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Merge pull request #387 from devimc/topic/cpuCgroups

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constraints: add cpu cgroups documentation
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constraints/cpu.md
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* [CPU constraints in Kata Containers](#cpu-constraints-in-kata-containers)
* [Default number of virtual CPUs](#default-number-of-virtual-cpus)
* [Virtual CPUs and Kubernetes pods](#virtual-cpus-and-kubernetes-pods)
* [Container lifecycle](#container-lifecycle)
* [Container without CPU constraint](#container-without-cpu-constraint)
* [Container with CPU constraint](#container-with-cpu-constraint)
* [Do not waste resources](#do-not-waste-resources)
* [CPU cgroups](#cpu-cgroups)
* [cgroups in the guest](#cgroups-in-the-guest)
* [CPU pinning](#cpu-pinning)
* [cgroups in the host](#cgroups-in-the-host)
# CPU constraints in Kata Containers
## Default number of virtual CPUs
@ -157,6 +170,84 @@ docker run --cpus 4 -ti debian bash -c "nproc; cat /sys/fs/cgroup/cpu,cpuacct/cp
400000 # cfs quota
```
## CPU cgroups
Kata Containers runs over two layers of cgroups, the first layer is in the guest where
only the workload is placed, the second layer is in the host that is more complex and
might contain more than one process and task (thread) depending of the number of
containers per POD and vCPUs per container. The following diagram represents a nginx container
created with `docker` with the default number of vcpus.
```
$ docker run -dt --runtime=kata-runtime nginx
.-------.
| nginx |
.--'-------'---. .------------.
| Guest Cgroup | | Kata agent |
.-'--------------'--'------------'. .-----------.
| Thread: Hypervisor's vCPU 0 | | Kata Shim |
.'---------------------------------'. .'-----------'.
| Tasks | | Processes |
.'-----------------------------------'--'-------------'.
| Host Cgroup |
'------------------------------------------------------'
```
The next sections explain the difference between processes and tasks and why only hypervisor
vCPUs are constrained.
### cgroups in the guest
Only the workload process including all its threads are placed into cpu cgroups, this means
that `kata-agent` and `systemd` run without constraints in the guest.
#### CPU pinning
Kata Containers tries to apply and honor the cgroups but sometimes that is not possible.
An example of this occurs with cpu cgroups when the number of virtual CPUs (in the guest)
does not match the actual number of physical host CPUs.
In Kata Containers to have a good performance and small memory footprint, the resources are
hot added when they are needed, therefore the number of virtual resources is not the same
as the number of physical resources. The problem with this approach is that it's not possible
to pin a process on a specific resource that is not present in the guest. To deal with this
limitation and to not fail when the container is being created, Kata Containers does not apply
the constraint in the first layer (guest) if the resource does not exist in the guest, but it
is applied in the second layer (host) where the hypervisor is running. The constraint is applied
in both layers when the resource is available in the guest and host. The next sections provide
further details on what parts of the hypervisor are constrained.
### cgroups in the host
In Kata Containers the workloads run in a virtual machine that is managed and represented by a
hypervisor running in the host. Like other processes the hypervisor might use threads to realize
several tasks, for example IO and Network operations. One of the most important uses for the
threads is as vCPUs. The processes running in the guest see these vCPUs as physical CPUs, while
in the host those vCPU are just threads that are part of a process. This is the key to ensure
workloads consumes only the amount of CPU resources that were assigned to it without impacting
other operations. From user perspective the easier approach to implement it would be to take the
whole hypervisor including its threads and move them into the cgroup, unfortunately this will
impact negatively the performance, since vCPUs, IO and Network threads will be fighting for
resources. The following table shows a random read performance comparison between a Kata Container
with all its hypervisor threads in the cgroup and other with only its hypervisor vCPU threads
constrained, the difference is huge.
| Bandwidth | All threads | vCPU threads | Units |
|:-------------:|:-------------:|:------------:|:-----:|
| 4k | 136.2 | 294.7 | MB/s |
| 8k | 166.6 | 579.4 | MB/s |
| 16k | 178.3 | 1093.3 | MB/s |
| 32k | 179.9 | 1931.5 | MB/s |
| 64k | 213.6 | 3994.2 | MB/s |
To have the best performance in Kata Containers only the vCPU threads are constrained.
[1]: https://docs.docker.com/config/containers/resource_constraints/#cpu
[2]: https://kubernetes.io/docs/tasks/configure-pod-container/assign-cpu-resource
[3]: https://kubernetes.io/docs/concepts/workloads/pods/pod/