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Monday, August 3, 2009

Linux Tune Network Stack (Buffers Size) To Increase Networking Performance


I have two servers located in two different data centers. Both servers deals with a lot of concurrent large file transfers. But network performance is very poor for large files and performance degradation take place with a large files. How do I tune TCP under Linux to solve this problem?

By default the Linux network stack is not configured for high speed large file transfer across WAN links. This is done to save memory resources. You can easily tune Linux network stack by increasing network buffers size for high-speed networks that connect server systems to handle more network packets.

The default maximum Linux TCP buffer sizes are way too small. TCP memory is calculated automatically based on system memory; you can find the actual values by typing the following commands:
$ cat /proc/sys/net/ipv4/tcp_mem

The default and maximum amount for the receive socket memory:
$ cat /proc/sys/net/core/rmem_default
$ cat /proc/sys/net/core/rmem_max


The default and maximum amount for the send socket memory:
$ cat /proc/sys/net/core/wmem_default
$ cat /proc/sys/net/core/wmem_max


The maximum amount of option memory buffers:
$ cat /proc/sys/net/core/optmem_max

Tune values

Set the max OS send buffer size (wmem) and receive buffer size (rmem) to 12 MB for queues on all protocols. In other words set the amount of memory that is allocated for each TCP socket when it is opened or created while transferring files:

WARNING! The default value of rmem_max and wmem_max is about 128 KB in most Linux distributions, which may be enough for a low-latency general purpose network environment or for apps such as DNS / Web server. However, if the latency is large, the default size might be too small. Please note that the following settings going to increase memory usage on your server.


# echo 'net.core.wmem_max=12582912' >> /etc/sysctl.conf
# echo 'net.core.rmem_max=12582912' >> /etc/sysctl.conf


You also need to set minimum size, initial size, and maximum size in bytes:
# echo 'net.ipv4.tcp_rmem= 10240 87380 12582912' >> /etc/sysctl.conf
# echo 'net.ipv4.tcp_wmem= 10240 87380 12582912' >> /etc/sysctl.conf


Turn on window scaling which can be an option to enlarge the transfer window:
# echo 'net.ipv4.tcp_window_scaling = 1' >> /etc/sysctl.conf

Enable timestamps as defined in RFC1323:
# echo 'net.ipv4.tcp_timestamps = 1' >> /etc/sysctl.conf

Enable select acknowledgments:
# echo 'net.ipv4.tcp_sack = 1' >> /etc/sysctl.conf

By default, TCP saves various connection metrics in the route cache when the connection closes, so that connections established in the near future can use these to set initial conditions. Usually, this increases overall performance, but may sometimes cause performance degradation. If set, TCP will not cache metrics on closing connections.

# echo 'net.ipv4.tcp_no_metrics_save = 1' >> /etc/sysctl.conf

Set maximum number of packets, queued on the INPUT side, when the interface receives packets faster than kernel can process them.

# echo 'net.core.netdev_max_backlog = 5000' >> /etc/sysctl.conf


Now reload the changes:

# sysctl -p


Use tcpdump to view changes for eth0:

# tcpdump -ni eth0

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