LockFreeStack¶
Usage
use LockFreeStack;
or
import LockFreeStack;
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 nilable Node(eltType)¶
-
proc
init
(val: ?eltType)¶
-
proc
init
(type eltType)
-
type
-
class
LockFreeStack
¶ -
type
objType
¶
-
var
_top
: AtomicObject(unmanaged nilable Node(objType), hasGlobalSupport = true, hasABASupport = false)¶
-
var
_manager
= new owned LocalEpochManager()¶
-
proc
objTypeOpt
type¶
-
proc
init
(type objType)¶
-
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
-
proc
tryReclaim
()¶
-
type