Research on IO-bound tasks about weave

[mratsim]

Also Rust coupled both IO and a task parallelism in the past and decided to avoid that:

https://github.com/aturon/rfcs/blob/remove-runtime/active/0000-remove-runtime.md

https://github.com/rust-lang/rfcs/blob/master/text/0230-remove-runtime.md

[mratsim]

An idea on how to play well with Asyncdispatch or Chronos or any future async/await library.

They all offer a poll() function that runs their event loop.

We can add a field pollHook*: proc() {.nimcall, gcsafe.} on each worker.
It would be setup by setPollingFunction(_: typedesc[Weave], poll: proc() {.nimcall, gcsafe.}) before Weave initialization (at first, can be relaxed later).

Then we modify loadBalance(), sync(), syncScope() to interleave pollHook calls before and after executing a task.

Note that loadBalance() is called in-between each parallelFor iterations:

• If loop is fine-grained, even if it's executed on the main thread, there are plenty of opportunities to handle IO event.
• If there is no hook, the if not pollHook.isNil: is very predictable and should be costless
• If there is a hook, the syscalls to handle the IO event will probably slow down fine-grained parallelism a lot and also completely flush the CPU caches with data loaded by the kernel. This is bad, there are some ways to "mitigate" that:
  • Document the tradeoff
  • Only install the hook on the main thread, so that worker threads are not polluted
  • Give the option to install the hook on either the main thread or all thread.

Note that worker threads will sleep if they have no tasks, but it does not make sense for them to try to handle IO events without a task.

A potential issue is that a task can be migrated or for a parallel loop, it can even be split and then executed on 2 different threads, i.e. are the async libraries using {.threadvar.} to manage some global state? Because that will not work.

[mratsim]

RFC #132 and its implementation with Weave as an independent background service is probably a better path forward #136

[mratsim]

See https://github.com/weavers-guild/weave-io