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SubjectRe: real-time threaded IO with low latency (audio)
Paul Barton-Davis wrote:
>
> >> a "real" FX processor can do real-time flanging with a 0.2ms
> >> delay. the idea that linux (or beos) for that matter can step into
> >> this kind of role is absurd, or close to it.
> >
> >Absurd? My greatest problem seems to be the PCI DMA burst size of the
> >card I'm using now.
>
> At a 48KHz sample rate, a 0.2ms flange is slightly under 10
> samples. If you specify a fragment size of 16 (just for the power-of-2
> fan club), that translates to 4800 interrupts a second from the
> soundcard just to say "i crossed a fragment boundary". I didn't see
> any posts to linux-kernel that really answered the question "how many
> cycles are burnt handling a timer interrupt", but I have a pretty good
> feeling that Linux wasn't meant to run well with a regular 4.8KHz
> interrupt rate from something requiring an interrupt handler as
> relatively complex as the typical DAC handler.

No, Linux doesn't stand a chance here. RTLinux OTOH, is designed for it.

> >Driving the PC speaker at 8 kHz interrupt rate or higher is a no brainer
>
> the PC speaker, as I recall, doesn't talk back to you. You have no
> idea of fragment boundary crossings. this won't work for real DAC's.

This is irrelevant. The point is that the interrupt handler gets
executed exactly when it's supposed to be. What it does is a completely
different matter.
> >with RTL. (I've done it at 85 kHz on a dual P-II 233 with periodic
> >scheduling, but that's burning lots of CPU power on scheduling, and the
> >jitter is [probably] bigger than the average latency.)
>
> Yes, but thats not a realistic example of what an FX unit does. You've
> got to copy data from the DAC output buffer into the host memory and
> back again. Believing that you can do this *and* the FX computation
> *and* run other things seems wrong to me.

People are doing things like this on a dayly basis under RTL...

> I also don't understand the appeal of using RTL for this, other than
> "it works" and "its not windows or macos" ...

And? AFAIK, getting the work done is the whole point. If there's a
better way of doing it, fine. Where's the kernel patch that makes it
possible? (Hopefully, it'll show up soon...)

> >Ok... Say you have the CPU processing data using 80% of the avaiable
> >cycles. That is, the minimum time it can take to process one buffer is
> >80% of the time it takes to play it.
> >
> >Now, for some reason, the processing task doesn't get started in time.
> >If this delay is within the maximum allowed limits, the engine will
> >still be available to deliver the output buffer in time.
> >
> >However, when the next buffer comes in, it has to *wait* until the
> >current (delayed) one is processed, thus causing the next buffer to be
> >delayed as well! The engine will cath up after a few buffers, as the
> >load is less than 100%.
> >
> >But what if your process loses the CPU once again before it has cathed
> >up...?
>
> All understood. But you're missing something here. I mentioned that my
> measurements said that 99% of the write()'s to the soundcard complete
> within 0.05ms of the specified time. Moreover, if you use ALSA (which
> properly allows non-blocking I/O to the ADAC), you'll find that the
> actual system call takes only 2-5 microseconds (on a PII-450). That
> means that in almost every case (on a quiescent system), your task
> *does* get scheduled in time.
>
> If you now add in SCHED_FIFO for the relevant thread, it is
> essentially *guaranteed* to be scheduled to run after the ADAC
> interrupts us to say that it just crossed a fragment boundary.
>
> So, we're almost there ... well, no, because as Benno's benchmark
> demonstrates, even though its scheduled to run, it can't because the
> update thread is busy with the page cache. Sigh.

So what am I missing here, actually? Deadlines are missed. Period.

If those few remaining misses go away, I'll be happy to join you in user
space. I'm not against nice solution; it's just that I like solutions
that gets the job done.

> >(...)
> >> >"Average latency" and "maximum latency" are two very different things.
> >>
> >> no - latency and jitter are two different things. what we have a
> >> problem with is jitter, not latency. the difference is subtle but its
> >> important because it affects how you think about solving it.
> >
> >True, but I was actually thinking about interrupt and scheduling
>
> Let me try to be precise in my usage of the terms:
>
> latency: the constant delay between the generation of an audio
> sample and its effect on the analog or digital output
> of an audio device. caused almost entirely by the
> fragment size requested by the application, and limited
> only by the h/w's minimum fragment size and maximum
> CPU throughput.
>
> jitter: variation in the delay between sample generation and sample
> output from the audio device caused by the host CPU.

Well, that explains the misunderstanding. :-)

> >latencies here. I'm being unclear again, obviously. Maximum latency =
> >average latency + maximum deviation due to jitter. Anyway, the problem
> >is that your code doesn't always get control in time, and/or that it can
> >sometimes lose control before it's done.
>
> Not if its SCHED_FIFO. If you wire down its pages (mlockall(2)), and
> there are no other SCHED_FIFO tasks, only interrupt servicing can get
> in your way. If you don't believe me, try running:
>
> while (1) { }
>
> as a SCHED_FIFO task. Your system will appear to hang, but its really
> just exceedingly busy :)

Well, at least one thing that's under full control then. :-)

> Now, interrupt servicing is clearly a problem, but what are you
> proposing ?

Proposing? Well, RTL is already preempting Linux interrupts. That's why
I don't have any problems with other drivers, or in fact ANYTHING within
the standard part of the Linux kernel.

> That we *don't* service an interrupt in a timely fashion ?

Certainly not. On the contrary, high priority interrupts should be able
to preempt lower priority ones when it comes to very time critical
stuff, in order to *improve* interrupt timing. However, now we're
talking microseconds. Shouldn'd matter to normal drivers, and I wouldn't
like this kind of mess in drivers not used for hard real time work.

> This is one of the reasons dedicated FX units work so reliably at low
> latencies: they have no "other interrupts".

This is not a problem. As I said, interrupts can be preempted, much like
normal tasks. This is done by all hard real time OSes, and also has to
be done by any [dedicated] system with more than one interrupt source,
unless all interrupt handlers are fast enough not to cause any problems.


//David

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