Requests and Limits
As described in kubernetes documentation, platform uses two kinds of resource “limits”. One is called Requests, and provides guaranteed amount of resource, the other one is called Limits and specifies never exceed limit of resource. User could specify Requests only, which enables guarantees without limit (which forbidden in our platform), or Limits only and in this case, Requests and Limits are equal. All resources are allocated from a single computing node, it means that they cannot be larger than capacity of a single node, e.g., requesting 1000 CPUs will never be satisfied. Resources between Requests and Limits are not guaranteed. Kubernetes scheduler consider only Requests when planning Pods to nodes, so it can happen that node receives disk or memory pressure caused by overbooking these kinds of resources. In this case, a Pod can be evicted from the node. For CPU resources between Requests and Limits, slow down can happen if node does not have free CPUs anymore.
General YAML fragment looks like this:
resources:
requests:
# resources
limits:
# resources
CPU
CPU resource can be requested in whole CPU units such as 2 for two CPUs, or in milli units such as 100m for 0.1 CPU. See complete example below.
Memory
Memory resource can be requested in bytes or multiples of binary bytes. Notation is 1000Mi or 1Gi for 1GB of memory. Amount of this resource comprises shared memory (see below) and all memory emptyDir volumes. The example below consumes up to 1GB of memory resource and if application requires e.g., 2GB of memory, user needs to request 3GB of memory resource.
volumes:
- name ramdisk
emptyDir:
medium: Memory
sizeLImit: 1Gi
Shared Memory
By default, each container runs with 64MB limit on shared memory. For many cases, this limit is enough but for some GPU applications or GUI applications this amount is not enough. In such a case, SHM size needs to be increased. This cannot be done in resource section, the only possibility is to mount additional memory volume using the following YAML fragment that increases SHM size to 1GB:
volumes:
- name: dshm
emptyDir:
medium: Memory
sizeLimit: 1Gi
volumeMounts:
- name: dshm
mountPath: /dev/shm
Name of the volume is not important and can be anything valid, the mountPath must be /dev/shm.
GPU
GPU resource can be requested in two distinct ways. User can request GPU(s) exclusively using nvidia.com/gpu: x where x is a number denoting number of requested GPUs. User can request also only a fraction of GPU using cerit.io/gpu-mem: x where x is a number of GB of GPU memory. In this case, user is given a possibly shared GPU. Fractions of GPUs are easily available, on the other hand, Linux has no technical power to enforce requested GPU memory limits, so if a user exceeds requested amount of GPU memory, there is a chance that the computation will fail for all the users sharing the GPU.
Storage
User should specify also required ephemeral-storage resource. Units are the same as for the memory resource. This resource denotes limit on a local storage that comprises size of running container and all local files created. Those files includes all temporary files within container such as files in /tmp or /var/tmp directories and also all emptyDir volumes that are not in memory.
Full Resource Example
The following example requests 1 GPU, 2 CPUs and 4GB of memory guaranteed, and 3 CPUs and 6GB of memory hard limit.
resources:
requests:
cpu: 2
memory: 4Gi
nvidia.com/gpu: 1
limits:
cpu: 3
memory: 6Gi
nvidia.com/gpu: 1
The following example requests 2 GB of GPU memory, 0.5 CPU and 4GB of memory guaranteed and also as hard limit.
resources:
requests:
cpu: 500m
memory: 4Gi
cerit.io/gpu-mem: 2
limits:
cpu: 500m
memory: 4Gi
cerit.io/gpu-mem: 2