[linux-audio-dev] lock-free ring buffer code

rm async at cc.gatech.edu
Fri Apr 4 07:38:01 UTC 2003


On Fri, Apr 04, 2003 at 05:03:54PM +0200, torbenh at gmx.de wrote:

> The problem i have with the current ringbuffer, is that it does not
> solve the synchronization of the non realtime thread to the realtime
> thread. what should be done when you cant write to the buffer ?
> usleep( a_sensible_value ); ?

you should design your system so that it never happens. (based on the
fact that most well behaved rt tasks are somewhat periodic and don't
take 100% of the cpu).  

the way i've done this in my applications is to use the fact that
in general, some item is given to the RT thread who later gives it
back. this pattern pops up alot because the RT thread shouldn't malloc
or free memory, so anything non-static generally comes from somewhere
outside. 

so before a writer puts something on the queue to the RT thread it
first dequeues everything returning from the RT thread, and then
enqueues its item. since these items are conserved, the RT thread can
never write back more items than it is given, and it may never be
given more items than the queue can hold. so as long as your queues
are equal length, it's generally all good.

this is specific to the tasks i've used it for though.

if you want to be woken up a soon as possible, maybe run a lower
priority RT thread which will guarantee you cpu time after the higher
priority one (which can then do some dirty work, possibly with locks
or file descriptors).  

(alternately find a better operating system that supports application
level scheduling (exo-kernel), scheduler activations, closures,
whatever).

i think though, that the waking-up issue is separate from the
lock-free-messaging issue and should be solved separately according to
the problem at hand. conflating the two will only end in bloodshed or at
the very least an upset tummy.

	rob


----
Robert Melby
Georgia Institute of Technology, Atlanta Georgia, 30332
uucp:     ...!{decvax,hplabs,ncar,purdue,rutgers}!gatech!prism!gt4255a
Internet: async at cc.gatech.edu



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