3 ms·

Do they have isolated heaps and can they be preempted, even if they spin in an infinite loop doing some CPU intensive things?

by rdtsc 2y ago

neonsunset 2y ago
Tasks are not processes, and that would be a wrong thing to do, and so would be "isolated heaps" given performance requirements faced by .NET - you do want to share memory through concurrent data structures (which e.g. channels are despite what go apologists say), and easily await them when you want to. CSP, while is nice on paper, has the same issues as e.g. partitioning in Kafka, just at a much lower level where it becomes critical bottleneck - you can't trivially "fork" and "join" the flows of execution, which well-implemented async model enables. It's not "what about x" but rather how you end up applying the concurrent model in practice, and C# tasks allow you to idiomatically mix in concurrency and/or parallelism in otherwise regular code (as you can see in the example). I'm just clarifying on the parent comment that concurrency in .NET is not like in Java/C++/Python (even if the latter does share similarities, there are constraints of Python itself).
- rdtsc 2y ago
  > and that would be a wrong thing to do, and so would be "isolated heaps" - you do want to share memory through concurrent data structures (which e.g. channels are despite what go apologists say), and easily await them when you want to. It depends on the context. In some contexts absolutely not. If we share memory, and these tasks start modifying global data or taking locks and then crash, can those tasks be safely restarted, can we reason about the state of the whole node any longer? > CSP, while is nice on paper Not sure if Erlang's module is CSP or Actor's (it started as neither actually) but it's not just nice on paper. We have nodes with millions of concurrent processes running comfortably, I know they can crash or I can restart various subsets of them safely. That's no small thing and it's not just paper-theoretical.
  - neonsunset 2y ago
    What value does isolated heap offer for memory-safe languages? Task exceptions can simply be handled via try-catch at the desired level. Millions of concurrently handled tasks is not that high of a number for .NET's threadpool. It's one thing among many that is "nothingburger" in .NET ecosystem which somehow ends up being sold as major advantage in other languages (you can see it with other features too - Nest.js as a "major improvement" for back-end, while it just looks like something we had 10 years ago, "structured concurrency" which is simple task interleaving, etc.). It's a different, lower-level model, but it comes with the fact that you are not locked into particular (even if good) way of doing concurrency in Erlang.
    - asabil 2y ago
      GC determinism is one of the things you get. Another one is non cooperative asynchronous termination.
      - neonsunset 2y ago
        Pretty much all efficient GC implementations are inherently non-deterministic, even if predictable. How can this improve predictability of GC impact?
        asabil 2y ago
        No global GC. Each erlang process does its own GC, and the GC only happens when the process runs out of space (ie. the heap and stack meet). You can for example configure a process to have enough initial memory so as not to ever run into GC, this is especially useful if you have a process that does a specific task before terminating. Once terminated the entire process memory is reclaimed.
        neonsunset 2y ago
        There is no free lunch in software - the tradeoff is binary serialization and/or data copying over simple function calls. The same goes for GC - for efficient GC, it has to come with quite involved state which has additional cost of spawning. At this point, might use bump allocator, or an arena. Either way, Gen0 (it's a generational GC) in .NET acts like one, STW pauses can be sub-millisecond and are pretty much non-issue, given that you don't even need to allocate that often compared to many other high-level languages.

Do they have isolated heaps and can they be preempted, even if they spin in an infinite loop doing some CPU intensive things?

by rdtsc 2y ago

Do they have isolated heaps and can they be preempted, even if they spin in an infinite loop doing some CPU intensive things?

neonsunset 2y ago

Tasks are not processes, and that would be a wrong thing to do, and so would be "isolated heaps" given performance requirements faced by .NET - you do want to share memory through concurrent data structures (which e.g. channels are despite what go apologists say), and easily await them when you want to. CSP, while is nice on paper, has the same issues as e.g. partitioning in Kafka, just at a much lower level where it becomes critical bottleneck - you can't trivially "fork" and "join" the flows of execution, which well-implemented async model enables. It's not "what about x" but rather how you end up applying the concurrent model in practice, and C# tasks allow you to idiomatically mix in concurrency and/or parallelism in otherwise regular code (as you can see in the example). I'm just clarifying on the parent comment that concurrency in .NET is not like in Java/C++/Python (even if the latter does share similarities, there are constraints of Python itself).

rdtsc 2y ago
> and that would be a wrong thing to do, and so would be "isolated heaps" - you do want to share memory through concurrent data structures (which e.g. channels are despite what go apologists say), and easily await them when you want to. It depends on the context. In some contexts absolutely not. If we share memory, and these tasks start modifying global data or taking locks and then crash, can those tasks be safely restarted, can we reason about the state of the whole node any longer? > CSP, while is nice on paper Not sure if Erlang's module is CSP or Actor's (it started as neither actually) but it's not just nice on paper. We have nodes with millions of concurrent processes running comfortably, I know they can crash or I can restart various subsets of them safely. That's no small thing and it's not just paper-theoretical.
- neonsunset 2y ago
  What value does isolated heap offer for memory-safe languages? Task exceptions can simply be handled via try-catch at the desired level. Millions of concurrently handled tasks is not that high of a number for .NET's threadpool. It's one thing among many that is "nothingburger" in .NET ecosystem which somehow ends up being sold as major advantage in other languages (you can see it with other features too - Nest.js as a "major improvement" for back-end, while it just looks like something we had 10 years ago, "structured concurrency" which is simple task interleaving, etc.). It's a different, lower-level model, but it comes with the fact that you are not locked into particular (even if good) way of doing concurrency in Erlang.
  - asabil 2y ago
    GC determinism is one of the things you get. Another one is non cooperative asynchronous termination.
    - neonsunset 2y ago
      Pretty much all efficient GC implementations are inherently non-deterministic, even if predictable. How can this improve predictability of GC impact?
      - asabil 2y ago
        No global GC. Each erlang process does its own GC, and the GC only happens when the process runs out of space (ie. the heap and stack meet). You can for example configure a process to have enough initial memory so as not to ever run into GC, this is especially useful if you have a process that does a specific task before terminating. Once terminated the entire process memory is reclaimed.
        neonsunset 2y ago
        There is no free lunch in software - the tradeoff is binary serialization and/or data copying over simple function calls. The same goes for GC - for efficient GC, it has to come with quite involved state which has additional cost of spawning. At this point, might use bump allocator, or an arena. Either way, Gen0 (it's a generational GC) in .NET acts like one, STW pauses can be sub-millisecond and are pretty much non-issue, given that you don't even need to allocate that often compared to many other high-level languages.