constantine/constantine/threadpool

Constantine Threadpool

API

The API spec follows https://github.com/nim-lang/RFCs/issues/347#task-parallelism-api

Overview

This implements a lightweight, energy-efficient, easily auditable multithreaded threadpool.

This threadpool will desirable properties are:

• Ease of auditing and maintenance.
• Resource-efficient. Threads spindown to save power, low memory use.
• Decent performance and scalability. The CPU should spent its time processing user workloads and not dealing with threadpool contention, latencies and overheads.

Compared to Weave, here are the tradeoffs:

• Constantine's threadpool provides spawn/sync (task parallelism) and optimized parallelFor for (data parallelism).
  It however does not provide precise in/out dependencies (events / dataflow parallelism).
• Constantine's threadpool has been significantly optimized to provide overhead lower than Weave's default (and as low as Weave "lazy" + "alloca" allocation scheme).

Compared to nim-taskpools, here are the tradeoffs:

• Constantine does not use std/tasks:
  • No external dependencies at runtime (apart from compilers, OS and drivers)
  • We can replace Task with an intrusive linked list
  • Furthermore we can embed tasks in their future
• Hence allocation/scheduler overhead is 3x less than nim-taskpools as we fuse the following allocations:
  • Task
  • The linked list of tasks
  • The future (Flowvar) result channel
• Contention improvement, Constantine is entirely lock-free while Nim-taskpools need a lock+condition variable for putting threads to sleep
• Powersaving improvement, threads sleep when awaiting for a task and there is no work available.
• Scheduling improvement, Constantine's threadpool incorporate Weave's adaptative scheduling policy with additional enhancement (leapfrogging)

See also design.md