LockFreeStack

Usage

use LockFreeStack;

or

import LockFreeStack;

Support for a lock-free Treiber stack.

Warning

This module relies on the AtomicObjects package module, which has several platform restrictions in its current state:

• It relies on Chapel extern code blocks and so requires that the Chapel compiler is built with LLVM enabled.

• The implementation relies on using either GCC style inline assembly (for x86-64) or a GCC/clang builtin, and so is restricted to a CHPL_TARGET_COMPILER value of gnu, clang, or llvm.

• The implementation does not work with CHPL_ATOMICS=locks.

An implementation of the Treiber Stack [1], a lock-free stack. Concurrent safe memory reclamation is handled by an internal EpochManager. Usage of the stack can be seen below.

var lfs = new LockFreeStack(int);
forall i in 1..N do lfs.push(i);
var total : int;
coforall tid in 1..here.maxTaskPar with (+ reduce total) {
  var (hasElt, elt) = lfs.pop();
  while hasElt {
    total += elt;
    (hasElt, elt) = lfs.pop();
  }
}

As an optimization, the user can register to receive a TokenWrapper, and pass this to the stack. This can provide significant improvement in performance by up to an order of magnitude by avoiding the overhead of registering and unregistering for each operation.

var lfs = new LockFreeStack(int);
forall i in 1..N with (var tok = lfs.getToken()) do lfs.push(i,tok);
var total : int;
coforall tid in 1..here.maxTaskPar with (+ reduce total) {
  var tok = lfs.getToken();
  var (hasElt, elt) = lfs.pop(tok);
  while hasElt {
    total += elt;
    (hasElt, elt) = lfs.pop(tok);
  }
}

Lastly, to safely reclaim memory, the user must explicitly invoke tryReclaim, or else there will be a memory leak. This must be explicitly invoked so that the user may tune how often reclamation will be attempted. Reclamation is concurrent-safe, but if called too frequently, it can add unnecessary overhead. A complete example of what would be considered ‘optimal’ usage of this lock-free stack.

var lfs = new LockFreeStack(int);
forall i in 1..N with (var tok = lfs.getToken()) do lfs.push(i,tok);
var total : int;
coforall tid in 1..here.maxTaskPar with (+ reduce total) {
  var tok = lfs.getToken();
  var (hasElt, elt) = lfs.pop(tok);
  var n : int;
  while hasElt {
    total += elt;
    (hasElt, elt) = lfs.pop(tok);
    n += 1;
    if n % GC_THRESHOLD == 0 then lfs.tryReclaim();
  }
}

Also provided, is a utility method for draining the stack of all elements, called drain. This iterator will implicitly call tryReclaim at the end and will optimally create one token per task.

var lfs = new LockFreeStack(int);
forall i in 1..N with (var tok = lfs.getToken()) do lfs.push(i,tok);
var total = + reduce lfs.drain();

[1]

Hendler, Danny, Nir Shavit, and Lena Yerushalmi. “A scalable lock-free stack algorithm.” Proceedings of the sixteenth annual ACM symposium on Parallelism in algorithms and architectures. ACM, 2004.

class Node

type eltType

var val : toNilableIfClassType(eltType)

var next : unmanaged Node(eltType)?

proc init(val: ?eltType)

proc init(type eltType)

class LockFreeStack

type objType

var _top : AtomicObject(unmanaged Node(objType)?, hasGlobalSupport = true, hasABASupport = false)

var _manager = new owned LocalEpochManager()

proc objTypeOpt type

proc init(type objType)

proc deinit()

proc getToken() : owned TokenWrapper

proc push(newObj: objType, tok: owned TokenWrapper = getToken())

proc pop(tok: owned TokenWrapper = getToken()) : (bool, objType)

iter drain() : objTypeOpt

iter drain(param tag: iterKind) : objTypeOpt  where tag == iterKind.standalone

proc tryReclaim()