Post on 16-Aug-2020
CILK/CILK++ AND REDUCERS YUNMING ZHANG
RICE UNIVERSITY
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OUTLINE • CILK and CILK++ Language Features and
Usages • Work stealing runtime • CILK++ Reducers • Conclusions
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IDEALIZED SHARED MEMORY ARCHITECTURE
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• Hardware model • Processors • Shared global
memory • Software model
• Threads • Shared variables • Communication • Synchronization
Slide from Comp 422 Rice University Lecture 4
CILK AND CILK++ DESIGN GOALS • Programmer friendly
• Dynamic tasking • Parallel extension to C
• Scalable performance • Efficient runtime system • Minimum program overhead
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CILK KEYWORDS • Cilk: a Cilk function • Spawn: call can execute asynchronously
in a concurrent thread • Sync: current thread waits for all locally-
spawned functions
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CILK EXAMPLE cilk int fib(n) {
if (n < 2) return n; else { int n1, n2; n1 = spawn fib(n-1); n2 = spawn fib(n-2); sync; return (n1 + n2); }
}
6 Borrowed from Comp 422 Rice University Lecture 4
CILK++ EXAMPLE int fib(n) {
if (n < 2) return n; else { int n1, n2; n1 = cilk_spawn fib(n-1); n2 = fib(n-2); cilk_sync; return (n1 + n2); }
}
7 Borrowed from Comp 422 Rice University Lecture 4
CILK++ EXAMPLE WITH DAG
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Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
OUTLINE • CILK and CILK++ Language Features and
Usages • Work stealing runtime • CILK++ Reducers • Conclusions
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WORK FIRST PRINCIPLE • Work: T1 • Critical path length: T∞ • Number of processor: P • Expected time
• Tp = T1/P + O(T∞) • Parallel slackness assumption
• T1/P >> C∞T∞
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WORK FIRST PRINCIPLE • Minimize scheduling overhead borne by
work at the expense of increasing critical path • Tp ≤ C1Ts/P + C∞T∞ ≈ C1Ts/P Minimize C1 even at the expense of a larger C∞
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WORK STEALING DESIGN GOALS • Minimizing contentions
• Decentralized task deque • Doubly linked deque
• Minimize communication • Steal work rather than push work
• Load balance across cores • Lazy task creation • Steal from the top of the deque
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CILK WORK STEALING SCHEDULER
13 Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
CILK WORK STEALING SCHEDULER
14 Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
CILK WORK STEALING SCHEDULER
15 Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
CILK WORK STEALING SCHEDULER
16 Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
CILK WORK STEALING SCHEDULER
17 Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
CILK WORK STEALING SCHEDULER
18 Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
CILK WORK STEALING SCHEDULER
Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
CILK WORK STEALING SCHEDULER
Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
CILK WORK STEALING SCHEDULER
21 Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
CILK WORK STEALING SCHEDULER
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Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
CILK WORK STEALING SCHEDULER
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Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
CILK WORK STEALING SCHEDULER
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Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
CILK WORK STEALING SCHEDULER
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Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
CILK WORK STEALING SCHEDULER
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Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
TWO CLONE STRATEGY • Fast clone
• Identical in most respects to the C elision of the Cilk program
• Very little execution overhead • Sync statements compile to no op • Allocates an continuation
• Program variables and instruction pointer • Slow clone
• Convert a spawn schedule to slow clone only when it is stolen
• Restores program state from activation frame that contains local variables, program counter and other parts of the procedure instance
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FAST CLONE
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SLOW CLONE Slow_fib(frame * _cilk_frame){
switch (_cilk_frame->header.entry) { fast_fib(_cilk_frame->n - 1 ); case 1: goto _cilk_sync1; fast_fib(_cilk_frame->n - 2 ); case 2: goto _cilk_sync2; sync (not a no op) case 3: goto _cilk_sync3; }
}
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FRAMES • C++ Main Frame
• Local variables of the procedure instance • Temporary variables • Linkage information for return values
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FRAMES • CILK++ Stack Frame
• Everything in C++ Main Frame • Continuation • Parent pointer • Have exactly one child • Used by Fast Clone • A worker can have multiple Stack Frames
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FRAMES • CILK++ Full Frame (used by slow clone)
• Everything in CILK++ Stack Frame • Lock • Join counter • List of children (has more than one
children) • A worker has at most one Full Frame
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EXTENDED DEQUE WITH CALL STACKS
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Stack frame
Full frame
Extended Deque
Call stack
FUNCTION CALL
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Stack frame
Full frame
Extended Deque (Before Function Call) Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
FUNCTION CALL
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Stack frame
Full frame
Extended Deque (After Function Call) Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
New stack frame
SPAWN
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Stack frame
Full frame
Extended Deque (Before Spawn Call) Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
SPAWN
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Stack frame
Full frame
Extended Deque (After Spawn Call) Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
Set continuation in last stack frame
RESUME FULL FRAME
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Stack frame
Full frame
Extended Deque Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
Set the full frame to be the only frame in the call stack, resume execution on the continuation
RANDOMLY STEAL
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Stack frame
Full frame
Extended Deque Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
Steal this call stack
RANDOMLY STEAL
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Stack frame
Full frame
Extended Deque Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
Steal this call stack 1 1 1
RANDOMLY STEAL
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Stack frame
Full frame
Extended Deque Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
1
1 1
PROVABLY GOOD STEAL
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Stack frame
Full frame
Extended Deque Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
0
UNCONDITIONALLY STEAL
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Stack frame
Full frame
Extended Deque Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
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FUNCTION CALL RETURN
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Stack frame
Full frame
Extended Deque (Before Return from a Call Case1) Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
FUNCTION CALL RETURN
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Stack frame
Full frame
Extended Deque (Return from a Call Case 1) Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
FUNCTION CALL RETURN
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Stack frame
Full frame
Extended Deque (Return from a Call Case2) Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
Worker executes an unconditional steal
SPAWN RETURN
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Stack frame
Full frame
Extended Deque (Before Spawn return Case 1) Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
SPAWN RETURN
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Stack frame
Full frame
Extended Deque (After Spawn return Case 1) Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
SPAWN RETURN
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Stack frame
Full frame
Extended Deque (Return from a SpawnCase2) Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
Worker executes an provably good steal
SYNC
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Stack frame
Full frame
Extended Deque (Sync Case 1) Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
Do nothing if it is a stack frame (No Op)
SYNC
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Stack frame
Full frame
Extended Deque (Sync Case 2) Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
Pop the frame, provably good steal
OUTLINE • CILK and CILK++ Language Features and
Usages • Work stealing runtime • CILK++ Reducers • Conclusions
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PROBLEMS WITH NON-LOCAL VARIABLES bool has_property(Node *) List<Node *> output_list; void walk(Node *x) {
if (x) { if (has_property(x)) output_list.push_back(x); cilk_spawn walk(x->left); walk(x->right); cilk_sync;
}
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REDUCER DESIGN GOALS • Support parallelization of programs
containing global variables • Enable efficient parallel scaling by
avoiding a single point of contention • Provide deterministic result for
associative reduce operations • Operate independently of any control
constructs
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REDUCER EXAMPLE bool has_property(Node *) List_append_reducer<Node *> output_list; void walk(Node *x) {
if (x) { if (has_property(x)) output_list.push_back(x); cilk_spawn walk(x->left); walk(x->right); cilk_sync;
}
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HYPER OBJECTS
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Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
REDUCER
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Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
SEMANTICS OF REDUCERS • The child strand owns the view owned by
parent function before cilk_spawn • The parent strand owns a new view,
initialized to identity view e, • A special optimization ensures that if a
view is unchanged when combined with the identity view 3
• Parent strand P own the view from completed child strands
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REDUCING OVER LIST CONCATENATION
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Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
REDUCING OVER LIST CONCATENATION
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Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
IMPLEMENTATION OF REDUCER • Each worker maintains a hypermap • Hypermap
• Maps reducers to the views • User
• The view of the current procedure • Children
• The view of the children procedures • Right
• The view of right sibling • Identity
• The default value of a view
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UNDERSTANDING HYPERMAPS bool has_property(Node *) List_append_reducer<Node *> output_list; void walk(Node *x) ß------------ Proc A {
if (x) { if (has_property(x)) output_list.push_back(x); cilk_spawn walk(x->left); ß---------proc B cilk_spawn walk(x->right); ß-------- proc C cilk_sync;
}
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LAZY CREATION • A new view will only be created
• after a steal • On demand
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HYPERMAP CREATION
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Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
HYPERMAP CREATION
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Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
HYPERMAP CREATION
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Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
HYPERMAP CREATION
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Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
HYPERMAP CREATION
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Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
LOOK UP FAILURE • Inserts a view containing an identity
element for the reducer into the hypermap. • Following the lazy principle
• Look up returns the newly inserted identity view
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RANDOM WORK STEALING A random steal operation steals a full frame P and replaces it with a new full frame C in the victim.
USERC ← USERP; U S E R P ← 0/ ; CHILDRENP←0/; RIGHTP←0/.
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RANDOM WORK STEALING
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Pictures from “Reducers and Other CILK+ HyperObjects” Talk by Matteo Frigo (Intel). Pablo Halpern ( Intel). Charles E. Leiserson (MIT). Stephen Lewin-Berlin (Intel).
RETURN FROM A CALL Let C be a child frame of the parent frame P that originally called C, and suppose that C returns.
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RETURN FROM A CALL
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Stack frame
Full frame
Extended Deque (Before Return from a Call Case1) Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
RETURN FROM A CALL
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Stack frame
Full frame
Extended Deque (Return from a Call Case 1) Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
RETURN FROM A CALL Let C be a child frame of the parent frame P that originally called C, and suppose that C returns.
• If C is a stack frame, do nothing,
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FUNCTION CALL RETURN
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Stack frame
Full frame
Extended Deque (Return from a Call Case2) Function call Spawn Call return Spawn return Sync Randomly steal Provably good steal Unconditionally steal Resume full frame
Worker executes an unconditional steal
RETURN FROM A CALL Let C be a child frame of the parent frame P that originally called C, and suppose that C returns.
• If C is a stack frame, do nothing, • If C is a full frame.
• Transfer ownership of view • Children and Right are empty • USERP ← USERC
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RETURN FROM A SPAWN Let C be a child frame of the parent frame P that originally spawned C, and suppose that C returns. • Always do USERC ← REDUCE(USERC,RIGHTC) • If C is a stack frame, do nothing • If C is a full frame
• If C has siblings, • RIGHTL ← REDUCE(RIGHTL,USERC)
• C is the leftmost child • CHILDRENP ←
REDUCE(CHILDRENP,USERC)
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RETURN FROM A SPAWN EXAMPLE bool has_property(Node *) List_append_reducer<Node *> output_list; void walk(Node *x) ß------------ Proc A {
if (x) { if (has_property(x)) output_list.push_back(x); cilk_spawn walk(x->left); ß---------proc B cilk_spawn walk(x->right); ß-------- proc C cilk_sync;
}
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RETURN FROM A SPAWN EXAMPLE bool has_property(Node *) List_append_reducer<Node *> output_list; void walk(Node *x) ß------------ Proc A {
if (x) { if (has_property(x)) output_list.push_back(x); cilk_spawn walk(x->left); ß---------proc B cilk_spawn walk(x->right); ß-------- proc C cilk_sync;
}
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RETURN FROM A SPAWN EXAMPLE bool has_property(Node *) List_append_reducer<Node *> output_list; void walk(Node *x) ß------------ Proc A {
if (x) { if (has_property(x)) output_list.push_back(x); cilk_spawn walk(x->left); ß---------proc B cilk_spawn walk(x->right); ß-------- proc C cilk_sync;
}
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SYNC A cilk_sync statement waits until all children have com- pleted. When frame P executes a cilk_sync, one of following two cases applies: • If P is a stack frame, do nothing. • If P is a full frame,
• USERP ← REDUCE(CHILDRENP,USERP).
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BENEFITS OF REDUCERS
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OUTLINE • CILK and CILK++ Language Features and
Usages • Work stealing runtime • CILK++ Reducers • Conclusions
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CONCLUSIONS • CILK and CILK++ provide a programmer
friendly programming model • Extension to C • Incremental parallelism • Scaling on future machines
• Non-compromising performance • Work stealing runtime • Minimizing overheads • Reducers
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FINAL NOTES • Designed for an idealized shared memory
model • Today’s architectures are typically NUMA
• Task creation can be lazier • http://ieeexplore.ieee.org/xpls/abs_all.jsp?
arnumber=6012915&tag=1 • Cilk_for
• Divide and conquer parallelization
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