Command-line tools


A benchmarking tool is provided to estimate the maximum throughput for UDP. There are two versions: one implemented in Python ( and one in C++ (spead2_bench), which are installed by the corresponding installers. The examples show the Python version, but the C++ version functions very similarly. However, they cannot be mixed: use the same version on each end of the connection.

On the receiver, pick a port number (which must be free for both TCP and UDP) and run slave <port>

Then, on the sender, run master [options] <host> <port>

where host is the hostname of the receiver. This script will run tests at a variety of speeds to determine the maximum speed at which the connection seems reliable most of the time. This speed is right at the edge of stability: for a totally reliable setup, you should use a lower speed.


There are also separate spead2_send and spead2_recv (and Python equivalents) programs. The former generates a stream of meaningless data, while the latter consumes an existing stream and reports the heaps and items that it finds. Apart from being useful for debugging a stream, spead2_recv has a similar plethora of command-line options for tuning that allow for exploration.


mcdump is a tool similar to tcpdump, but specialised for high-speed capture of multicast UDP traffic using hardware that supports the Infiniband Verbs API. It has only been tested on Mellanox ConnectX-3 NICs. Like gulp, it uses a separate thread for disk I/O and CPU core affinity to achieve reliable performance. With a sufficiently fast disk subsystem, it is able to capture line rate from a 40Gb/s adapter.

It is not limited to capturing SPEAD data. It is included with spead2 rather than released separately because it reuses a lot of the spead2 code.


The tool is automatically compiled and installed with spead2, provided that libibverbs support is detected at configure time.

It may also be necessary to configure the system to work with ibverbs. See Support for ibverbs for more information.


The simplest incantation is

mcdump -i xx.xx.xx.xx output.pcap yy.yy.yy.yy:zzzz

which will capture on the interface with IP address xx.xx.xx.xx, for the multicast group yy.yy.yy.yy on UDP port zzzz. mcdump will take care of subscribing to the multicast group. Note that only IPv4 is supported. Capture continues until interrupted by Ctrl-C. You can also list more group:port pairs, which will all stored in the same pcap file.

You can also specify - in place of the filename to suppress the write to file. This is useful to simply count the bytes/packets received without being limited by disk throughput.

Unfortunately, unlike tcpdump, it is not possible to directly tell whether packets were dropped. NIC counters (on Linux, accessed with ethtool -S) can give an indication, although sometimes packets are dropped during the shutdown process.

These options are important for performance:

-N <cpu>, -C <cpu>, -D <cpu>

Set CPU core IDs for various threads. The -D option can be repeated multiple times to use multiple threads for disk I/O. By default, the threads are not bound to any particular core. It is recommended that these cores be on the same CPU socket as the NIC.


Use the O_DIRECT flag to open the file. This bypasses the kernel page cache, and can in some cases yield higher performance. However, not all filesystems support it, and it can also reduce performance when capturing a small enough amount of data that it will fit into RAM.

--count <count>

Stop after <count> packets have been received. Without this option, mcdump will run until SIGINT (Ctrl-C) is received.


  • Timestamps are only collected if Mellanox extensions to the verbs API are detected at compile time. Otherwise, all packets have a zero timestamp in the file.
  • Only IPv4 multicast is supported.
  • It is not optimised for small packets (below about 1KB). Packet capture rates top out around 6Mpps for current hardware.