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OpenBSD network performance tunning


Many of today’s desktop systems and servers come with on board gigabit network controllers. After some simple speeds tests you will soon find out that you are not be able to transfer data over the network much faster than you did with a 100MB link. There are many factors which affect network performance including hardware, operating systems and network stack options. The purpose of this page is to explain how you can achieve up to 930 megabits per second transfer rates over a gigabit link using OpenBSD as a firewall or transparent bridge.

It is important to remember that you can not expect to reach gigabit speeds using slow hardware or an unoptimized firewall rule set. Speed and efficiency are key to our goal. Lets start with the most important aspect of the equation, hardware.


No matter what operating system you choose, the machine you run on will determine the theoretical speed limit you can expect to achieve. When people talk about how fast a system is they always mention CPU clock speed. We would expect an AMD64 2.4GHz to run faster than a Pentium3 1.0 GHz, but CPU speed is not the key, motherboard bus speed is.

In terms of a firewall or bridge we are looking to move data through the system as fast as possible. This means we need to have a PCI bus that is able to move data quickly between network interfaces. To do this the machine must have a wide bus and high bus speed. CPU clock speed is a very minor part of the equation.

The quality of a network card is key to high though put. As a very general rule, using the on-board network card is going to be much slower than an add in PCI card. The reason is that most desktop motherboard manufacturers use cheap on-board network chip sets that use CPU processing time instead of handling TCP traffic by themselves. This leads to very slow network performance and high CPU load.

A gigabit network controller built on board using the CPU will slow the entire system down. More than likely the system will not even be able to sustain 100MB speeds while also pegging the CPU at 100%. A network controller that is able to negotiate as a gigabit is _very_ different from a controller that can transfer a gigabit of data per second.

Ideally you want to use a server based add on card with a TCP offload engine or TCP accelerator. We have seen very good speeds with the Intel Pro/1000 MT series (em4) cards. They are not too expensive and all OS’s have support.

Not to say that all on-board chip sets are bad. Supermicro server boards use an Intel 82546EB Gigabit Ethernet Controller on their server motherboards. It offers two(2) copper gigabit ports through a single chip set offering a 133MHz PCI-X, 128 bit wide bus, pre-fetching up to 64 packet descriptors and has two 64 KB on-chip packet buffers. This is an exceptionally fast chip and it saves space by being built onto the server board.

Now, in order to move data in and out of the network cards as fast as possible we need a bus with a wide bit rate and high clock speed. For example, a PCI-X 64bit slot is wider than a PCI-X 32bit as is a 66MHz bus is faster than a 33MHz bus. Wide is good, fast is good, but wide and fast are better.




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