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Is it better to have two stacks?
Author: Hubert Lamontagne Date: 2016-06-17 10:07
Agner wrote:
Hubert Lamontagne wrote:
the catch is that a thread switch goes through the thread scheduler. That's totally fine if you're doing something that doesn't need any tight timing (ex: server software), but it's a problem in very real time apps
You are right. We need to think about real time applications. Is there anything special we need to do to support real time operating systems? It would be nice to be able to reserve one or more CPU cores in a multicore processor for a particular critical task, but that is an operating system issue. The hardware just needs to support fast task switching. We can have extremely fast task switching if the CPU has multiple on-chip memory maps that it can switch between.
It's a bit of an application-specific thing.

For instance, I'm 99% sure MacOS has a special case in its scheduler for the audio thread, so that when the sound hardware sends a "the sound buffer is 50% empty, fill the other half" interrupt, the sound code runs straight off the interrupt and literally outprioritizes EVERYTHING ELSE (including hardware drivers!), completely disables priority inversion, completely bypasses any sort fair time slice allocator and so forth. If the audio thread falls on a locked mutex, it might even elevate the thread holding that mutex to this crazy "screw everything else" priority - this is necessary to use mutexes or things locked by mutexes like malloc() in the audio thread, otherwise trying to lock the mutex might result in a task switch to a low priority GUI thread which then gets preempted, and then you get an audio glitch.

In Windows, the scheduler is generally designed for networking so you generally get screwed over: at very least, you need to install a special app to get low latency audio (ASIO4ALL), you need to spend half an hour finetuning buffer length, any badly written driver can completely make it impossible (for instance there was an Intel network driver that would periodically lock everything for 5ms), and even when it does work, all other applications get no sound because you have to bypass the sound mixer because it runs at way too low priority (and on way too large blocks). In Linux, afaik this sort of scheduling is possible (although it's not really meant for it), but requires special flags in the thread scheduler priority and in the mutexes (and possibly a kernel patch), and there's a video from Android OS engineers about getting a low latency audio path in Android using that (tdlr version: it took some work and many versions!).

Another case I can think of is game consoles... if you have 8 cores, does it make any sense to run ANYTHING ELSE than the currently played game on 7 of those cores?

 
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