Shared Memory

Overview

There are two types of shared memory objects in Linux: System V and POSIX. The containers in a pod share the IPC namespace of the pod infrastructure container and so are able to share the System V shared memory objects. This document describes how they can also share POSIX shared memory objects.

POSIX Shared Memory

POSIX shared memory requires that a tmpfs be mounted at /dev/shm. The containers in a pod do not share their mount namespaces so we use volumes to provide the same /dev/shm into each container in a pod. The following example shows how to set up POSIX shared memory between two containers.

shared-memory.yaml

  1. ---
  2. apiVersion: v1
  3. id: hello-openshift
  4. kind: Pod
  5. metadata:
  6.   name: hello-openshift
  7.   labels:
  8.     name: hello-openshift
  9. spec:
  10.   volumes:                          (1)
  11.     - name: dshm
  12.       emptyDir:
  13.         medium: Memory
  14.   containers:
  15.     - image: kubernetes/pause
  16.       name: hello-container1
  17.       ports:
  18.         - containerPort: 8080
  19.           hostPort: 6061
  20.       volumeMounts:                 (2)
  21.         - mountPath: /dev/shm
  22.           name: dshm
  23.     - image: kubernetes/pause
  24.       name: hello-container2
  25.       ports:
  26.         - containerPort: 8081
  27.           hostPort: 6062
  28.       volumeMounts:                 (3)
  29.         - mountPath: /dev/shm
  30.           name: dshm
1specifies the tmpfs volume dshm.
2enables POSIX shared memory for hello-container1 via dshm.
3enables POSIX shared memory for hello-container2 via dshm.

Create the pod using the shared-memory.yaml file:

  1. $ oc create -f shared-memory.yaml