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

Christopher Obbard chris at 64studio.com
Tue Mar 27 00:12:23 CEST 2018

Hi Chris,

>> 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?

Yeah, BeagleBone Black. It's quite a nice board!

> If so I run the RT kernels from the RCN repo, never had any problem with
> those so might be a good starting point.

Ah, I've been running mainline stable 4.14, applied CONFIG_PREEMPT and
ran a cyclictest on three threads with no extra load. It was giving me
"ok" average latency, and terrible maximum latency. It happened to be
every few seconds there would be a massive spike. No wonder I've been
stumped !!!

using 4.14-rt branch from https://github.com/beagleboard/linux/, the
same cyclictest without load was giving me a maximum of 46. So this is
looking promising!

Started jackd, all looking OK.
Can get jack periods down to 64 now, even 32! But some dreaded errors occur.
This time I have a printk message of "GBLCTL write error" every few us.
Good news! Looks like a bug in the mcasp driver.
Message didn't appear on Mainline, it's getting late here so time to
squish tomorrow.
I tried my client with the jackd -adummy, I know it's not a real test
but it managed to run down to 32 frames.

Lesson learnt: use vendor BSP (unless vendor is Chinese. That has
bitten me before ;-) )

>> 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.

Thanks for your detailed explanation! It really helps for me, been
diving deep into the kernel for the past year or so.
Been software/electronic engineer at a higher level for a few years,
but this is just super interesting to me.

>> 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.

Yeah, the system is all setup for Audio to have realtime. I've managed
to set similar stuff up on a number of different AMD64 systems before,
but not so much on ARM embedded boards.

> 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:
> http://www.rncbc.org/jack/
> (link is toward the bottom)

Yeah, the rtirq script is handy. It will need some tweaking for arm
boards, I think.

> There may be some useful info here:
> https://wiki.linuxaudio.org/wiki/system_configuration
> 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.

This is somewhere I want to improve docs after figuring it out a bit better :-).

> 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
> 50.
> 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
> set.

Thanks, that kind of matches what I was thinking.

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

the TI defconfig for RT sets 250Hz, and it seems to do OK.

>>> 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.
> http://processors.wiki.ti.com/index.php/Sitara_Linux_Audio_Driver_Overview
> 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.

Exactly that, nothing super clever :-).

>> 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.

This is the kind of experienced knowledge that is really helpful to me!

>> 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
> application....
>> 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.

That makes sense!

>> 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.
> https://wiki.linuxfoundation.org/realtime/documentation/start

Hopefully, the bug I have uncovered with the GBLCTL was making me
loose the plot a little bit :-)

Really appreciate your in-depth replies.



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