host will be calculated, and the VM calculated shares will be verified if it does not surpass the host's value. Likewise, the migration of VMs will have a similar new verification. Lastly, the scale of VMs will also have the same verification for the VM's host.
To determine the max shares of a given host, we will use the same equation currently used in ACS for calculating the shares of VMs, presented in Section 1. When Equation 1 is used to determine the maximum shares of a host, CPU is the number of cores of the host, and speed is the nominal CPU speed, i.e., considering the CPU's base frequency.
It is important to note that these changes are only for hosts with the KVM hypervisor using cgroup v2 for now.