Operations

    Learn how to tune the OS for more performance, configure the open files limit, the kernel and the network

    IPv4 vs IPv6

    IPv6 (enabled by default on Java) was found to sometimes cause some performance issues, so the launch scripts disable it with the following options:

    -Djava.net.preferIPv4Stack=true
-Djava.net.preferIPv6Addresses=false

    If you really need to prefer IPv6, please edit the launch scripts.

    OS tuning

    Gatling can consume a very large number of open file handles during normal operation. Typically, operating systems limit this number, so you may have to tweak a few options in your chosen OS so that you can open many new sockets and achieve heavy load.

    Kernel and network tuning

    Consider tuning kernel and network by adding settings such as the following in /etc/sysctl.d/99-gatling.conf:

    fs.nr_open = 1073741816

net.ipv4.ip_local_port_range = 1024 65535
net.ipv4.tcp_fin_timeout = 15
net.ipv4.tcp_max_tw_buckets = 8192
net.ipv4.tcp_tw_reuse = 1

net.ipv4.tcp_slow_start_after_idle = 0

net.core.optmem_max = 131072
net.ipv4.tcp_rmem = 4096 131072 33554432
net.ipv4.tcp_wmem = 4096 16384 4194304

    In newer kernel (6.1+), as fs.file-max defaults to the largest signed 64 bits integer, it doesn’t need to be changed. If it is not the case, you can add it to the previous configuration. This can be checked with:

    $ cat /proc/sys/fs/file-max
9223372036854775807

    Apply the configuration by using:

    sudo sysctl -p /etc/sysctl.d/99-gatling.conf

    Open files limit

    Most operating systems can change the open-files limit using the ulimit -n command. Example:

    ulimit -n 1048576

    However, this only changes the limit for the current shell session. Changing the limit on a system-wide, permanent basis varies more between systems.

    To permanently set the soft and hard values for all users of the system to allow for up to 1048576 open files; create or edit /etc/security/limits.d/99-gatling.conf and add the following two lines:

    *         soft   nofile 1048576
*         hard   nofile 1048576

    Save the file. Start a new session so that the limits take effect. You can now verify with ulimit -n that the limits are correctly set.

    Also, if accessing the machine via SSH, be sure to have UsePAM yes in /etc/ssh/sshd_config.

    Systemd limits

    The SSH daemon can also be limited by Systemd, which can be checked with:

    $ systemctl show sshd -p LimitNOFILE
LimitNOFILE=65535

    If you see this number is below our settings, you can either edit the sshd service file and add LimitNOFILE=1048576 in the Service section or create a file called /etc/systemd/system.conf.d/99-gatling.conf with the following content:

    [Manager]
DefaultLimitNOFILE=1048576:1048576

    The /etc/systemd/system.conf.d may not exist yet and can be created with:

    sudo mkdir /etc/systemd/system.conf.d

    In Amazon Linux 2023, there is an existing file with default limits you can edit: /etc/systemd/system.conf.d/50-limits.conf.

    Don’t forget to reload the Systemd daemon after any modification, then check the limit again:

    $ sudo systemctl daemon-reload
$ systemctl show sshd -p LimitNOFILE
LimitNOFILE=1048576

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