[Jack-Devel] Fwd: connecting to JACKD2 with low buffer sizes

Chris Caudle chris at chriscaudle.org
Mon Mar 26 21:06:24 CEST 2018

On Mon, March 26, 2018 9:38 am, Christopher Obbard wrote:
> Big typo! I am running on a TI 1GHz AM3358 processor.

Ok, three orders of magnitude is a pretty significant difference.  So is
this a BeagleBone Black or something similar?
If so I run the RT kernels from the RCN repo, never had any problem with
those so might be a good starting point.

> I've been using CONFIG_PREEMPT also in other places, I have been
> wondering whether the full RT patch will cause less throughput for
> the jack process.

The design of jackd is not about throughput, it is about low latency.  If
you want better throughput, use larger period sizes.  That is not really
specific to jack, that is just a general principle in computing, if you
want the best throughput using a general purpose computer design you
should bundle your data into larger groups so that the processor has to
handle fewer interrupts and the DMA controller(s) can run in the most
efficient configuration.  If you want the lowest latency you will have to
trade off some throughput for that.
If you want really low latency and high throughput you will likely have to
design custom hardware with pipelined processing (e.g. some of the
cut-through switch designs used for Infiniband and very low latency HPC
Ethernet switches).  That is a more advanced topic than how to get jackd
running on an off the shelf processor.

> Both jack and my application are requesting SCHED_FIFO, I am not sure
> of the priority of the application but I am thinking of setting them
both to 70-80.

Why not start off reading the linux audio  and jack FAQs that explain how
you should configure your RT system?  It sounds like you are getting ready
to re-discover all of that information the long and hard way.  Might be
entertaining and educational but probably is no the best use of your time.

I thought there was a page somewhere with suggestions on RT priority
settings, but I do not see it on the jackaudio FAQ or wiki pages.   The
rtirq script from Rui might be useful:
(link is toward the bottom)

There may be some useful info here:
but unfortunately the low latency page is from 2000, there have been quite
a few changes in kernel setup since then, so I don't know how useful the
low latency wiki page actually is on linuxaudio.org these days.

The short version is you want the interrupt for your sound hardware to be
highest, then jack right after that.  There probably won't be any other RT
tasks on a single use system, so it probably won't matter whether you set
the sound IRQ and jackd priorities to 90 and 89, or 70 and 69, just make
sure they are higher than the default for non-RT tasks, which I think is

The applications using jackd actually run the RT audio thread in jackd
context, so unless I misunderstood something it should not in principle
matter what the priority of the (main thread) of the other applications is

> What about HZ, I currently have this set to HZ_100. Would HZ_1000 be any

Could be better for scheduling latency, but for IRQ driven tasks like
handling the sound hardware I don't know  if it matters or not.

>> What audio hardware are you running?
>> Are you using an ALSA driver from the current kernel tree?
> It's an 6-channel in, 6-channel out card with a simple ALSA driver
written by me.

There appears to be a TI McASP driver in the kernel tree already, any
reason you are not using that?  Just asking, I have not tried to base
anything on that driver yet myself.

>From that page it appears you would just need to write the codec driver
that handles specifics of the platform driver settings needed, and any
control interfaces.

> The driver just binds the codecs to the CPU, nothing latent in there.
> All of the FIFO stuff is taken care of by the TI McASP driver.

OK, it sounds like maybe that is actually what you did, just wrote the
codec portion and used the existing McASP driver to handle the I2S/TDM
interface pieces.

> Currently the driver reads & writes to 8 channels but two of the
> out channels are unused, I can possibly try to gain some
> performance here.

Not worth the trouble at this point.   If you get a system that is mostly
working and just has an occasional problem it might be worth optimizing,
but it sounds like right now the system is pretty broken, so look  for big
problems, not small optimizations you could make.

> Sorry, I missed out the first part of the log. The LockedTimedWait
> error came when my client tried to connect. I think it's to do
> with the Kernel scheduling.

Make sure you have RT scheduling enabled for whatever user account you are
using, try again and get the entire startup log.

> It manages to open and all performs fine at 128 frames.

OK, then make sure  you are really clear about when you are trying to
debug problems with e.g. 32 frame setting vs. problems with the 128 frames
setting, since it looks like you have different problems with the
different period sizes.

> I've not had much luck with PREEMPT_RT in the past, I don't
> think I've set the priorities of interrupts properly.

Then start with Rui's rtirq script. Some of the devices in the default
script are x86 specific, and even some of those are probably outdated, but
the framework is great, put the devices you want in the configuration file
and run the script, it will set everything for you.

> I think I need to chrt the soundcard IRQ highest, then JACK, then my
> AFAIK, jack already does this with the -R argument & sets the
> scheduler priority to SCHED_FIFO.

I don't think the application RT priority matters, if I understand
correctly jackd will be allocating the thread that the application uses
for audio callback, so that thread of the application should run at jackd
priority, so for a simple embedded system you would just need to set the
sound hardware IRQ the highest, then jackd after that (with the -R
argument, default values is probably OK), then everything else can run at
non-RT default.  I think non-RT default is 50 and  jackd default is 70
based on what I have seen on x86 machines.

> Have you got any links to information, books etc on RT patch?

Just this, but honestly the original jackd developers took care of almost
everything for you, you shouldn't need to know much about the low level
details just to get it running.

Chris Caudle

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