Basic Linux IPC

• Wikipedia - Inter-process communication

• [Linux.conf.au 2013] - An Introduction to Linux IPC Facilities

  • [Linux.conf.au 2013] - An Introduction to Linux IPC Facilities - slide

• sockets

• FIFOs

• shared memory

D-Bus

D-Bus is a message bus system for inter-process communication (IPC)

• D-Bus

• Wikipedia - D-Bus

• Freedesktop - Introduction to D-Bus

• Linux From Scratch - D-Bus

• D-Bus Specification

D-Bus is enabled automatically when using systemd because dbus is a dependency of systemd.

What’s D-Bus

• a powerful IPC system

• the closest thing to a standard in this area as can be found on Linux

• provides a nice method-call transaction mechanism

• has fundamental inefficiencies of the user-space implementation

  • well suited to control tasks

    • works well to tell a sound server to change the volume

  • less so for anything that has to carry significant amounts of data

    • one would not want to try to send the actual audio data over the bus

• In D-Bus a call-return message requires 10 message copies, 4 message validations, 4 context switches

• D-Bus has no timestamps on messages

• not available at early boot

• We need a better implementation -> kdbus

D-Bus - Architecture

• libdbus

• dbus-daemon

• wrapper libraries based on particular application frameworks

Interesting :

In 2013 the systemd project rewrote libdbus in an effort to simplify the code, but it turned out to significantly increase the performance of D-Bus as well.
In preliminary benchmarks, BMW found that the systemd D-Bus library increased performance by 360%.

kdbus

• D-Bus in the kernel [linux.conf.au 2014]

  • D-Bus in the Kernel - slide

• ALS: Linux interprocess communication and kdbus (May 30, 2013)

• The unveiling of kdbus (Jan 13, 2014)

  • Linux Kernel only have primitives IPC : sockets, FIFOs, and shared memory

  • kdbus is a in-kernel implementation of D-Bus

    • can carry large amounts of data

      • even used for gigabyte-sized message streams

    • have zero-copy message passing

    • worst case : 2 copy operations, 2 validations, 2 context switches

    • all messages carry timestamps

    • full credential information (user ID, process ID, SELinux label, control group information, capabilities, and much more) is passed with each message

    • always available to the system (no need to wait for the D-Bus daemon to be started)

    • Linux security modules can hook into it directly

    • various race conditions have been fixed

    • API has simplified

    • Kdbus is implemented as a character device in the kernel

    • signal broadcasting mechanism has been rewritten to use Bloom filters to select recipients

    • There is a user-space proxy server that can be used by older code that has not been rewritten to use the new API, so everything should just work on a kdbus-enabled system with no code changes required.

  • the new memfd syscall was merged into Linux kernel 3.17

    • memfd is a mechanism similar to Android’s ashmem that allows zero-copy message passing in KDBUS.

  • Android “ashmem” subsystem

  • Android Binder

• Kdbus meets linux-kernel (Nov 4, 2014)

• D-Bus, FreeDesktop, and lots of madness

  • Hacker News - D-Bus, FreeDesktop, and lots of madness

• Kdbus Details - Greg Kroah-Hartman

  • Binder vs. kdbus

  • Binder is bound to the CPU, D-Bus (and hence kdbus), is bound to RAM

• kdbus.txt