4 ms·

So how would you raw-structure a performance critical (audio) program that needs some flexible path for the data processing, like with a 128 sample buffer and a

by iammyIP 10y ago

So how would you raw-structure a performance critical (audio) program that needs some flexible path for the data processing, like with a 128 sample buffer and about 3-10 choices along the processing path. Processing path should be switchable per buffer frequency. The function pointers were the best solution i could come up with, but maybe that doesnt really eliminate branching cache misses and stalling and its impact on performance, but just hides it in plain-text.

vardump 10y ago
How many permutations? If not too many, maybe just compile them all. Sometimes you do need to use function pointers. It's just if it's in a performance critical path, you might want to consider alternatives.
- iammyIP 10y ago
  Permutations are for my own case pretty low, it's just a replacement for a handful of switches, otherwise the flow is pretty straightforward. But i see that if one wants to restructure the whole program flow on-the-fly it might lead to problems. What are the alternatives? The link you provided is for graphics, and does not seem to be generalised. Do you suggest a complete switch to a jit interpreted codebase?
  - vardump 10y ago
    Graphics and audio are usually pretty similar, other than the volume of data is usually quite a bit larger for graphics than for audio. There's no silver bullet how to structure such flexible, dynamically configurable high performance system.
    - iammyIP 10y ago
      I disagree with 'pretty similar' and would suggest 'challenging in completely different ways'. Graphics in a minimum 60 fps context is about 700 times less time sensitive. That's almost three magnitudes difference. On top of that it isn't a major fail if you don't meet the 60 on time, but in audio it is. Furthermore audio tends to be more single thread focused, while graphics is much more multi thread friendly (see your link to llvmpipe and general gpu architectures). Graphics need to shovel around about some hundreds of megabytes per second and is less time critical, whereas audio is ultimately time critical and only needs to produce about 350kb per second for 32 bit stereo (the data difference you already mentioned). That's again some three orders of magnitude of difference. I wouldn't call that similar. Since you can't provide any advise for an alternative to function pointers i must challenge your expertise in audio programming since you seem to project your view from a graphics programming point.
      - vardump 10y ago
        Well, I haven't done audio programming for quite a while. I did get audio hardware behave pretty nicely in 1998, though. I believe low latency audio was much harder back then. I'm pretty surprised if your deadline is really that 23 microseconds (60 fps / 7000). You can't even reliably dispatch an IRQ request in that time! > I wouldn't call that similar If you have a long audio filter chain it's not all that different from a long fragment shader. You can think of a fragment shader as a filter chain. > Since you can't provide any advise for an alternative to function pointers Uh. That's pretty hard without seeing the source code and full problem description. Especially because you seem to be expecting some magical solution. Well, those don't exist, it's just hard work. Besides, like I said earlier, sometimes function pointers are appropriate for the task. Just understand the cost. Do keep in mind audio filter chains can be pretty complicated. If your filter chain has many concurrent channels with hundreds of dynamic filter steps for each sample to pass through, yeah, you might have some performance problems. > i must challenge your expertise in audio programming since you seem to project your view from a graphics programming point. Be my guest. Just wanted to share something I know about and give new ideas. Btw, I'm not doing graphics, but mostly I/O with micro/millisecond level deadlines.
        iammyIP 10y ago
        Getting audio hardware to behave nicely for audio is pretty nice :) The calculation for the time factor difference is simply 44100/60 which is about 733, i don't know how the 23 microseconds you got there relate to this. Your comparision to graphics seem to relate to long FIR filters, which i dont't use because of cache reasons (big kernel/lookup table). FIR filters can however sound pretty nice with static parameters but get extremely expensive for realtime modulation. For realtime modulation i suggest IIR filters (Finite Impulse Response vs Infinite Impulse Response filter architecture). Of course it's hard to give advice into the blue without any source, but i atleast expected some general hint, like 'avoid branches, use function pointers' on a next-level, since you were so opposed to function pointers to begin with. I am also not completely sure what's the best approach in this regard, and i surely did not enough profiling in this regard. That said, i remain sceptical regarding your critique of function pointers in a single threaded audio application, because of a lack of an alternaltive. I/O on a microsecond critical deadline certainly seems to be in a neigbourhood to these problems. Do you use some arcane javascript asm.js with some jit auto-reconfiguring optimising for this, or how would you describe your approach?

by iammyIP 10y ago

vardump 10y ago

How many permutations? If not too many, maybe just compile them all. Sometimes you do need to use function pointers. It's just if it's in a performance critical path, you might want to consider alternatives.

iammyIP 10y ago
Permutations are for my own case pretty low, it's just a replacement for a handful of switches, otherwise the flow is pretty straightforward. But i see that if one wants to restructure the whole program flow on-the-fly it might lead to problems. What are the alternatives? The link you provided is for graphics, and does not seem to be generalised. Do you suggest a complete switch to a jit interpreted codebase?
- vardump 10y ago
  Graphics and audio are usually pretty similar, other than the volume of data is usually quite a bit larger for graphics than for audio. There's no silver bullet how to structure such flexible, dynamically configurable high performance system.
  - iammyIP 10y ago
    I disagree with 'pretty similar' and would suggest 'challenging in completely different ways'. Graphics in a minimum 60 fps context is about 700 times less time sensitive. That's almost three magnitudes difference. On top of that it isn't a major fail if you don't meet the 60 on time, but in audio it is. Furthermore audio tends to be more single thread focused, while graphics is much more multi thread friendly (see your link to llvmpipe and general gpu architectures). Graphics need to shovel around about some hundreds of megabytes per second and is less time critical, whereas audio is ultimately time critical and only needs to produce about 350kb per second for 32 bit stereo (the data difference you already mentioned). That's again some three orders of magnitude of difference. I wouldn't call that similar. Since you can't provide any advise for an alternative to function pointers i must challenge your expertise in audio programming since you seem to project your view from a graphics programming point.
    - vardump 10y ago
      Well, I haven't done audio programming for quite a while. I did get audio hardware behave pretty nicely in 1998, though. I believe low latency audio was much harder back then. I'm pretty surprised if your deadline is really that 23 microseconds (60 fps / 7000). You can't even reliably dispatch an IRQ request in that time! > I wouldn't call that similar If you have a long audio filter chain it's not all that different from a long fragment shader. You can think of a fragment shader as a filter chain. > Since you can't provide any advise for an alternative to function pointers Uh. That's pretty hard without seeing the source code and full problem description. Especially because you seem to be expecting some magical solution. Well, those don't exist, it's just hard work. Besides, like I said earlier, sometimes function pointers are appropriate for the task. Just understand the cost. Do keep in mind audio filter chains can be pretty complicated. If your filter chain has many concurrent channels with hundreds of dynamic filter steps for each sample to pass through, yeah, you might have some performance problems. > i must challenge your expertise in audio programming since you seem to project your view from a graphics programming point. Be my guest. Just wanted to share something I know about and give new ideas. Btw, I'm not doing graphics, but mostly I/O with micro/millisecond level deadlines.
      - iammyIP 10y ago
        Getting audio hardware to behave nicely for audio is pretty nice :) The calculation for the time factor difference is simply 44100/60 which is about 733, i don't know how the 23 microseconds you got there relate to this. Your comparision to graphics seem to relate to long FIR filters, which i dont't use because of cache reasons (big kernel/lookup table). FIR filters can however sound pretty nice with static parameters but get extremely expensive for realtime modulation. For realtime modulation i suggest IIR filters (Finite Impulse Response vs Infinite Impulse Response filter architecture). Of course it's hard to give advice into the blue without any source, but i atleast expected some general hint, like 'avoid branches, use function pointers' on a next-level, since you were so opposed to function pointers to begin with. I am also not completely sure what's the best approach in this regard, and i surely did not enough profiling in this regard. That said, i remain sceptical regarding your critique of function pointers in a single threaded audio application, because of a lack of an alternaltive. I/O on a microsecond critical deadline certainly seems to be in a neigbourhood to these problems. Do you use some arcane javascript asm.js with some jit auto-reconfiguring optimising for this, or how would you describe your approach?