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Chapter 1. Tuning network adapter settings

In high-speed networks with 40 Gbps and faster, certain default values of network adapter-related kernel settings can be a cause of packet drops and performance degradation. Tuning these settings can prevent such problems.

Increase the size of an Ethernet device’s ring buffers if the packet drop rate causes applications to report a loss of data, timeouts, or other issues.

Receive ring buffers are shared between the device driver and network interface controller (NIC). The card assigns a transmit (TX) and receive (RX) ring buffer. As the name implies, the ring buffer is a circular buffer where an overflow overwrites existing data. There are two ways to move data from the NIC to the kernel, hardware interrupts and software interrupts, also called SoftIRQs.

The kernel uses the RX ring buffer to store incoming packets until the device driver can process them. The device driver drains the RX ring, typically by using SoftIRQs, which puts the incoming packets into a kernel data structure called an sk_buff or skb to begin its journey through the kernel and up to the application that owns the relevant socket.

The kernel uses the TX ring buffer to hold outgoing packets which should be sent to the network. These ring buffers reside at the bottom of the stack and are a crucial point at which packet drop can occur, which in turn will adversely affect network performance.

Procedure

  1. Display the packet drop statistics of the interface: 

    # ethtool -S enp1s0
    ...
    rx_queue_0_drops: 97326
    rx_queue_1_drops: 63783
    ...

    Note that the output of the command depends on the network card and the driver.

    High values in discard or drop counters indicate that the available buffer fills up faster than the kernel can process the packets. Increasing the ring buffers can help to avoid such loss.

  2. Display the maximum ring buffer sizes: 

    # ethtool -g enp1s0
 Ring parameters for enp1s0:
 Pre-set maximums:
 RX:             4096
 RX Mini:        0
 RX Jumbo:       16320
 TX:             4096
 Current hardware settings:
 RX:             255
 RX Mini:        0
 RX Jumbo:       0
 TX:             255

    If the values in the Pre-set maximums section are higher than in the Current hardware settings section, you can change the settings in the next steps.

  3. Identify the NetworkManager connection profile that uses the interface: 

    # nmcli connection show
NAME                UUID                                  TYPE      DEVICE
Example-Connection  a5eb6490-cc20-3668-81f8-0314a27f3f75  ethernet  enp1s0

  4. Update the connection profile, and increase the ring buffers:

    • To increase the RX ring buffer, enter: 

      # nmcli connection modify Example-Connection ethtool.ring-rx 4096

    • To increase the TX ring buffer, enter: 

      # nmcli connection modify Example-Connection ethtool.ring-tx 4096

  5. Reload the NetworkManager connection: 

    # nmcli connection up Example-Connection
    Important

    Depending on the driver your NIC uses, changing in the ring buffer can shortly interrupt the network connection.

Increase the size of an Ethernet device’s ring buffers if the packet drop rate causes applications to report a loss of data, timeouts, or other issues.

Ring buffers are circular buffers where an overflow overwrites existing data. The network card assigns a transmit (TX) and receive (RX) ring buffer. Receive ring buffers are shared between the device driver and the network interface controller (NIC). Data can move from NIC to the kernel through either hardware interrupts or software interrupts, also called SoftIRQs.

The kernel uses the RX ring buffer to store incoming packets until the device driver can process them. The device driver drains the RX ring, typically by using SoftIRQs, which puts the incoming packets into a kernel data structure called an sk_buff or skb to begin its journey through the kernel and up to the application that owns the relevant socket.

The kernel uses the TX ring buffer to hold outgoing packets which should be sent to the network. These ring buffers reside at the bottom of the stack and are a crucial point at which packet drop can occur, which in turn will adversely affect network performance.

You configure ring buffer settings in the NetworkManager connection profiles. By using Ansible and the network RHEL system role, you can automate this process and remotely configure connection profiles on the hosts defined in a playbook.

Warning

You cannot use the network RHEL system role to update only specific values in an existing connection profile. The role ensures that a connection profile exactly matches the settings in a playbook. If a connection profile with the same name already exists, the role applies the settings from the playbook and resets all other settings in the profile to their defaults. To prevent resetting values, always specify the whole configuration of the network connection profile in the playbook, including the settings that you do not want to change.

Prerequisites

  • You have prepared the control node and the managed nodes.
  • You are logged in to the control node as a user who can run playbooks on the managed nodes.
  • The account you use to connect to the managed nodes has sudo permissions for these nodes.
  • You know the maximum ring buffer sizes that the device supports.

Procedure

  1. Create a playbook file, for example, ~/playbook.yml, with the following content: 

    ---
- name: Configure the network
  hosts: managed-node-01.example.com
  tasks:
    - name: Ethernet connection profile with dynamic IP address setting and increased ring buffer sizes
      ansible.builtin.include_role:
        name: redhat.rhel_system_roles.network
      vars:
        network_connections:
          - name: enp1s0
            type: ethernet
            autoconnect: yes
            ip:
              dhcp4: yes
              auto6: yes
            ethtool:
              ring:
                rx: 4096
                tx: 4096
            state: up

    The settings specified in the example playbook include the following:

    rx: <value>
    Sets the maximum number of received ring buffer entries.
    tx: <value>
    Sets the maximum number of transmitted ring buffer entries.

    For details about all variables used in the playbook, see the /usr/share/ansible/roles/rhel-system-roles.network/README.md file on the control node.

  2. Validate the playbook syntax: 

    $ ansible-playbook --syntax-check ~/playbook.yml

    Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

  3. Run the playbook: 

    $ ansible-playbook ~/playbook.yml

Verification

  • Display the maximum ring buffer sizes: 

    # ansible managed-node-01.example.com -m command -a 'ethtool -g enp1s0'
managed-node-01.example.com | CHANGED | rc=0 >>
...
Current hardware settings:
RX:             4096
RX Mini:        0
RX Jumbo:       0
TX:             4096

When a network card receives packets and before the kernel protocol stack processes them, the kernel stores these packets in backlog queues. The kernel maintains a separate queue for each CPU core.

If the backlog queue for a core is full, the kernel drops all further incoming packets that the netif_receive_skb() kernel function assigns to this queue. If the server contains a 10 Gbps or faster network adapter or multiple 1 Gbps adapters, tune the backlog queue size to avoid this problem.

Prerequisites

  • A 10 Gbps or faster or multiple 1 Gbps network adapters

Procedure

  1. Determine whether tuning the backlog queue is needed, display the counters in the /proc/net/softnet_stat file: 

    # awk '{for (i=1; i<=NF; i++) printf strtonum("0x" $i) (i==NF?"\n":" ")}' /proc/net/softnet_stat | column -t
221951548  0      0  0  0  0  0  0  0  0  0  0  0
192058677  18862  0  0  0  0  0  0  0  0  0  0  1
455324886  0      0  0  0  0  0  0  0  0  0  0  2
...

    This awk command converts the values in /proc/net/softnet_stat from hexadecimal to decimal format and displays them in table format. Each line represents a CPU core starting with core 0.

    The relevant columns are:

    • First column: The total number of received frames
    • Second column: The number of dropped frames because of a full backlog queue
    • Last column: The CPU core number

  2. If the values in the second column of the /proc/net/softnet_stat file increment over time, increase the size of the backlog queue:

    1. Display the current backlog queue size: 

      # sysctl net.core.netdev_max_backlog
net.core.netdev_max_backlog = 1000

    2. Create the /etc/sysctl.d/10-netdev_max_backlog.conf file with the following content: 

      net.core.netdev_max_backlog = 2000

      Set the net.core.netdev_max_backlog parameter to a double of the current value.

    3. Load the settings from the /etc/sysctl.d/10-netdev_max_backlog.conf file: 

      # sysctl -p /etc/sysctl.d/10-netdev_max_backlog.conf

Verification

  • Monitor the second column in the /proc/net/softnet_stat file: 

    # awk '{for (i=1; i<=NF; i++) printf strtonum("0x" $i) (i==NF?"\n":" ")}' /proc/net/softnet_stat | column -t

    If the values still increase, double the net.core.netdev_max_backlog value again. Repeat this process until the packet drop counters no longer increase.

The kernel stores packets in a transmit queue before transmitting them. The default queue length (1000 packets) is often sufficient for 10 Gbps networks. However, in faster networks, or if you encounter an increasing number of transmit errors on an adapter, increase the queue length.

Procedure

  1. Display the current transmit queue length: 

    # ip -s link show enp1s0
2: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000
...

    In this example, the transmit queue length (qlen) of the enp1s0 interface is 1000.

  2. Monitor the dropped packets counter of a network interface’s software transmit queue: 

    # tc -s qdisc show dev enp1s0
qdisc fq_codel 0: root refcnt 2 limit 10240p flows 1024 quantum 1514 target 5ms interval 100ms memory_limit 32Mb ecn drop_batch 64
 Sent 16889923 bytes 426862765 pkt (dropped 191980, overlimits 0 requeues 2)
...

  3. If you encounter a high or increasing transmit error count, set a higher transmit queue length:

    1. Identify the NetworkManager connection profile that uses this interface: 

      # nmcli connection show
NAME                UUID                                  TYPE      DEVICE
Example-Connection  a5eb6490-cc20-3668-81f8-0314a27f3f75  ethernet  enp1s0

    2. Update the connection profile, and increase the transmit queue length: 

      # nmcli connection modify Example-Connection link.tx-queue-length 2000

      Set the queue length to double of the current value.

    3. Apply the changes: 

      # nmcli connection up Example-Connection

Verification

  1. Display the transmit queue length: 

    # ip -s link show enp1s0
2: enp1s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 2000
...

  2. Monitor the dropped packets counter: 

    # tc -s qdisc show dev enp1s0

    If the dropped counter still increases, double the transmit queue length again. Repeat this process until the counter no longer increases.

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