[linux-audio-dev] Re: lock-free data structures

[Paul Davis]

>>How this all is done in Ardour? I browsed the source but there are
>>a lot of stuff there. How about LinuxSampler?
>>  
>>
>
>We use a very simple approach in LinuxSampler: in the disk thread if all 

Ardour also uses a simple mechanism, but the overall result is a bit
more complex. Basically, the disk/butler thread blocks on a
pthread_cond. Yes, I know, I said they weren't safe for RT use :)
I have made several attempts at using a FIFO, but every time I ran
into a subtlety that I had failed to anticipate. I would still like to
do this at some point.

Ardour has a lot (*) of things for the butler/disk thread to
do. Transport requests and disk i/o are intimately related and require
a fairly tight interlocking. When the user requests a change in the
transport state, the GUI queues an event in a (lock free)
ringbuffer. The audio thread checks this ringbuffer every process()
cycle, and when it finds a request, it divides into up to 3 distinct
parts (function calls) "pre" RT, post-RT part and "final" RT part. The
audio thread executes the pre RT part, and then sets a bitfield to
indicate the work required for the non-RT part, and then signals the
butler to start up again. Whenever the butler is awake, it is
constantly checking to see if there is non-disk-I/O work to be done
(i.e. in between reads/writes for particular tracks). If it finds that
there is work to do, it does it, and then marks the work as done. On
the next process() cycle after this, the audio thread will execute the
final-RT function (if there is one, and there generally isn't), and at
that point, the transport state change is done.

code is in libs/ardour/session_{butler,transport,events}.cc

(*) These are things that have to be divided across the audio and
butler threads:

  PostTransportStop
  PostTransportDisableRecord
  PostTransportPosition
  PostTransportDidRecord
  PostTransportDuration
  PostTransportLocate
  PostTransportRoll
  PostTransportAbort
  PostTransportOverWrite
  PostTransportSpeed
  PostTransportAudition
  PostTransportScrub
  PostTransportReverse
  PostTransportInputChange
  PostTransportCurveRealloc