Linux Tuning

This page contains a quick reference guide for Linux 2.6+ 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 of OC3 (155 Mbps) or less, such as home users on Cable/DSL connections.

For a detailed explanation of some of the advice on this page, see the Linux Tuning Expert page. 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.

If you are trying to optimize for a single flow, see the tuning advice for test/measurement hosts page.

 General Approach

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'.

 TCP tuning

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 commands for different types of hosts.

For a host with a 10G NIC, optimized for network paths up to 100ms RTT, add this to /etc/sysctl.conf

# increase TCP max buffer size settable using setsockopt()
net.core.rmem_max = 16777216 
net.core.wmem_max = 16777216 
# increase Linux autotuning TCP buffer limit 
net.ipv4.tcp_rmem = 4096 87380 16777216
net.ipv4.tcp_wmem = 4096 65536 16777216
# increase the length of the processor input queue
net.core.netdev_max_backlog = 30000
# recommended default congestion control is htcp 
net.ipv4.tcp_congestion_control=htcp
# recommended for hosts with jumbo frames enabled
net.ipv4.tcp_mtu_probing=1

    /sbin/ifconfig ethN txqueuelen 10000

For a host with a 10G NIC optimized for network paths up to 200ms RTT, or a 40G NIC up on paths up to 50ms RTT:

# increase TCP max buffer size settable using setsockopt()
net.core.rmem_max = 67108864 
net.core.wmem_max = 67108864 
# increase Linux autotuning TCP buffer limit
net.ipv4.tcp_rmem = 4096 87380 67108864
net.ipv4.tcp_wmem = 4096 65536 67108864
# increase the length of the processor input queue
net.core.netdev_max_backlog = 250000
# recommended default congestion control is htcp 
net.ipv4.tcp_congestion_control=htcp
# recommended for hosts with jumbo frames enabled
net.ipv4.tcp_mtu_probing=1

    /sbin/ifconfig ethN txqueuelen 10000

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 disagree with that recommendation, as we have observed that the default value of 1 helps in more cases than it hurts. But if you are extremely CPU bound you might want to experiment with turning those off.

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:

sysctl net.ipv4.tcp_available_congestion_control

If cubic and/or htcp are not listed try the following, as most distributions include them as loadable kernel modules:

/sbin/modprobe tcp_htcp
/sbin/modprobe tcp_cubic

NOTE: There seem to be bugs in both bic and cubic for a number of versions of the Linux kernel up to version 2.6.33. We recommend using htcp with older kernels to be safe. To set the congestion control do:

sysctl -w net.ipv4.tcp_congestion_control=htcp

If you are using Jumbo Frames, we recommend setting tcp_mtu_probing = 1 to help avoid the problem of MTU black holes. Setting it to 2 sometimes causes performance problems.

 UDP Tuning

The TCP window size actually effects UDP as well on Linux. Be sure to use the setsockopt() call to increase SO_SNDBUF/SO_RCVBUF to around 4MB if you want to do UDP streams that are faster than 3-4 Gbps. The optimal data payload size is the MTU size minus IP and UDP header size, or 1472 for a 1500 byte MTU and 8972 for a 9000 byte MTU.

 NIC Tuning

This can be added to /etc/rc.local to be run at boot time:

    # increase txqueuelen for 10G NICS
    /sbin/ifconfig ethN txqueuelen 10000

Note that this might have adverse affects for a 10G host sending to a 1G host or slower.

 Virtual Machine Tuning