108 lines
2.8 KiB
Text
108 lines
2.8 KiB
Text
IPC - in cinit and in general
|
|
=============================
|
|
Nico Schottelius <nico-linux-cinit__@__schottelius.org>
|
|
0.1, Initial Version from 2006-07-09
|
|
:Author Initials: NS
|
|
|
|
IPC - Inter process communication
|
|
|
|
Introduction
|
|
------------
|
|
This document describes the IPC methods used and/or tested for
|
|
cinit.
|
|
|
|
It does not describe in detail, how the different methods work
|
|
(this is already done many times, there's great documentation
|
|
available online), but more the advantages and disadvantages
|
|
(especially for an init system).
|
|
|
|
What is IPC?
|
|
~~~~~~~~~~~~
|
|
IPC describes methods to communicate between different processes
|
|
(programs).
|
|
|
|
IPC as described by SUSV3 (The Single UNIX Specification Version 3)
|
|
aka IEEE Std 1003.1, 2004 Edition aka POSIX only defines
|
|
MSQ, SHM and Semaphores as IPC. This document also covers
|
|
Sockets and FIFOs.
|
|
|
|
|
|
What is not (yet) covered by this document?
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
Speed of different IPC methods, different behaviour on different
|
|
unices.
|
|
|
|
|
|
Why do you need IPC for an init system?
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
|
|
cinit IPC history + analysis
|
|
----------------------------
|
|
|
|
|
|
FIFOs
|
|
~~~~~
|
|
First in - first out
|
|
File on the filesystem
|
|
Always need two files for two way communication
|
|
|
|
Hints
|
|
^^^^^
|
|
The first idea for IPC in cinit was to use two FIFOs, like minit does.
|
|
Wrong assumption:
|
|
runit also uses fifos, but two fifos for each services. That way
|
|
more parallel transmissions is possible.
|
|
You have to pay attention: Maximum number of opened files!
|
|
(On Linux 2.6 this is 1024, which makes a maximum of 512 services.
|
|
This limit is most likely never reached, but you have to keep it
|
|
in mind).
|
|
|
|
Sockets
|
|
~~~~~~~
|
|
Clean and beautiful. They allow easy two way communication.
|
|
|
|
If you created a socket, fs is r/o, you cannot use it, although
|
|
there is SO_REUSE.
|
|
|
|
First method: Using memory mapped part (tmpfs).
|
|
Second method: use interal communication (pipes!) and external after
|
|
/etc/cinit/ becomes writable.
|
|
|
|
|
|
Current IPC configuration
|
|
--------------------------
|
|
|
|
- switchable (conf/ipc_method)
|
|
- each ipc implementation needs:
|
|
|
|
ipc.h -> for global variables and ipc specific things
|
|
prefix variables with ipc_
|
|
int cinit_ipc_init(void); -> general initialization
|
|
return 1 on success, 0 on failure
|
|
int cinit_ipc_listen(void); -> begin to listen for messages
|
|
int cinit_ipc_send(void *data) -> send data to a client
|
|
|
|
--------------------------------------------------------------------------------
|
|
TO SORT:
|
|
|
|
1. message queues
|
|
o clients schreiben rein
|
|
o datenpaket gross genug fuer antwort?
|
|
|
|
2. shared memory
|
|
o client kann direkt rauslesen, was gestartet werden muss
|
|
o client kann direkt abhaengigkeiten starten, muss diese aber
|
|
eintragen
|
|
|
|
Ausprobieren:
|
|
- Mutexe
|
|
- Message Queues
|
|
-
|
|
|
|
Ideen:
|
|
Message queue
|
|
1. Client geht auf die normale
|
|
als id = pid (cast!)
|
|
dann rueck via anderer queue
|
|
|