This page contains a quick reference guide for Linux tuning for Data Transfer hosts connected at speeds of 1Gbps or higher. Note that most of the tuning settings described here will actually decrease performance of hosts connected at rates less than 1Gbps, such as most home users on Cable/DSL connections.
Note that the settings on this page are not attempting to achieve full 10G with a single flow. These settings assume you are using tools that support parallel streams, or have multiple data transfers occurring in parallel, and want to have fair sharing between the flows. As such the maximum values are 2 to 4 times less than what would be required to support a single stream. As an example, a 10Gbps flow across a 100ms network requires 120MB of buffering (see the BDP calculator). Most data movement applications, such as GridFTP, would employ 2-8 streams to do this efficiently and to guard against congestive packet loss. Setting your 10Gbps capable host to consume a maximum of 32M - 64M per socket ensures that parallel streams work well, and do not consume a majority of system resources.
If you are trying to optimize for a single flow, see the tuning advice for test / measurement hosts page.
To check what setting your system is using, use 'sysctl name' (e.g.: 'sysctl net.ipv4.tcp_rmem'). To change a setting use 'sysctl -w'. To make the setting permanent add the setting to the file 'sysctl.conf'.
Like most modern OSes, Linux now does a good job of auto-tuning the TCP buffers, but the default maximum Linux TCP buffer sizes are still too small. Here are some example sysctl.conf configurations for different types of hosts.
For a host with a 10G NIC, optimized for network paths up to 100ms RTT, and for friendliness to single and parallel stream tools, add this to /etc/sysctl.conf
# allow testing with buffers up to 64MB
We also strongly recommend reducing the maximum flow rate to avoid bursts of packets that could overflow switch and receive host buffers.
For example for a 10G host, add this to a boot script:
/sbin/tc qdisc add dev ethN root fq maxrate 8gbit
For for a host running data transfer tools that use 4 parallel streams, do this:
/sbin/tc qdisc add dev ethN root fq maxrate 2gbit
Where 'ethN' is the name of the ethernet device on your system.
For a host with a 10G NIC optimized for network paths up to 200ms RTT, and for friendliness to single and parallel stream tools, or a 40G NIC up on paths up to 50ms RTT:
# allow testing with buffers up to 128MB
Notes: you should leave net.tcp_mem alone, as the defaults are fine. A number of performance experts say to also increase net.core.optmem_max to match net.core.rmem_max and net.core.wmem_max, but we have not found that makes any difference. Some experts also say to set net.ipv4.tcp_timestamps and net.ipv4.tcp_sack to 0, as doing that reduces CPU load. We strongly disagree with that recommendation for WAN performance, as we have observed that the default value of 1 helps in more cases than it hurts, and can help a lot.
Linux supports pluggable congestion control algorithms. To get a list of congestion control algorithms that are available in your kernel (kernal 2.6.20+), run:
cubic is usually the default in most linux distribution, but we have found htcp usually works better. You might also want to try BBR if its available on your system.
To set the default congestion control algorithm, do:
sysctl -w net.ipv4.tcp_congestion_control=htcp