How to debug program latency?

Question

I'm trying to run amsynth on a rpi. Latest version of raspbian, fresh install.

I tested amsynth on my x86 desktop and it works great, no latency.

On the rpi however, I get a delay of around 1 sec between sending a midi command (using vkeybd) and hearing the tone.

Setting a better niceness doesn't seem to help.

I wrote a small script in python using the mido library to control amsynth also but I still hear the lag.

How would I go about trying to debug what is causing this? I am basically just an end user.

I'm guessing either the midi command is getting delayed somehow, or amsynth itself is lagging (seems unlikely).

Load average is 0.4 during these tests, vs 0.1 idling.

top shows amsynth hovering at 12% CPU utilization.

A low latency kernel shouldn't be needed for this right? I would imagine that application would be for getting the latency down from 50 ms to 10 ms, for example, vs what I want to do here is just get the latency down from 1000 ms to 50 ms.

Edit: Using alsa, and not getting any sort of audio lag with any other programs, ie, playing audio from youtube in chromium works in real time. When I press play and pause, the delay is not even noticeable.

Answer 1

Try to determine what you use with amsynth and vkeybd: Jack, alsa, etc Then you can try:

• https://www.alsa-project.org/main/index.php/Low_latency_howto
• https://wiki.linuxaudio.org/wiki/jack_latency_tests
• possibly try kernel with PREEMPT_RT patch (keep in mind you will be need build custom kernel)

Hard way it's use:

• perf util https://perf.wiki.kernel.org/index.php/Tutorial
• strace https://strace.io/

Answer 2

Make sure the process has realtime privileges (this is typically done per-user).

As mentioned by Ivan, a kernel with the PREEMPT_RT patchset maybe more suitable.

In my experience, JACK is better than ALSA for low latency audio.

http://jackaudio.org/faq/linux_rt_config.html