This repo contains an implementation of the Homa transport protocol as a Linux kernel module.

• For more information on Homa in general, see the Homa Wiki.

• More information about this implementation and its performance are available in the paper A Linux Kernel Implementation of the Homa Transport Protocol, which appeared in the USENIX Annual Technical Conference in July, 2021.

• A synopsis of the protocol implemented by this module is available in protocol.md.

• As of August 2020, Homa has complete functionality for running real applications, and its tail latency is more than 10x better than TCP for all workloads I have measured (Homa's 99-th percentile latency is usually better than TCP's mean latency). Here is a list of the most significant functionality that is still missing:

  • The incast optimization from Section 3.6 of the SIGCOMM paper has not been implemented yet. If you would like to test Homa under large incasts, let me know and I will implement this feature.

• Please contact me if you have any problems using this repo; I'm happy to provide advice and support.

• To build and install Homa, see instructions in INSTALL.md.

• A collection of man pages is available in the "man" subdirectory. The API for Homa is different from TCP sockets.

• Related work that you may find useful:

  • Preliminary support for using Homa with gRPC
  • A Go client that works with this module

• NIC support: Homa is known to work with the following NICs:

  • Mellanox ConnectX-4, ConnectX-5, and ConnectX-6
  • Intel E810 series (ice) Please let me know if you find other NICs that work (or don't work).

• The subdirectory "test" contains unit tests, which you can run by typing "make" in that subdirectory.

• The subdirectory "util" contains an assortment of utility programs that you may find useful in exercising and benchmarking Homa. Compile them by typing make in that subdirectory. Here are some examples of benchmarks you might find useful:

  • The cp_node program can be run stand-alone on clients and servers to run simple benchmarks. For a simple latency test, run cp_node server on node1 of the cluster, then run cp_node client on node 0. The client will send continuous back-to-back short requests to the server and output timing information. Or, run cp_node client --workload 500000 on the client: this will send continuous 500 KB messages for a simple througput test. Type cp_node --help to learn about other ways you can use this program.
  • The cp_bench script uses cp_node to run any of a variety of benchmarks; it was used to generate the data for Figures 3 and 4 in the Homa ATC paper. Here is an example command:

    cp_bench -n 10 -w w4 -b 20 -B homa_vs_tcp
    

    The -B option selects one of several benchmarks; homa_vs_tcp will run both Homa and TCP on the same workload and generate graphs comparing them. When invoked on node0, this will run the W4 workload from the ATC paper, running on 10 nodes and generating 20 Gbps of offered load (80% network load on a 25 Gbps network). Type cp_bench --help for information on all available options.

• Some additional tools you might find useful:

  • Homa collects various metrics about its behavior, such as the size distribution of incoming messages. You can access these through the file /proc/net/homa_metrics. The script util/metrics.py will collect metrics and print out all the numbers that have changed since its last run.
  • Homa exports a collection of configuration parameters through the sysctl mechanism. For details, see the man page homa.7.

Significant changes

• March 2026: backported Homa to Red Hat Enterprise Linux versions 8 and 9.5, using the branches rhel8 and rhel9.5. Future changes made to the main branch are likely to be reflected in these branches also.
• January 2026: introduced new 'homa_qdisc' queuing discpline to improve performance when TCP and Homa run simultaneously. Results on c6620 CloudLab cluster (100 Gbps network):
  • Without homa_qdisc, if Homa and TCP run together, Homa performance suffers (4x increase for P99 for short messages) but TCP performance improves.
  • Homa_qdisc improves performance for both Homa and TCP, whether running stand-alone or together.
  • homa_qdisc improves Homa's performance when running with TCP to almost what is is when running without TCP.
  • TCP performance improves when running together with Homa, with or without homa_qdisc.
• November 2025: upgraded to Linux 6.17.8.
• October 2025: added the HOMAIOCINFO ioctl for retrieving status information about a Homa socket. See man/homa.7 for details.
• May 2025: homa_api.c has been removed, so the functions homa_abort, homa_reply, homa_replyv, homa_send, and homa_sendv no longer exist.
• May 2025: added support for network namespaces.
• May 2025: reworked support for peers to cap peer memory usage.
• April 2025: upgraded to Linux 6.13.9.
• April 2025: major refactoring of grant management (more efficient, remove complexity that was causing an unending stream of bugs).
• March 2025: added memory cap on memory for outgoing messages: send requests can block if memory limit is reached.
• March 2025: implemented private RPCs, resulting in API changes. HOMA_RECVMSG_REQUEST and HOMA_RECVMSG_RESPONSE flags no longer exist and struct homa_sendmsg_args now has a flags field with one defined flag: HOMA_SENDMSG_PRIVATE.
• February 2025: by default, incoming requests for a socket are rejected unless the socket has been bound. setsockopt can be used with SO_HOMA_SERVER to enable or disable incoming requests for any socket.
• October 2024: the process of upstreaming Homa into the Linux kernel has begun. The reviewing process is likely to result in API changes. Upstreaming will occur in stages, so the first version to appear in Linux will not be either functionally complete or performant. The sources in this repository contain '#ifndef STRIP' directives, which separate functionality being upstreamed from functionality that is not currently upstreamed (some things, such as development aids, may never be upstreamed).
• October 2024: Homa now has an official IANA IP protocol number (146).
• August 2024: upgraded to Linux 6.10.6.
• July 2024: introduced "TCP hijacking", where Homa packets are sent as legitimate TCP segments (using TCP as the IP protocol) and then reclaimed from TCP on the destination. This allows Homa to make better use of TSO and RSS.
• June 2024: refactored sk_buff management to use frags; improves efficiency significantly.
• April 2024: replaced master branch with main
• July 2023: upgraded to Linux 6.1.38.
• December 2022: Version 2.0. This includes a new mechanism for managing buffer space for incoming messages, which improves throughput by 50-100% in many situations. In addition, Homa now uses the sendmsg and recvmsg system calls, rather than ioctls, for sending and receiving messages. The API for receiving messages is incompatible with 1.01.
• November 2022: implemented software GSO for Homa.
• September 2022: added support for IPv6, as well as completion cookies. This required small but incompatible changes to the API. Many thanks to Dan Manjarres for contributing these improvements.
• September 2022: Homa now works on Linux 5.18 as well as 5.17.7
• June 2022: upgraded to Linux 5.17.7.
• November 2021: changed semantics to at-most-once (servers can no longer see multiple instances of the same RPC).
• August 2021: added new versions of the Homa system calls that support iovecs; in addition, incoming messages can be read incrementally across several homa_recv calls.
• November 2020: upgraded to Linux 5.4.3.
• June 2020: implemented busy-waiting during homa_recv: shaves 2 microseconds off latency.
• June 2020: several fixes to prevent RPCs from getting "stuck", where they never make progress.
• May 2020: got priorities working correctly using the DSCP field of IP headers.
• December 2019: first versions of cperf ("cluster performance") benchmark.
• December 2019 - June 2020: many improvements to the GRO mechanism, including better hashing and batching across RPCs; improves both throughput and latency.
• Fall 2019: many improvements to pacer, spread over a couple of months.
• November 6, 2019: reworked locking to use RPC-level locks instead of socket locks for most things (significantly reduces socket lock. contention). Many more refinements to this in subsequent commits.
• September 25, 2019: reworked timeout mechanism to eliminate over-hasty timeouts. Also, limit the rate at which RESENDs will be sent to an overloaded machine.
• August 1, 2019: GSO and GRO are now working.
• March 13, 2019: added support for shutdown kernel call, plus poll, select, and epoll. Homa now connects will all of the essential Linux plumbing.
• March 11, 2019: extended homa_recv API with new arguments: flags, id.
• February 16, 2019: added manual entries in the subdirectory "man".
• February 14, 2019: output queue throttling now seems to work (i.e., senders implement SRPT properly).
• November 6, 2019: timers and packet retransmission now work.