I have no idea how to solve this. I am not using many local variables, yet I am hitting the 512 KiB stack size limit for secondary threads. My app has already been close to hitting this, now it's definitely hitting it.
How the heck do I trace this down? Instruments has nothing about stack memory usage.
For some reason, a function that has barely any local variables incurs a local stack frame difference like this:
sp = 0x000000016ec7eb40
sp = 0x000000016ebfea40
Oh, I forgot, that function has some significant stack usage.
Whee, I guess I shouldn't allocate temporary audio buffers on the stack. Fixed.