[SDL] Audio callback function is not called in time sometimes

[Mine]

On Dec 5, 2007 4:29 AM, David Olofson <david at olofson.net> wrote:

> On Tuesday 04 December 2007, Mine wrote:
> [...]
> > I did further investigation, it looks like SDL_Cond_Wait/Signal
> > takes the most extra time, and SDL_Lock/Unlock_Mutex takes less
> > extra time when the problem occurs. I now use a while loop &
> > SDL_Lock/Unlock_Mutext instead of SDL_Cond_Wait/Signal, then it
> > works better, there's less audio drop.
>
> What are you using these for? You might want to try some lock-free
> solution for passing "commands" to the audio thread, if that's where
> the problem is...

I use these stuff to synchronize the buffer read & write. I have two thread,
one for decoding and write the audio data to my buffer, the other for
reading the audio data from the buffer, so I use mutex/cond to protect the
buffer.
I would like to send the buffer to audio device with nearly 0 latency, which
means I need my audio callback function works as fast as possible, but I
find it sometimes takes more than 40 ticks, stuck in the SDL_Lock/Unlock.
Maybe I should not use any protection in my read thread so it would work
pretty fast?

>
>
>
> > Is it possible that SDL_Lock/Unlock_Mutex will take about 30~40
> > ticks?
>
> Well, not really, but they certainly increase the risk of you being
> blocked, and whenever that happens, there's a quite substantial risk
> of various other threads running for a while before you get the CPU
> back. You're not on a real time OS, after all.
>
> Also note that due to this dynamic priority deal that any normal OS
> will have, using locks or similar to protect data actually gives you
> a sort of priority inversion problem. It doesn't help that the audio
> thread is behaving and given high priority, if it has to wait on a
> lock held by your CPU hog main thread...!
>
> (As opposed to an RTOS, a normal OS doesn't implement priority
> inheritance and similar solutions to this problem, as priorities are
> just considered a way of tweaking the timesharing a little,
> basically. So, you have to work around it one way or another.)
>
>
> //David Olofson - Programmer, Composer, Open Source Advocate
>
> .-------  http://olofson.net - Games, SDL examples  -------.
> |        http://zeespace.net - 2.5D rendering engine       |
> |       http://audiality.org - Music/audio engine          |
> |     http://eel.olofson.net - Real time scripting         |
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