1. CILK: An Efficient Multithreaded Runtime System

2. People Project at MIT & now at UT Austin
Bobby Blumofe (now UT Austin, Akamai)
Chris Joerg
Brad Kuszmaul (now Yale)
Charles Leiserson (MIT, Akamai)
Keith Randall (Bell Labs)
Yuli Zhou (Bell Labs)

3. Outline Introduction Programming environment
The work-stealing thread scheduler
Performance of applications
Modeling performance
Proven Properties
Conclusions

4. Introduction Why multithreading? Cilk programmer optimizes:
To implement dynamic, asynchronous, concurrent programs.
Cilk programmer optimizes:
total work
critical path
A Cilk computation is viewed as a dynamic, directed acyclic graph (dag)

5. Introduction ...

6. Introduction ... Cilk program is a set of procedures
A procedure is a sequence of threads
Cilk threads are:
represented by nodes in the dag
Non-blocking: run to completion: no waiting or suspension: atomic units of execution

7. Introduction ... Threads can spawn child threads
downward edges connect a parent to its children
A child & parent can run concurrently.
Non-blocking threads  a child cannot return a value to its parent.
The parent spawns a successor that receives values from its children

8. Introduction ...
A thread & its successor are parts of the same Cilk procedure.
connected by horizontal arcs
Children’s returned values are received before their successor begins:
They constitute data dependencies.
Connected by curved arcs

9. Introduction ...

10. Introduction: Execution Time
Execution time of a Cilk program using P processors depends on:
Work (T1): time for Cilk program with 1 processor to complete.
Critical path (T): the time to execute the longest directed path in the dag.
TP >= T1 / P (not true for some searches)
TP >= T

11. Introduction: Scheduling
Cilk uses run time scheduling called work stealing.
Works well on dynamic, asynchronous, MIMD-style programs.
For “fully strict” programs, Cilk achieves asymptotic optimality for:
space, time, & communication

12. Introduction: language
Cilk is an extension of C
Cilk programs are:
preprocessed to C
linked with a runtime library

13. Programming Environment
Declaring a thread:
thread T ( <args> ) { <stmts> }
T is preprocessed into a C function of 1 argument and return type void.
The 1 argument is a pointer to a closure

14. Environment: Closure A closure is a data structure that has:
a pointer to the C function for T
a slot for each argument (inputs & continuations)
a join counter: count of the missing argument values
A closure is ready when join counter == 0.
A closure is waiting otherwise.
They are allocated from a runtime heap

15. Environment: Continuation
A Cilk continuation is a data type, denoted by the keyword cont.
cont int x;
It is a global reference to an empty slot of a closure.
It is implemented as 2 items:
a pointer to the closure; (what thread)
an int value: the slot number. (what input)

16. Environment: Closure

17. Environment: spawn spawn T (<args> ) spawn T (k, ?x);
To spawn a child, a thread creates its closure:
spawn T (<args> )
creates child’s closure
sets available arguments
sets join counter
To specify a missing argument, prefix with a “?”
spawn T (k, ?x);

18. Environment: spawn_next
A successor thread is spawned the same way as a child, except the keyword spawn_next is used:
spawn_next T(k, ?x)
Children typically have no missing arguments; successors do.

19. Explicit continuation passing
Nonblocking threads  a parent cannot block on children’s results.
It spawns a successor thread.
This communication paradigm is called explicit continuation passing.
Cilk provides a primitive to send a value from one closure to another.

20. send_argument Cilk provides the primitive send_argument( k, value )
sends value to the argument slot of a waiting closure specified by continuation k.
spawn_next
successor
parent
spawn
send_argument
child

21. Cilk Procedure for computing a Fibonacci number
thread int fib ( cont int k, int n ) {
if ( n < 2 ) send_argument( k, n );
else { cont int x, y;
spawn_next sum ( k, ?x, ?y );
spawn fib ( x, n - 1 );
spawn fib ( y, n - 2 ); }
thread sum ( cont int k, int x, int y ) {
send_argument ( k, x + y );

22. Nonblocking Threads: Advantages
Shallow call stack.
Simplify runtime system:
Completed threads leave C runtime stack empty.
Portable runtime implementation

23. Nonblocking Threads: Disdvantages
Burdens programmer with explicit continuation passing.

24. Work-Stealing Scheduler
The concept of work-stealing goes at least as far back as 1981.
Work-stealing:
a process with no work selects a victim from which to get work.
it gets the shallowest thread in the victim’s spawn tree.
In Cilk, thieves choose victims randomly.

25. Thread Level

26. Stealing Work: The Ready Deque
Each closure has a level:
level( child ) = level( parent ) + 1
level( successor ) = level( parent )
Each processor maintains a ready deque:
Contains ready closures
The Lth element contains the list of all ready closures whose level is L.

27. Ready deque if ( ! readyDeque .isEmpty() ) take deepest thread else
steal shallowest thread from readyDeque of randomly selected victim

28. Why Steal Shallowest closure?
Shallow threads probably produce more work, therefore, reduce communication.
Shallow threads more likely to be on critical path.

29. Readying a Remote Closure
If a send_argument makes a remote closure ready,
put closure on sending processor’s readyDeque
 extra communication.
Done to make scheduler provably good
Putting on local readyDeque works well in practice.

30. Performance of Application
Tserial = time for C program
T1 = time for 1-processor Cilk program
Tserial /T1 = efficiency of the Cilk program
Efficiency is close to 1 for programs with moderately long threads: Cilk overhead is small.

31. Performance of Applications
T1/TP = speedup
T1/ T = average parallelism
If average parallelism is large
then speedup is nearly perfect.
If average parallelism is small
then speedup is much smaller.

32. Performance Data

33. Performance of Applications
Application speedup = efficiency X speedup
= ( Tserial /T1 ) X ( T1/TP ) = Tserial / TP

34. Modeling Performance TP >= max( T , T1 / P )
A good scheduler should come close to these lower bounds.

35. Modeling Performance Empirical data suggests that for Cilk:
TP  c1 T1 / P + c  T ,
where c1  & c   1.042
If T1 / T > 10P
then critical path does not affect TP.

36. Proven Property: Time Time: Including overhead, TP = O( T1/P + T ),
which is asymptotically optimal

37. Conclusions
We can predict the performance of a Cilk program by observing machine-independent characteristics:
Work
Critical path
when the program is fully-strict.
Cilk’s usefulness is unclear for other kinds of programs (e.g., iterative programs).

38. Conclusions ... Explicit continuation passing a nuisance.
It subsequently was removed (with more clever pre-processing).

39. Conclusions ... Great system research has a theoretical underpinning.
Such research identifies important properties
of the systems themselves, or
of our ability to reason about them formally.
Cilk identified 3 significant system properties:
Fully strict programs
Non-blocking threads
Randomly choosing a victim.

40. END

41. The Cost of Spawns
A spawn is about an order of magnitude more costly than a C function call.
Spawned threads running on parent’s processor can be implemented more efficiently than remote spawns.
This usually is the case.
Compiler techniques can exploit this distinction.

42. Communication Efficiency
A request is an attempt to steal work (the victim may not have work).
Requests/processor & steals/processor both grow as the critical path grows.

43. Proven Properties: Space
A fully strict program’s threads send arguments only to its parent’s successors.
For such programs, space, time, & communication bounds are proven.
Space: SP <= S1 P.
There exists a P-processor execution for which this is asymptotically optimal.

44. Proven Properties: Communication
Communication: The expected # of bits communicated in a P-processor execution is:
O( T P SMAX )
where SMAX denotes its largest closure.
There exists a program such that, for all P, there exists a P-processor execution that communicates k bits, where k > c T P SMAX, for some constant, c.