To obtain the maximum possible TCP throughput, it is usually necessary to tune
the TCP settings. In order to tune the host, one must understand a bit about
TCP buffers.
Please read our short overview of TCP buffers. A complete explanation
is available in the PSC TCP Tuning Guide.
Below is a brief summary of how to check and set the maximum TCP
buffer size. These examples set the maximum buffer size to 4 MB, which
is a reasonable value for most of today's high-speed networks.
For Linux/FreeBSD/MacOSX/Solaris, you can run this script
to check your TCP buffer size.
More details on how to
determine the best buffer size and other host tuning options are described
in the TCP Tuning Guide.
Linux:
To check the maximum buffer size, type the following, and make sure the values
are at least 4 MB:
sysctl -a | grep "net.core.*max"
sysctl -a | grep "net.ipv4.*mem"
To set the maximum buffer size, add these lines to the file /etc/sysctl.conf, and then run "sysctl -p".
# increase TCP max buffer size
net.core.rmem_max = 4194304
net.core.wmem_max = 4194304
# increase Linux autotuning TCP buffer limits
net.ipv4.tcp_rmem = 4096 87380 4194304
net.ipv4.tcp_wmem = 4096 65536 4194304
Windows XP:
The easiest way to tune TCP under Windows XP is to get
DrTCP from "DSL Reports".
Set the "Tcp Receive Window" to your computed BDP (e.g. 4000000), turn on "Window Scaling"
"Selective Acks", and "Time Stamping".
A more powerful (and more complicated) tool for tuning Windows XP TCP is the
SG TCP Optimizer.
Windows Vista:
Default maximum TCP buffer in Vista is 16 MB, so no tuning is needed.
Mac OSX and FreeBSD:
To check the maximum buffer size, type the following, and make sure the values
is at least 4 MB:
sysctl kern.ipc.maxsockbuf
To set the maximum buffer size, add these lines to the file /etc/sysctl.conf:
kern.ipc.maxsockbuf=4194304
and then run:
sysctl -w kern.ipc.maxsockbuf=4194304
Solaris:
To check the maximum buffer size, type the following, and make sure the values
are at least 4 MB:
ndd /dev/tcp tcp_max_buf
ndd /dev/tcp tcp_cwnd_max
To set the maximum buffer size, create and execute a boot script with the following lines:
ndd -set /dev/tcp tcp_max_buf 4194304
ndd -set /dev/tcp tcp_cwnd_max 4194304
More detailed information on host tuning is available at
http://dsd.lbl.gov/TCP-tuning/
and http://www.psc.edu/networking/projects/tcptune/.
Warning: TCP Buffers that are too large can cause some applications to behave poorly (typically causing sluggish
interactive response) and risk running the system out of memory. Therefore one should be careful using large
default socket buffers on OSes without TCP autotuning.
Also, Setting the TCP buffers to the wrong value (0: for example) can
effectively disable some systems. ESnet takes no responsibility for any problems
you encounter due to changing your system settings.
