Which amount of CPU and memory are recommended to reserve for the system in OpenShift 4 nodes?

Solution Verified - Updated -

Environment

  • Red Hat OpenShift Container Platform (RHOCP)
    • 4
  • Red Hat Enterprise Linux CoreOS (RHCOS)
    • 4

Issue

  • Recommended systemReserved values for OpenShift 4 clusters.

Resolution

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The systemReserved CPU and memory resources avoid that scheduled pods consume the whole resources of a node, causing the node to hang. The recommended values depends on specific node resources.

IMPORTANT NOTE: the recommended values for the systemReserved could be different by OpenShift version. Refer to the dynamic_memory_sizing and dynamic_cpu_sizing functions code for each specific release.

Recommended reserved Memory

At the time of writing, recommended values are:

  • For nodes with 1 GiB of memory or less

    • 255 MiB.

  • For nodes with more than 1 GiB of memory

    • 25% of the first 4 GiB of memory.
    • 20% of the next 4 GiB of memory (between 4 GiB and 8 GiB).
    • 10% of the next 8 GiB of memory (between 8 GiB and 16 GiB).
    • 6% of the next 112 GB of memory (up to 128GB)
    • 2% of the remaining memory.

Recommended reserved CPU

  • The recommended values till OpenShift 4.16 are as follows:

    • 6% of the first core.
    • 1% of the second core.
    • 0.5% of the next 2 cores.
    • 0.25% of any remaining core.

  • Starting with OpenShift 4.17, new recommendations are in place introduced by modify auto tuned system reserved cpu recommendation:

    Raw
            # Base allocation for 1 CPU in fractions of a core (60 millicores = 0.06 CPU core)
        base_allocation_fraction=0.06
        # Increment per additional CPU in fractions of a core (12 millicores = 0.012 CPU core)
        increment_per_cpu_fraction=0.012
        if ((total_cpu > 1)); then
            # Calculate the total system-reserved CPU in fractions, starting with the base allocation
            # and adding the incremental fraction for each additional CPU
            recommended_systemreserved_cpu=$(awk -v base="$base_allocation_fraction" -v increment="$increment_per_cpu_fraction" -v cpus="$total_cpu" 'BEGIN {printf "%.2f\n", base + increment * (cpus - 1)}')
        else
            # For a single CPU, use the base allocation
            recommended_systemreserved_cpu=$base_allocation_fraction
        fi

    Note: it is also possible to use a script to obtain the memory and CPU values. Refer to the "Diagnostic Steps" section.

Configuration

Default system reservation values are explained in what is the default setting value of reserved and eviction in OpenShift, but those defaults are not the recommended values for all node sizes.

Starting with OpenShift 4.8, the reservation can be configured to be calculated automatically based on the resources in each node. See automatically allocating resources for nodes for more information. This feature is disabled by default.

It is also possible to manually allocate resources for nodes.

Note: The values recommended above are the ones applied when the calculation is automatic as per the source code. If they are not suitable for specific cluster for any reason, please contact Red Hat Support for advice.

Example of the old recomendation

Let's say that there is a node with the following hardware:

  • CPU: 16 cores.
  • Memory: 32 GiB.

Values recommended for the above example for OpenShift 4.16 and older:

  • Recommended reserved CPU = 60m + 10m + (2 * 5m) + (12 * 2.5m) = 110m.

    • Consequent allocatable CPU = 16000m- 110m= 15890m.

  • Recommended reserved memory = 25% of 4 GiB + 20% of 4 GiB + 10% of 8 GiB + 6% of 16 GiB = 3.56 GiB.

    • Consequent allocatable memory = 32 GiB - 3.56 GiB = 28.44 GiB.

Note: the m stands for milicore. 1000m = 1 core.

Root Cause

The systemReserved CPU and memory resources avoid that scheduled pods consume the whole resources of a node, at the expense of the system critical processes like cri-o, kubelet, sshd, NetworkManager. The entire list of processes using the systemReserved resources can be shown as explained in the "Diagnostic Steps" section.

Diagnostic Steps

  • Check the entire list of processes included in the resource reservation:

    Raw
    $ oc debug node/[node_name]
[...]
sh-4.4# chroot /host bash
# systemd-cgls /system.slice/

  • If the automatic allocation of resources is enabled, it is possible to check the values in use checking the /etc/node-sizing.env file within the nodes:

    Raw
    $ oc debug node/[node_name]
[...]
sh-4.4# chroot /host bash
# cat /etc/node-sizing.env

  • If the automatic allocation of resources is NOT enabled, it is possible to check the values it will generate for specific node using the script already included in the MachineConfigs starting with 00 in current OpenShift releases the following way:

    Raw
    $ oc debug node/[node_name]
[...]
sh-4.4# chroot /host bash
# NODE_SIZES_ENV=/tmp/node-sizing.txt /usr/local/sbin/dynamic-system-reserved-calc.sh true
# cat /tmp/node-sizing.txt

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