The high packet rates of today's high speed interfaces (up to 14.88 Mpps on 10 GigE interfaces) make it very difficult to do software packet processing at wire rate. An important reason is that the APIs and software architecture that we use is the same we had 20-30 years ago when "fast" was 1000 times slower.
netmap is a very efficient framework for line-rate raw packet I/O from user space, which is capable to support 14.88Mpps on an ordinary PC and OS. Netmap integrates some known ideas into a novel, robust and easy to use framework that is available on FreeBSD and Linux without the need of special hardware or proprietary software.
With netmap, it takes as little as 60-65 clock cycles to move one packet between the user program and the wire. As an example, a single core running at 900 MHz can generate the 14.8 Mpps that saturate a 10 GigE interface. This is a 10-20x improvement over the use of a standard device driver.
The rest of this page gives a high level description of the project.
Papers and presentations
Here are a few papers and submissions describing netmap and applications using it:
Note that we are actively developing and improving the code. The performance data below refers to the July 2011 version, and in many cases we have achieved some speedups (10-20% and more in some cases).
netmap uses some well known performance-boosting techniques, such as memory-mapping the card's packet buffers, I/O batching, and modeling the send and receive queues as circular buffers to match what the hardware implements. Unlike other systems, applications using netmap cannot crash the OS, because they run in user space and have no direct access to critical resources (device registers, kernel memory pointers, etc.). The programming model is extremely simple (circular rings of fixed size buffers), and applications use only standard system calls: non-blocking ioctl() to synchronize with the hardware, and poll()-able file descriptors to wait for packet receptions or transmissions on individual queues.
netmap can generate traffic at line rate (14.88Mpps) on a 10GigE link with just a single core running at 900Mhz. This equals to about 60-65 clock cycles per packet, and scales well with cores and clock frequency (with 4 cores, line rate is achieved at less than 450 MHz). Similar rates are reached on the receive side. In the graph below, the two top curves (green and red) indicate the performance of netmap on FreeBSD with 1 and 4 cores, respectively (Intel 82599 10Gbit card). The blue curve is the fastest available packet generator on Linux (pktgen, works entirely in the kernel), while the purple curve on the bottom shows the performance of a user-space generator on FreeBSD using udp sockets.
netmap scales well to multicore systems: individual file descriptors can be associated to different cards or queues of a multi-queue card, and move packets between queues without the need to synchronize with each other.
netmap implements a special device, /dev/netmap, which is the gateway to switch one or more network cards to netmap mode, where the card's datapath is disconnected from the operating system. open("/dev/netmap") returns a file descriptor that can be used with ioctl(fd, NIOCREG, ...) to switch an interface to netmap mode. A subsequent mmap() exports to userspace a replica of the TX and RX rings of the card, and the actual packet buffers. Each "shadow" ring indicates the number of available buffers, the current read or write index, and the address and length each buffer (buffers have fixed size and are preallocated by the kernel).
Two ioctl() synchronize the state of the rings between kernel
and userspace: ioctl(fd, NIOCRXSYNC) tells
the kernel which buffers have been read by userspace, and informs
userspace of any newly received packets. On the TX side, ioctl(fd, NIOCTXSYNC) tells the kernel about new packets to
transmit, and reports to userspace how many free slots are available.
Receiving a packet is as simple as reading from the buffer in the mmapped region; eventually, ioctl(fd, NIOCRXSYNC) is used to release one or more buffers at once. Writing to the network requires to fill one or more buffers with data, set the lengths, and eventually invoke the ioctl(fd, NIOCTXSYNC) to issue the appropriate commands to the card.
The memory mapped region contains all rings and buffers of all cards in netmap mode, so it is trivial to implement packet forwarding between interfaces. Zero-copy operation is also possible, by simply writing the address of the received buffer into the in the transmit ring.
Talking to the host stack
In addition to the "hardware" rings, each card in netmap mode exposes two additional rings that connect to the host stack. Packets coming from the stack are put in an RX ring where they can be processed in the same way as those coming from the network. Similarly, packets written to the additional TX ring are passed up to the host stack when the ioctl(fd, NIOCTXSYNC) is invoked. Zero-copy bridging between the host stack and the card is then possible in the same way as between two cards. In terms of performance, using the card in netmap mode and bridging in software is often more efficient than using standard mode, because the driver uses simpler and faster code paths.
Programs using netmap do not need any special library or knowledge of the inner details of the network controller. Not only the ring and buffer format is independent of the card itself, but any operation that requires to program the card is done entirely within the kernel.
netmap is currently available for FreeBSD 8 and HEAD, and supports the ixgbe (Intel 10GigE), e1000 (Intel) and re (Realtek) 1GigE drivers. Support for other cards is coming, as well as a Linux port.
The code consists of about 2000 lines for a kernel module, a 400-500 line diff for each individual driver (mostly mechanical modifications). The simplicity of the programming model makes it possible to use netmap without any userspace library.
Example of use
Below is a code snapshot to set a device in netmap mode and read packets from it. Macros are used to assign pointers because the shared memory region contains kernel virtual addresses.
Detailed performance data
When talking about performance it is important to understand what are the relevant metrics. I won't enter into a long discussion here, please have a look at the papers for a more detailed discussion and up to date numbers.
Transmit and receive speed is shown in the previous section, and is relatively uninteresting as we go at line rate even with a severely underclocked CPU.
More interesting is what happens when you touch the data.
netmap can forward packets at line rate (14.88 Mpps) at 1.7 GHz without touching data, and slightly slower with full data copies. As a comparison, native packet forwarding using the in-kernel bridge does about 700Kpps on the same hardware. Though the comparison is a bit unfair because our bridge and testpcap don't do address lookups; however we have some real forwarding code (a modified version of openvswitch) that does almost 3Mpps using netmap.