Luke Gorrie's blog

21st Century Network Hacking

Snabb Switch's LuaJIT Ethernet Device Driver

I am having fun while writing the Snabb Switch Ethernet device driver. This is like Intel’s standard E1000/IGB/IGBE driver for Linux except that:

  • It is written in LuaJIT. (It’s my first LuaJIT program, I’m a newbie.)
  • It runs in a normal Linux userspace process but talks directly to hardware.
  • It is tailor made for one application, a simple hypervisor-friendly ethernet switch.

I have been optimizing the LuaJIT selftest code to transmit ethernet packets in a loop. I am pretty encouraged by the performance that I see: 3.1% CPU utilization on a low-end Hetzner EX6 machine to saturate a 1Gbps ethernet port with tiny packets. That is 28 nanoseconds of CPU time per packet.

I hope the details will be interesting. It is not so often that people write about low-level networking in high-level dynamic programming languages, is it? So, to give a quick taste, the driver source code is in intel.lua and the selftest main loop works like this:

local deadline = C.get_time_ns() + 10000000000LL
   while tx_load() > 0.75 do C.usleep(10000) end
   for i = 1, tx_available() do
      add_txbuf(dma_phys, 60)
until C.get_time_ns() > deadline

and here is what that means:

  • Sleep while the hardware transmit queue is at least 75% full (tx_load).
  • Fill up the transmit queue with 60-byte packets (add_txbuf).
  • Tell the NIC hardware to process the transmit queue (flush_tx).
  • Stop after 10 seconds have elapsed (get_time_ns).

This is all accomplished by directly controlling the NIC using memory-mapped register I/O and DMA with shared memory. The only operating system calls here are to sleep and check the time.

This is a really fun sort of programming to be doing!

Going forward I am really excited to see how much of a production quality Ethernet switch can be written in a high-level dynamic programming language, and how neatly any parts that are ultimately written in C can be integrated into the whole. This is an open source project and you are welcome to join in the fun too!

(Comments welcome on the Snabb Switch Reddit.)