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https://github.com/status-im/nim-taskpools

Lightweight, energy-efficient, easily auditable threadpool
https://github.com/status-im/nim-taskpools

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Lightweight, energy-efficient, easily auditable threadpool

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# Taskpools



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



This taskpools will be used in a highly security-sensitive blockchain application

targeted at resource-restricted devices hence desirable properties are:



- Ease of auditing and maintenance.

  - Formally verified synchronization primitives are highly-sought after.

  - Otherwise primitives are implemented from papers or ported from proven codebases

    that can serve as reference for auditors.

- 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.



## Example usage



```Nim

# Demo of API using a very inefficient π approcimation algorithm.



import

  std/[strutils, math, cpuinfo],

  taskpools



# From https://github.com/nim-lang/Nim/blob/v1.6.2/tests/parallel/tpi.nim

# Leibniz Formula https://en.wikipedia.org/wiki/Leibniz_formula_for_%CF%80

proc term(k: int): float =

  if k mod 2 == 1:

    -4'f / float(2*k + 1)

  else:

    4'f / float(2*k + 1)



proc piApprox(tp: Taskpool, n: int): float =

  var pendingFuts = newSeq[FlowVar[float]](n)

  for k in 0 ..< pendingFuts.len:

    pendingFuts[k] = tp.spawn term(k) # Schedule a task on the threadpool a return a handle to retrieve the result.

  for k in 0 ..< pendingFuts.len:

    result += sync pendingFuts[k]     # Block until the result is available.



proc main() =

  var n = 1_000_000

  var nthreads = countProcessors()



  var tp = Taskpool.new(num_threads = nthreads) # Default to the number of hardware threads.



  echo formatFloat(tp.piApprox(n))



  tp.syncAll()                                  # Block until all pending tasks are processed (implied in tp.shutdown())

  tp.shutdown()



# Compile with nim c -r -d:release --threads:on --outdir:build example.nim

main()

```



## API



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



The following types and procedures are exposed:



- Taskpool:

  - ```Nim

    type Taskpool* = ptr object

      ## A taskpool schedules procedures to be executed in parallel

    ```

  - ```Nim

    proc new(T: type Taskpool, numThreads = countProcessor()): T

      ## Initialize a threadpool that manages `numThreads` threads.

      ## Default to the number of logical processors available.

    ```

  - ```Nim

    proc syncAll*(pool: Taskpool) =

      ## Blocks until all pending tasks are completed.

      ##

      ## This MUST only be called from

      ## the root thread that created the taskpool

    ```

  - ```Nim

    proc shutdown*(tp: var TaskPool) =

      ## Wait until all tasks are completed and then shutdown the taskpool.

      ##

      ## This MUST only be called from

      ## the root scope that created the taskpool.

    ```

  - ```Nim

    macro spawn*(tp: TaskPool, fnCall: typed): untyped =

      ## Spawns the input function call asynchronously, potentially on another thread of execution.

      ##

      ## If the function calls returns a result, spawn will wrap it in a Flowvar.

      ## You can use `sync` to block the current thread and extract the asynchronous result from the flowvar.

      ## You can use `isReady` to check if result is available and if subsequent

      ## `spawn` returns immediately.

      ##

      ## Tasks are processed approximately in Last-In-First-Out (LIFO) order

    ```

    In practice the signature is one of the following

    ```Nim

    proc spawn*(tp: TaskPool, fnCall(args) -> T): Flowvar[T]

    proc spawn*(tp: TaskPool, fnCall(args) -> void): void

    ```

- Flowvar, a handle on an asynchronous computation scheduled on the threadpool

  - ```Nim

    type Flowvar*[T] = object

      ## A Flowvar is a placeholder for a future result that may be computed in parallel

    ```

  - ```Nim

    func isSpawned*(fv: Flowvar): bool =

      ## Returns true if a flowvar is spawned

      ## This may be useful for recursive algorithms that

      ## may or may not spawn a flowvar depending on a condition.

      ## This is similar to Option or Maybe types

    ```

  - ```Nim

    func isReady*[T](fv: Flowvar[T]): bool =

      ## Returns true if the result of a Flowvar is ready.

      ## In that case `sync` will not block.

      ## Otherwise the current will block to help on all the pending tasks

      ## until the Flowvar is ready.

    ```

  - ```Nim

    proc sync*[T](fv: sink Flowvar[T]): T =

      ## Blocks the current thread until the flowvar is available

      ## and returned.

      ## The thread is not idle and will complete pending tasks.

    ```



### Non-goals



The following are non-goals:



- Supporting GC-ed types with Nim default GC (sequences and strings). Using no GC or --gc:arc, --gc:orc or --gc:boehm (any GC that doesn't have thread-local heaps).

- Having async-awaitable tasks

- Running on environments without dynamic memory allocation

- High-Performance Computing specificities (distribution on many machines or GPUs or machines with 200+ cores or multi-sockets)



### Comparison with Weave



Compared to [Weave](https://github.com/mratsim/weave), here are the tradeoffs:

- Taskpools only provide spawn/sync (task parallelism).\

  There is no (extremely) optimized parallel for (data parallelism)\

  or precise in/out dependencies (events / dataflow parallelism).

- Weave can handle trillions of small tasks that require only 10µs per task. (Load Balancing overhead)

- Weave maintains an adaptive memory pool to reduce memory allocation overhead,

  Taskpools allocations are as-needed. (Scheduler overhead)



## License



Licensed and distributed under either of



* MIT license: [LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT

* Apache License, Version 2.0, ([LICENSE-APACHEv2](LICENSE-APACHEv2) or http://www.apache.org/licenses/LICENSE-2.0)



at your option. This file may not be copied, modified, or distributed except according to those terms.