osmo_io¶

osmo_io is the new (2023) way of ho we do I/O in osmocom projects. It moves from a raw select/poll abstraction (where every application performs its own read/write once the fd becomes readble/writable) to a submission/completion model, where the actual I/O is performed by the library, an the application just submits write requests (as struct msgb) and receives write-completion and read-completion call-backs.

Apart from having more code reuse and a cleaner split between application and I/O library, this has the added advantage that the underlying operating system I/O mechanism can be swapped out without touching each and every application. Specifically, we have currently the following two back-ends implemented in libosmocore:

• POLL: classic osmo_fd based implementation using poll(2) (default)
• IO_URING: a backend using the Linux kernel io_uring(7) API

The choice of back-end is done via setting the LIBOSMO_IO_BACKEND environment variable. If you set it to IO_URING when starting an osmocom program, all sub-systems / interfaces ported over to the osmo_io API will make use of the io_uring(7) API (via liburing).

The main advantage of io_uring is performance. Benchmarking has shown that a classic, poll-based production osmo-bsc loaded with 400 TRX is spending about 40% of its CPU cycles in the system call overhad (sock_poll, tcp_poll and do_sys_poll) and a lot of system call entry/exit in general, due to the many small read/write/recvmsg/sendmsg/recfrom/sendto and poll calls.

For more information, see:

• the original discussion in #5751
• FIXME

API reference at https://ftp.osmocom.org/api/latest/libosmocore/core/html/osmo__io_8c.html#details

Implementation status¶

The below tables indicate the implementation status of the osmo_io migration:

Libraries¶

library/interface	protocol	transport	users	commit	issue
libosmocore	gsmtap	UDP	everyone	libosmocore	#6213
libosmo-mgcp-client	MGCP	UDP	osmo-bsc, osmo-msc, osmo-hnbgw	osmo-mgw	#5754
libosmogb ns2	NS	UDP	osmo-pcu, osmo-gbproxy, osmo-sgsn	libosmocore	#5751
libosmo-netif	stream_cli	TCP	libosmo-sigtran(ipa), osmo-bsc(cbsp), osmo-cbc(cbsp)	libosmo-netif	#5753
libosmo-netif	stream_cli	SCTP	libosmo-sigtran	libosmo-netif	#5753
libosmo-sigtran	xUA ASP (SUA, M3UA)	SCTP	osmo-bsc, osmo-msc, osmo-sgsn, osmo-hnbgw, osmo-smlc	libosmo-sigtran	#5752

Program¶

program	Interface/protocol	transport	commit	issue
osmo-gbproxy	Gb/NS	UDP	indirect via libosmogb ns2
osmo-bsc	CBSP	TCP client+server	osmo-bsc	#6170
osmo-bsc	meas_feed	UDP	osmo-bsc
osmo-bsc	MGCP	UDP	indirect via libosmo-mgcp-client	#5755
osmo-bsc	M3UA/SUA/IPA	SCTP/TCP	indirect via libosmo-sigtran	#5755
osmo-hnbgw	MGCP	UDP	indirect via libosmo-mgcp-client
osmo-hnbgw	M3UA/SUA/IPA	SCTP/TCP	indirect via libosmo-sigtran
osmo-msc	MGCP	UDP	indirect via libosmo-mgcp-client
osmo-msc	M3UA/SUA/IPA	SCTP/TCP	indirect via libosmo-sigtran
osmo-sgsn	M3UA/SUA/IPA	SCTP/TCP	indirect via libosmo-sigtran
osmo-sgsn	Gb/NS	UDP	indirect via libosmogb ns2
osmo-smlc	M3UA/SUA/IPA	SCTP/TCP	indirect via libosmo-sigtran
osmo-stp	M3UA/SUA/IPA	SCTP/TCP	indirect via libosmo-sigtran	#5752
osmo-mgw	RTP/RTCP	UDP		#6387

Further reading¶

• wikipedia page on io_uring
• Efficient IO with io_uring by the io_uring creator Jens Axboe.
• io_uring and networking in 2023