In Kata Containers, workloads run in a virtual machine that is managed by a virtual machine monitor (VMM) running on the host. As a result, Kata Containers run over two layers of cgroups. The first layer is in the guest where the workload is placed, while the second layer is on the host where the VMM and associated threads are running.
The OCI runtime specification provides guidance on where the container cgroups should be placed:
cgroupsPath: (string, OPTIONAL) path to the cgroups. It can be used to either control the cgroups hierarchy for containers or to run a new process in an existing container
Cgroups are hierarchical, and this can be seen with the following pod example:
-
Pod 1:
cgroupsPath=/kubepods/pod1- Container 1:
cgroupsPath=/kubepods/pod1/container1 - Container 2:
cgroupsPath=/kubepods/pod1/container2
- Container 1:
-
Pod 2:
cgroupsPath=/kubepods/pod2- Container 1:
cgroupsPath=/kubepods/pod2/container1 - Container 2:
cgroupsPath=/kubepods/pod2/container2
- Container 1:
Depending on the upper-level orchestration layers, the cgroup under which the pod is placed is managed by the orchestrator or not. In the case of Kubernetes, the pod cgroup is created by Kubelet, while the container cgroups are to be handled by the runtime. Kubelet will size the pod cgroup based on the container resource requirements, to which it may add a configured set of pod resource overheads.
Kata Containers introduces a non-negligible resource overhead for running a sandbox (pod). Typically, the Kata shim, through its underlying VMM invocation, will create many additional threads compared to process based container runtimes: the para-virtualized I/O back-ends, the VMM instance or even the Kata shim process, all of those host processes consume memory and CPU time not directly tied to the container workload, and introduces a sandbox resource overhead. In order for a Kata workload to run without significant performance degradation, its sandbox overhead must be provisioned accordingly. Two scenarios are possible:
- The upper-layer orchestrator takes the overhead of running a sandbox into account when sizing the pod cgroup.
For example, Kubernetes
PodOverheadfeature lets the orchestrator add a configured sandbox overhead to the sum of all its containers resources. In that case, the pod sandbox is properly sized and all Kata created processes will run under the pod cgroup defined constraints and limits. - The upper-layer orchestrator does not take the sandbox overhead into account and the pod cgroup is not
sized to properly run all Kata created processes. With that scenario, attaching all the Kata processes to the sandbox
cgroup may lead to non-negligible workload performance degradations. As a consequence, Kata Containers will move
all processes but the vCPU threads into a dedicated overhead cgroup under
/kata_overhead. The Kata runtime will not apply any constraints or limits to that cgroup, it is up to the infrastructure owner to optionally set it up.
Those 2 scenarios are not dynamically detected by the Kata Containers runtime implementation, and thus the
infrastructure owner must configure the runtime according to how the upper-layer orchestrator creates and sizes the
pod cgroup. That configuration selection is done through the sandbox_cgroup_only flag within the Kata Containers
configuration file.
Setting sandbox_cgroup_only to true from the Kata Containers configuration file means that the pod cgroup is
properly sized and takes the pod overhead into account. This is ideal, as all the applicable Kata Containers processes
can simply be placed within the given cgroup path.
In the context of Kubernetes, Kubelet can size the pod cgroup to take the overhead of running a Kata-based sandbox
into account. This has been supported since the 1.16 Kubernetes release, through the
PodOverhead feature.
┌─────────────────────────────────────────┐
│ │
│ ┌──────────────────────────────────┐ │
│ │ │ │
│ │ ┌─────────────────────────────┐ │ │
│ │ │ │ │ │
│ │ │ ┌─────────────────────┐ │ │ │
│ │ │ │ vCPU threads │ │ │ │
│ │ │ │ I/O threads │ │ │ │
│ │ │ │ VMM │ │ │ │
│ │ │ │ Kata Shim │ │ │ │
│ │ │ │ │ │ │ │
│ │ │ │ /kata_<sandbox_id> │ │ │ │
│ │ │ └─────────────────────┘ │ │ │
│ │ │Pod 1 │ │ │
│ │ └─────────────────────────────┘ │ │
│ │ │ │
│ │ ┌─────────────────────────────┐ │ │
│ │ │ │ │ │
│ │ │ ┌─────────────────────┐ │ │ │
│ │ │ │ vCPU threads │ │ │ │
│ │ │ │ I/O threads │ │ │ │
│ │ │ │ VMM │ │ │ │
│ │ │ │ Kata Shim │ │ │ │
│ │ │ │ │ │ │ │
│ │ │ │ /kata_<sandbox_id> │ │ │ │
│ │ │ └─────────────────────┘ │ │ │
│ │ │Pod 2 │ │ │
│ │ └─────────────────────────────┘ │ │
│ │ │ │
│ │/kubepods │ │
│ └──────────────────────────────────┘ │
│ │
│ Node │
└─────────────────────────────────────────┘
When sandbox_cgroup_only is enabled, the Kata shim will create a per pod
sub-cgroup under the pod's dedicated cgroup. For example, in the Kubernetes context,
it will create a /kata_<PodSandboxID> under the /kubepods cgroup hierarchy.
On a typical cgroup v1 hierarchy mounted under /sys/fs/cgroup/, the memory cgroup
subsystem for a pod with sandbox ID 12345678 would live under
/sys/fs/cgroup/memory/kubepods/kata_12345678.
In most cases, the /kata_<PodSandboxID> created cgroup is unrestricted and inherits and shares all
constraints and limits from the parent cgroup (/kubepods in the Kubernetes case). The exception is
for the cpuset and devices cgroup subsystems, which are managed by the Kata shim.
After creating the /kata_<PodSandboxID> cgroup, the Kata Containers shim will move itself to it, before starting
the virtual machine. As a consequence all processes subsequently created by the Kata Containers shim (the VMM itself, and
all vCPU and I/O related threads) will be created in the /kata_<PodSandboxID> cgroup.
And why not directly adding the per sandbox shim directly to the pod cgroup (e.g.
/kubepods in the Kubernetes context)?
The Kata Containers shim implementation creates a per-sandbox cgroup
(/kata_<PodSandboxID>) to support the Docker use case. Although Docker does not
have a notion of pods, Kata Containers still creates a sandbox to support the pod-less,
single container use case that Docker implements. Since Docker does create any
cgroup hierarchy to place a container into, it would be very complex for Kata to map
a particular container to its sandbox without placing it under a /kata_<containerID>>
sub-cgroup first.
Keeping all Kata Containers processes under a properly sized pod cgroup is ideal and makes for a simpler Kata Containers implementation. It also helps with gathering accurate statistics and preventing Kata workloads from being noisy neighbors.
If the Kata caller wants to know the resource usage on the host it can get statistics from the pod cgroup. All cgroups stats in the hierarchy will include the Kata overhead. This gives the possibility of gathering usage-statics at the pod level and the container level.
Because the Kata runtime will place all the Kata processes in the pod cgroup, the resource limits that the caller applies to the pod cgroup will affect all processes that belong to the Kata sandbox in the host. This will improve the isolation in the host preventing Kata to become a noisy neighbor.
If the cgroup provided to Kata is not sized appropriately, Kata components will consume resources that the actual container workloads expect to see and use. This can cause instability and performance degradations.
To avoid that situation, Kata Containers creates an unconstrained overhead
cgroup and moves all non workload related processes (Anything but the virtual CPU
threads) to it. The name of this overhead cgroup is /kata_overhead and a per
sandbox sub cgroup will be created under it for each sandbox Kata Containers creates.
Kata Containers does not add any constraints or limitations on the overhead cgroup. It is up to the infrastructure owner to either:
- Provision nodes with a pre-sized
/kata_overheadcgroup. Kata Containers will load that existing cgroup and move all non workload related processes to it. - Let Kata Containers create the
/kata_overheadcgroup, leave it unconstrained or resize it a-posteriori.
┌────────────────────────────────────────────────────────────────────┐
│ │
│ ┌─────────────────────────────┐ ┌───────────────────────────┐ │
│ │ │ │ │ │
│ │ ┌─────────────────────────┼────┼─────────────────────────┐ │ │
│ │ │ │ │ │ │ │
│ │ │ ┌─────────────────────┐ │ │ ┌─────────────────────┐ │ │ │
│ │ │ │ vCPU threads │ │ │ │ VMM │ │ │ │
│ │ │ │ │ │ │ │ I/O threads │ │ │ │
│ │ │ │ │ │ │ │ Kata Shim │ │ │ │
│ │ │ │ │ │ │ │ │ │ │ │
│ │ │ │ /kata_<sandbox_id> │ │ │ │ /<sandbox_id> │ │ │ │
│ │ │ └─────────────────────┘ │ │ └─────────────────────┘ │ │ │
│ │ │ │ │ │ │ │
│ │ │ Pod 1 │ │ │ │ │
│ │ └─────────────────────────┼────┼─────────────────────────┘ │ │
│ │ │ │ │ │
│ │ │ │ │ │
│ │ ┌─────────────────────────┼────┼─────────────────────────┐ │ │
│ │ │ │ │ │ │ │
│ │ │ ┌─────────────────────┐ │ │ ┌─────────────────────┐ │ │ │
│ │ │ │ vCPU threads │ │ │ │ VMM │ │ │ │
│ │ │ │ │ │ │ │ I/O threads │ │ │ │
│ │ │ │ │ │ │ │ Kata Shim │ │ │ │
│ │ │ │ │ │ │ │ │ │ │ │
│ │ │ │ /kata_<sandbox_id> │ │ │ │ /<sandbox_id> │ │ │ │
│ │ │ └─────────────────────┘ │ │ └─────────────────────┘ │ │ │
│ │ │ │ │ │ │ │
│ │ │ Pod 2 │ │ │ │ │
│ │ └─────────────────────────┼────┼─────────────────────────┘ │ │
│ │ │ │ │ │
│ │ /kubepods │ │ /kata_overhead │ │
│ └─────────────────────────────┘ └───────────────────────────┘ │
│ │
│ │
│ Node │
└────────────────────────────────────────────────────────────────────┘
When sandbox_cgroup_only is disabled, the Kata Containers shim will create a per pod
sub-cgroup under the pods dedicated cgroup, and another one under the overhead cgroup.
For example, in the Kubernetes context, it will create a /kata_<PodSandboxID> under
the /kubepods cgroup hierarchy, and a /<PodSandboxID> under the /kata_overhead one.
On a typical cgroup v1 hierarchy mounted under /sys/fs/cgroup/, for a pod which sandbox
ID is 12345678, create with sandbox_cgroup_only disabled, the 2 memory subsystems
for the sandbox cgroup and the overhead cgroup would respectively live under
/sys/fs/cgroup/memory/kubepods/kata_12345678 and /sys/fs/cgroup/memory/kata_overhead/12345678.
Unlike when sandbox_cgroup_only is enabled, the Kata Containers shim will move itself
to the overhead cgroup first, and then move the vCPU threads to the sandbox cgroup as
they're created. All Kata processes and threads will run under the overhead cgroup except for
the vCPU threads.
With sandbox_cgroup_only disabled, Kata Containers assumes the pod cgroup is only sized
to accommodate for the actual container workloads processes. For Kata, this maps
to the VMM created virtual CPU threads and so they are the only ones running under the pod
cgroup. This mitigates the risk of the VMM, the Kata shim and the I/O threads going through
a catastrophic out of memory scenario (OOM).
Running all non vCPU threads under an unconstrained overhead cgroup could lead to workloads potentially consuming a large amount of host resources.
On the other hand, running all non vCPU threads under a dedicated overhead cgroup can provide accurate metrics on the actual Kata Container pod overhead, allowing for tuning the overhead cgroup size and constraints accordingly.
Kata Containers currently only supports cgroups v1.
In the following sections each cgroup is described briefly.
Cgroups V1 are under a tmpfs filesystem mounted at /sys/fs/cgroup, where each cgroup is
mounted under a separate cgroup filesystem. A Cgroups v1 hierarchy may look like the following
diagram:
/sys/fs/cgroup/
├── blkio
│ ├── cgroup.procs
│ └── tasks
├── cpu -> cpu,cpuacct
├── cpuacct -> cpu,cpuacct
├── cpu,cpuacct
│ ├── cgroup.procs
│ └── tasks
├── cpuset
│ ├── cgroup.procs
│ └── tasks
├── devices
│ ├── cgroup.procs
│ └── tasks
├── freezer
│ ├── cgroup.procs
│ └── tasks
├── hugetlb
│ ├── cgroup.procs
│ └── tasks
├── memory
│ ├── cgroup.procs
│ └── tasks
├── net_cls -> net_cls,net_prio
├── net_cls,net_prio
│ ├── cgroup.procs
│ └── tasks
├── net_prio -> net_cls,net_prio
├── perf_event
│ ├── cgroup.procs
│ └── tasks
├── pids
│ ├── cgroup.procs
│ └── tasks
└── systemd
├── cgroup.procs
└── tasks
A process can join a cgroup by writing its process id (pid) to cgroup.procs file,
or join a cgroup partially by writing the task (thread) id (tid) to the tasks file.
Kata Containers only supports v1.
To know more about cgroups v1, see cgroupsv1(7).
Cgroups v2 are also known as unified cgroups, unlike cgroups v1, the cgroups are
mounted under the same cgroup filesystem. A Cgroups v2 hierarchy may look like the following
diagram:
/sys/fs/cgroup/system.slice
├── cgroup.controllers
├── cgroup.events
├── cgroup.freeze
├── cgroup.max.depth
├── cgroup.max.descendants
├── cgroup.procs
├── cgroup.stat
├── cgroup.subtree_control
├── cgroup.threads
├── cgroup.type
├── cpu.max
├── cpu.pressure
├── cpu.stat
├── cpu.weight
├── cpu.weight.nice
├── io.bfq.weight
├── io.latency
├── io.max
├── io.pressure
├── io.stat
├── memory.current
├── memory.events
├── memory.events.local
├── memory.high
├── memory.low
├── memory.max
├── memory.min
├── memory.oom.group
├── memory.pressure
├── memory.stat
├── memory.swap.current
├── memory.swap.events
├── memory.swap.max
├── pids.current
├── pids.events
└── pids.max
Same as cgroups v1, a process can join the cgroup by writing its process id (pid) to
cgroup.procs file, or join a cgroup partially by writing the task (thread) id (tid) to
cgroup.threads file.
Kata Containers does not support cgroups v2 on the host.
Many Linux distributions do not yet support cgroups v2, as it is quite a recent addition.
For more information about the status of this feature see issue #2494.