1. Barrier Synchronization
Companion slides for
The Art of Multiprocessor Programming
by Maurice Herlihy & Nir Shavit

2. Art of Multiprocessor Programming
Simple Video Game
Prepare frame for display
By graphics coprocessor
“soft real-time” application
Need at least 35 frames/second
OK to mess up rarely
Art of Multiprocessor Programming

3. Art of Multiprocessor Programming
Simple Video Game
while (true) {
frame.prepare();
frame.display(); }
Art of Multiprocessor Programming

4. Art of Multiprocessor Programming
Simple Video Game
while (true) {
frame.prepare();
frame.display(); }
What about overlapping work?
1st thread displays frame
2nd prepares next frame
Art of Multiprocessor Programming

5. Art of Multiprocessor Programming
Two-Phase Rendering
while (true) {
if (phase) {
frame[0].display();
} else {
frame[1].display(); }
phase = !phase;
while (true) {
if (phase) {
frame[1].prepare();
} else {
frame[0].prepare(); }
phase = !phase;
Art of Multiprocessor Programming

6. Art of Multiprocessor Programming
Two-Phase Rendering
while (true) {
if (phase) {
frame[0].display();
} else {
frame[1].display(); }
phase = !phase;
while (true) {
if (phase) {
frame[1].prepare();
} else {
frame[0].prepare(); }
phase = !phase;
Even phases
Art of Multiprocessor Programming

7. Art of Multiprocessor Programming
Two-Phase Rendering
while (true) {
if (phase) {
frame[0].display();
} else {
frame[1].display(); }
phase = !phase;
while (true) {
if (phase) {
frame[1].prepare();
} else {
frame[0].prepare(); }
phase = !phase;
odd phases
Art of Multiprocessor Programming

8. Synchronization Problems
How do threads stay in phase?
Too early?
“we render no frame before its time”
Too late?
Recycle memory before frame is displayed
Art of Multiprocessor Programming

9. Ideal Parallel Computation1 1 1
Art of Multiprocessor Programming

10. Ideal Parallel Computation2 2 2 1 1 1
Art of Multiprocessor Programming

11. Real-Life Parallel Computation
zzz… 1 1
Art of Multiprocessor Programming

12. Real-Life Parallel Computation2
zzz… 1 1
Uh, oh
Art of Multiprocessor Programming

13. Barrier Synchronization
barrier
Art of Multiprocessor Programming

14. Barrier Synchronization1 1 1
Art of Multiprocessor Programming

15. Barrier Synchronization
Until every thread has left here
No thread enters here
Art of Multiprocessor Programming

16. Art of Multiprocessor Programming
Why Do We Care?
Mostly of interest to
Scientific & numeric computation
Elsewhere
Garbage collection
Less common in systems programming
Still important topic
Art of Multiprocessor Programming

17. Art of Multiprocessor Programming
Duality
Dual to mutual exclusion
Include others, not exclude them
Same implementation issues
Interaction with caches …
Invalidation?
Local spinning?
Art of Multiprocessor Programming

18. Example: Parallel Prefixb c d
before a
a+b
a+b+c
a+b+c +d
after
Art of Multiprocessor Programming

19. Art of Multiprocessor Programming
Parallel Prefix
One thread
Per entry a b c d
Art of Multiprocessor Programming

20. Parallel Prefix: Phase 1b c d a
a+b
b+c
c+d
Art of Multiprocessor Programming

21. Parallel Prefix: Phase 2b c d a
a+b
a+b+c
a+b+c +d
Art of Multiprocessor Programming

22. Art of Multiprocessor Programming
Parallel Prefix
N threads can compute
Parallel prefix
Of N entries
In log2 N rounds
What if system is asynchronous?
Why we need barriers
Art of Multiprocessor Programming

23. Art of Multiprocessor Programming
Prefix
class Prefix extends Thread {
private int[] a;
private int i;
private Barrier b;
public Prefix(int[] a,
Barrier b, int i) {
a = a;
b = b;
i = i; }
Art of Multiprocessor Programming

24. Art of Multiprocessor Programming
Prefix
class Prefix extends Thread {
private int[] a;
private int i;
private Barrier b;
public Prefix(int[] a,
Barrier b, int i) {
a = a;
b = b;
i = i; }
Array of input values
Art of Multiprocessor Programming

25. Art of Multiprocessor Programming
Prefix
class Prefix extends Thread {
private int[] a;
private int i;
private Barrier b;
public Prefix(int[] a,
Barrier b, int i) {
a = a;
b = b;
i = i; }
Thread index
Art of Multiprocessor Programming

26. Art of Multiprocessor Programming
Prefix
class Prefix extends Thread {
private int[] a;
private int i;
private Barrier b;
public Prefix(int[] a,
Barrier b, int i) {
a = a;
b = b;
i = i; }
Shared barrier
Art of Multiprocessor Programming

27. Art of Multiprocessor Programming
Prefix
class Prefix extends Thread {
private int[] a;
private int i;
private Barrier b;
public Prefix(int[] a,
Barrier b, int i) {
a = a;
b = b;
i = i; }
Initialize fields
Art of Multiprocessor Programming

28. Where Do the Barriers Go?
public void run() {
int d = 1, sum = 0;
while (d < N) {
if (i >= d)
sum = a[i-d];
a[i] += sum;
d = d * 2;
}}}
Art of Multiprocessor Programming

29. Where Do the Barriers Go?
public void run() {
int d = 1, sum = 0;
while (d < N) {
if (i >= d)
sum = a[i-d];
b.await();
a[i] += sum;
d = d * 2;
}}}
Art of Multiprocessor Programming

30. Where Do the Barriers Go?
public void run() {
int d = 1, sum = 0;
while (d < N) {
if (i >= d)
sum = a[i-d];
b.await();
a[i] += sum;
d = d * 2;
}}}
Make sure everyone reads before anyone writes
Art of Multiprocessor Programming

31. Where Do the Barriers Go?
public void run() {
int d = 1, sum = 0;
while (d < N) {
if (i >= d)
sum = a[i-d];
b.await();
a[i] += sum;
d = d * 2;
}}}
Make sure everyone reads before anyone writes
Art of Multiprocessor Programming

32. Where Do the Barriers Go?
public void run() {
int d = 1, sum = 0;
while (d < N) {
if (i >= d)
sum = a[i-d];
b.await();
a[i] += sum;
d = d * 2;
}}}
Make sure everyone reads before anyone writes
Make sure everyone writes before anyone reads
Art of Multiprocessor Programming

33. Barrier Implementations
Cache coherence
Spin on locally-cached locations?
Spin on statically-defined locations?
Latency
How many steps?
Symmetry
Do all threads do the same thing?
Art of Multiprocessor Programming

34. Art of Multiprocessor Programming
Barriers
public class Barrier {
AtomicInteger count;
int size;
public Barrier(int n){
count = AtomicInteger(n);
size = n; }
public void await() {
if (count.getAndDecrement()==1) {
count.set(size);
} else {
while (count.get() != 0);
}}}}
Art of Multiprocessor Programming

35. Number threads not yet arrived
Barriers
public class Barrier {
AtomicInteger count;
int size;
public Barrier(int n){
count = AtomicInteger(n);
size = n; }
public void await() {
if (count.getAndDecrement()==1) {
count.set(size);
} else {
while (count.get() != 0);
}}}}
Number threads not yet arrived
Art of Multiprocessor Programming

36. Number threads participating
Barriers
public class Barrier {
AtomicInteger count;
int size;
public Barrier(int n){
count = AtomicInteger(n);
size = n; }
public void await() {
if (count.getAndDecrement()==1) {
count.set(size);
} else {
while (count.get() != 0);
}}}}
Number threads participating
Art of Multiprocessor Programming

37. Art of Multiprocessor Programming
Barriers
public class Barrier {
AtomicInteger count;
int size;
public Barrier(int n){
count = AtomicInteger(n);
size = n; }
public void await() {
if (count.getAndDecrement()==1) {
count.set(size);
} else {
while (count.get() != 0);
}}}}
Initialization
Art of Multiprocessor Programming

38. Art of Multiprocessor Programming
Barriers
public class Barrier {
AtomicInteger count;
int size;
public Barrier(int n){
count = AtomicInteger(n);
size = n; }
public void await() {
if (count.getAndDecrement()==1) {
count.set(size);
} else {
while (count.get() != 0);
}}}}
Principal method
Art of Multiprocessor Programming

39. If I’m last, reset fields for next time
Barriers
public class Barrier {
AtomicInteger count;
int size;
public Barrier(int n){
count = AtomicInteger(n);
size = n; }
public void await() {
if (count.getAndDecrement()==1) {
count.set(size);
} else {
while (count.get() != 0);
}}}}
If I’m last, reset fields for next time
Art of Multiprocessor Programming

40. Otherwise, wait for everyone else
Barriers
public class Barrier {
AtomicInteger count;
int size;
public Barrier(int n){
count = AtomicInteger(n);
size = n; }
public void await() {
if (count.getAndDecrement()==1) {
count.set(size);
} else {
while (count.get() != 0);
}}}}
Otherwise, wait for everyone else
Art of Multiprocessor Programming

41. What’s wrong with this protocol?
Barriers
public class Barrier {
AtomicInteger count;
int size;
public Barrier(int n){
count = AtomicInteger(n);
size = n; }
public void await() {
if (count.getAndDecrement()==1) {
count.set(size);
} else {
while (count.get() != 0);
}}}}
What’s wrong with this protocol?
Art of Multiprocessor Programming

42. Art of Multiprocessor Programming
Reuse
Barrier b = new Barrier(n);
while ( mumble() ) {
work();
b.await() }
Do work
repeat
synchronize
Art of Multiprocessor Programming

43. Art of Multiprocessor Programming
Barriers
public class Barrier {
AtomicInteger count;
int size;
public Barrier(int n){
count = AtomicInteger(n);
size = n; }
public void await() {
if (count.getAndDecrement()==1) {
count.set(size);
} else {
while (count.get() != 0);
}}}}
Art of Multiprocessor Programming

44. Waiting for Phase 1 to finish
Barriers
public class Barrier {
AtomicInteger count;
int size;
public Barrier(int n){
count = AtomicInteger(n);
size = n; }
public void await() {
if (count.getAndDecrement()==1) {
count.set(size);
} else {
while (count.get() != 0);
}}}}
Waiting for Phase 1 to finish
Art of Multiprocessor Programming

45. Waiting for Phase 1 to finish
Barriers
Phase 1 is so over
public class Barrier {
AtomicInteger count;
int size;
public Barrier(int n){
count = AtomicInteger(n);
size = n; }
public void await() {
if (count.getAndDecrement()==1) {
count.set(size);
} else {
while (count.get() != 0);
}}}}
Waiting for Phase 1 to finish
Art of Multiprocessor Programming

46. Art of Multiprocessor Programming
Barriers
Prepare for phase 2
public class Barrier {
AtomicInteger count;
int size;
public Barrier(int n){
count = AtomicInteger(n);
size = n; }
public void await() {
if (count.getAndDecrement()==1) {
count.set(size);
} else {
while (count.get() != 0);
}}}}
ZZZZZ….
Art of Multiprocessor Programming

47. Waiting for Phase 2 to finish Waiting for Phase 1 to finish
Uh-Oh
public class Barrier {
AtomicInteger count;
int size;
public Barrier(int n){
count = AtomicInteger(n);
size = n; }
public void await() {
if (count.getAndDecrement()==1) {
count.set(size);
} else {
while (count.get() != 0);
}}}}
Waiting for Phase 2 to finish
Waiting for Phase 1 to finish
Art of Multiprocessor Programming

48. Art of Multiprocessor Programming
Basic Problem
One thread “wraps around” to start phase 2
While another thread is still waiting for phase 1
One solution:
Always use two barriers
Art of Multiprocessor Programming

49. Sense-Reversing Barriers
public class Barrier {
AtomicInteger count;
int size;
boolean sense = false;
threadSense = new ThreadLocal<boolean>…
public void await {
boolean mySense = threadSense.get();
if (count.getAndDecrement()==1) {
count.set(size); sense = !mySense
} else {
while (sense != mySense) {} }
threadSense.set(!mySense)}}}
Art of Multiprocessor Programming

50. Sense-Reversing Barriers
public class Barrier {
AtomicInteger count;
int size;
boolean sense = false;
threadSense = new ThreadLocal<boolean>…
public void await {
boolean mySense = threadSense.get();
if (count.getAndDecrement()==1) {
count.set(size); sense = !mySense
} else {
while (sense != mySense) {} }
threadSense.set(!mySense)}}}
Completed odd or even-numbered phase?
Art of Multiprocessor Programming

51. Sense-Reversing Barriers
public class Barrier {
AtomicInteger count;
int size;
boolean sense = false;
threadSense = new ThreadLocal<boolean>…
public void await {
boolean mySense = threadSense.get();
if (count.getAndDecrement()==1) {
count.set(size); sense = !mySense
} else {
while (sense != mySense) {} }
threadSense.set(!mySense)}}}
Store sense for next phase
Art of Multiprocessor Programming

52. Sense-Reversing Barriers
public class Barrier {
AtomicInteger count;
int size;
boolean sense = false;
threadSense = new ThreadLocal<boolean>…
public void await {
boolean mySense = threadSense.get();
if (count.getAndDecrement()==1) {
count.set(size); sense = !mySense
} else {
while (sense != mySense) {} }
threadSense.set(!mySense)}}}
Get new sense determined by last phase
Art of Multiprocessor Programming

53. Sense-Reversing Barriers
public class Barrier {
AtomicInteger count;
int size;
boolean sense = false;
threadSense = new ThreadLocal<boolean>…
public void await {
boolean mySense = threadSense.get();
if (count.getAndDecrement()==1) {
count.set(size); sense = !mySense
} else {
while (sense != mySense) {} }
threadSense.set(!mySense)}}}
If I’m last, reverse sense for next time
Art of Multiprocessor Programming

54. Sense-Reversing Barriers
public class Barrier {
AtomicInteger count;
int size;
boolean sense = false;
threadSense = new ThreadLocal<boolean>…
public void await {
boolean mySense = threadSense.get();
if (count.getAndDecrement()==1) {
count.set(size); sense = !mySense
} else {
while (sense != mySense) {} }
threadSense.set(!mySense)}}}
Otherwise, wait for sense to flip
Art of Multiprocessor Programming

55. Sense-Reversing Barriers
public class Barrier {
AtomicInteger count;
int size;
boolean sense = false;
threadSense = new ThreadLocal<boolean>…
public void await {
boolean mySense = threadSense.get();
if (count.getAndDecrement()==1) {
count.set(size); sense = !mySense
} else {
while (sense != mySense) {} }
threadSense.set(!mySense)}}}
Prepare sense for next phase
Art of Multiprocessor Programming

56. Combining Tree Barriers
Art of Multiprocessor Programming

57. Combining Tree Barriers
Art of Multiprocessor Programming

58. Combining Tree Barrier
public class Node{
AtomicInteger count; int size;
Node parent; Volatile boolean sense;
public void await() {…
if (count.getAndDecrement()==1) {
if (parent != null) {
parent.await()}
count.set(size);
sense = mySense
} else {
while (sense != mySense) {}
}…}}}
Art of Multiprocessor Programming

59. Combining Tree Barrier
Parent barrier in tree
public class Node{
AtomicInteger count; int size;
Node parent; Volatile boolean sense;
public void await() {…
if (count.getAndDecrement()==1) {
if (parent != null) {
parent.await()}
count.set(size);
sense = mySense
} else {
while (sense != mySense) {}
}…}}}
Art of Multiprocessor Programming

60. Combining Tree Barrier
Am I last?
public class Node{
AtomicInteger count; int size;
Node parent; Volatile boolean sense;
public void await() {…
if (count.getAndDecrement()==1) {
if (parent != null) {
parent.await()}
count.set(size);
sense = mySense
} else {
while (sense != mySense) {}
}…}}}
Art of Multiprocessor Programming

61. Combining Tree Barrier
Proceed to parent barrier
public class Node{
AtomicInteger count; int size;
Node parent; Volatile boolean sense;
public void await() {…
if (count.getAndDecrement()==1) {
if (parent != null) {
parent.await();}
count.set(size);
sense = mySense
} else {
while (sense != mySense) {}
}…}}}
Art of Multiprocessor Programming

62. Combining Tree Barrier
public class Node{
AtomicInteger count; int size;
Node parent; Volatile boolean sense;
public void await() {…
if (count.getAndDecrement()==1) {
if (parent != null) {
parent.await()}
count.set(size);
sense = mySense
} else {
while (sense != mySense) {}
}…}}}
Prepare for next phase
Art of Multiprocessor Programming

63. Combining Tree Barrier
Notify others at this node
public class Node{
AtomicInteger count; int size;
Node parent; Volatile boolean sense;
public void await() {…
if (count.getAndDecrement()==1) {
if (parent != null) {
parent.await()}
count.set(size);
sense = mySense
} else {
while (sense != mySense) {}
}…}}}
Art of Multiprocessor Programming

64. Combining Tree Barrier
public class Node{
AtomicInteger count; int size;
Node parent; Volatile boolean sense;
public void await() {…
if (count.getAndDecrement()==1) {
if (parent != null) {
parent.await()}
count.set(size);
sense = mySense
} else {
while (sense != mySense) {}
}…}}}
I’m not last, so wait for notification
Art of Multiprocessor Programming

65. Combining Tree Barrier
No sequential bottleneck
Parallel getAndDecrement() calls
Low memory contention
Same reason
Cache behavior
Local spinning on bus-based architecture
Not so good for NUMA
Art of Multiprocessor Programming

66. Art of Multiprocessor Programming
Remarks
Everyone spins on sense field
Local spinning on bus-based (good)
Network hot-spot on distributed architecture (bad)
Not really scalable
Art of Multiprocessor Programming

67. Tournament Tree Barrier
If tree nodes have fan-in 2
Don’t need to call getAndDecrement()
Winner chosen statically
At level i
If i-th bit of id is 0, move up
Otherwise keep back
Art of Multiprocessor Programming

68. Tournament Tree Barriers
root
winner
loser
winner
loser
winner
loser
Art of Multiprocessor Programming

69. Tournament Tree Barriers
All flags blue
Art of Multiprocessor Programming

70. Tournament Tree Barriers
Loser thread sets winner’s flag
Art of Multiprocessor Programming

71. Tournament Tree Barriers
Loser spins on own flag
Art of Multiprocessor Programming

72. Tournament Tree Barriers
Winner spins on own flag
Art of Multiprocessor Programming

73. Tournament Tree Barriers
Winner sees own flag, moves up, spins
Art of Multiprocessor Programming

74. Tournament Tree Barriers
Bingo!
Art of Multiprocessor Programming

75. Tournament Tree Barriers
Sense-reversing: next time use blue flags
Art of Multiprocessor Programming

76. Art of Multiprocessor Programming
Tournament Barrier
class TBarrier {
boolean flag;
TBarrier partner;
TBarrier parent;
boolean top; … }
Art of Multiprocessor Programming

77. Notifications delivered here
Tournament Barrier
class TBarrier {
boolean flag;
TBarrier partner;
TBarrier parent;
boolean top; … }
Notifications delivered here
Art of Multiprocessor Programming

78. Other thead at same level
Tournament Barrier
class TBarrier {
boolean flag;
TBarrier partner;
TBarrier parent;
boolean top; … }
Other thead at same level
Art of Multiprocessor Programming

79. Parent (winner) or null (loser)
Tournament Barrier
class TBarrier {
boolean flag;
TBarrier partner;
TBarrier parent;
boolean top; … }
Parent (winner) or null (loser)
Art of Multiprocessor Programming

80. Art of Multiprocessor Programming
Tournament Barrier
class TBarrier {
boolean flag;
TBarrier partner;
TBarrier parent;
boolean top; … }
Am I the root?
Art of Multiprocessor Programming

81. Art of Multiprocessor Programming
Tournament Barrier
void await(boolean mySense) {
if (top) {
return;
} else if (parent != null) {
while (flag != mySense) {};
parent.await(mySense);
partner.flag = mySense;
} else {
}}}
Art of Multiprocessor Programming

82. Parameter to save space…
Tournament Barrier
Sense is a
Parameter to save space…
void await(boolean mySense) {
if (top) {
return;
} else if (parent != null) {
while (flag != mySense) {};
parent.await(mySense);
partner.flag = mySense;
} else {
}}}
Le root, c’est moi
Art of Multiprocessor Programming

83. Art of Multiprocessor Programming
Tournament Barrier
void await(boolean mySense) {
if (top) {
return;
} else if (parent != null) {
while (flag != mySense) {};
parent.await(mySense);
partner.flag = mySense;
} else {
}}}
I am already a winner
Art of Multiprocessor Programming

84. Art of Multiprocessor Programming
Tournament Barrier
void await(boolean mySense) {
if (top) {
return;
} else if (parent != null) {
while (flag != mySense) {};
parent.await(mySense);
partner.flag = mySense;
} else {
}}}
Wait for partner
Art of Multiprocessor Programming

85. Art of Multiprocessor Programming
Tournament Barrier
void await(boolean mySense) {
if (top) {
return;
} else if (parent != null) {
while (flag != mySense) {};
parent.await(mySense);
partner.flag = mySense;
} else {
}}}
Synchronize upstairs
Art of Multiprocessor Programming

86. Art of Multiprocessor Programming
Tournament Barrier
void await(boolean mySense) {
if (top) {
return;
} else if (parent != null) {
while (flag != mySense) {};
parent.await(mySense);
partner.flag = mySense;
} else {
}}}
Inform partner
Art of Multiprocessor Programming

87. Art of Multiprocessor Programming
Tournament Barrier
void await(boolean mySense) {
if (top) {
return;
} else if (parent != null) {
while (flag != mySense) {};
parent.await(mySense);
partner.flag = mySense;
} else {
}}}
Natural-born loser
Art of Multiprocessor Programming

88. Art of Multiprocessor Programming
Tournament Barrier
void await(boolean mySense) {
if (top) {
return;
} else if (parent != null) {
while (flag != mySense) {};
parent.await(mySense);
partner.flag = mySense;
} else {
}}}
Tell partner I’m here
Art of Multiprocessor Programming

89. Wait for notification from partner
Tournament Barrier
void await(boolean mySense) {
if (top) {
return;
} else if (parent != null) {
while (flag != mySense) {};
parent.await(mySense);
partner.flag = mySense;
} else {
}}}
Wait for notification from partner
Art of Multiprocessor Programming

90. Art of Multiprocessor Programming
Remarks
No need for read-modify-write calls
Each thread spins on fixed location
Good for bus-based architectures
Good for NUMA architectures
Art of Multiprocessor Programming

91. Dissemination Barrier
At round i
Thread A notifies thread A+2i (mod n)
Requires log n rounds
Art of Multiprocessor Programming

92. Dissemination Barrier+1 +2 +4
Art of Multiprocessor Programming

93. Art of Multiprocessor Programming
Remarks
Great for lovers of combinatorics
Not so much fun for those who track memory footprints
Art of Multiprocessor Programming

94. Art of Multiprocessor Programming
Ideas So Far
Sense-reversing
Reuse without reinitializing
Combining tree
Like counters, locks …
Tournament tree
Optimized combining tree
Dissemination barrier
Intellectually Pleasing (matter of taste)
Art of Multiprocessor Programming

95. Which is best for Multicore?
On a cache coherent multicore chip: none of the above…
Use a static tree barrier!
Art of Multiprocessor Programming

96. Art of Multiprocessor Programming
Static Tree Barrier
All spin on
cached copy
of Done flag 2
Counter
in parent:
Num of
children
that have
not
arrived 2
We describe the static tree using counters in the nodes but could use simple array of locations, each written by a child, all spun on (repeatedly read) by the parent
Art of Multiprocessor Programming

97. Art of Multiprocessor Programming
Static Tree Barrier
All spin on
cached copy
of Done flag
All detect
change in
Done flag 1 2 1 2
We describe the static tree using counters in the nodes but could use simple array of locations, each written by a child, all spun on (repeatedly read) by the parent
Art of Multiprocessor Programming

98. Art of Multiprocessor Programming
Remarks
Very little cache traffic
Minimal space overhead
Can be laid out optimally on NUMA
Instead of Done bit, notification must be performed by passing sense down the static tree
Art of Multiprocessor Programming

99. So…How will we make use of multicores?
Back to Amdahl’s Law:
Speedup = 1/(ParallelPart/N + SequentialPart)
Pay for N = 8 cores
SequentialPart = 25%
Speedup = only 2.9 times!
Must parallelize applications on a very fine grain!

100. Need Fine-Grained Locking
The reason
we get
only 2.9 speedup
Coarse
Grained
Fine
Grained c
25%
Shared
25%
Shared c c c
75%
Unshared
75%
Unshared

101. Traditional Scaling Process7x
Speedup
3.6x
1.8x
User code
Recall the traditional scaling process for software: write it once, trust Intel to make the CPU faster to improve performance.
Traditional
Uniprocessor
Time: Moore’s law
Art of Multiprocessor Programming

102. Multicore Scaling Process
Speedup
1.8x 7x
3.6x
User code
Multicore
With multicores, we will have to parallelize the code to make software faster, and we cannot do this automatically (except in a limited way on the level of individual instructions).
As noted, not so simple…
Art of Multiprocessor Programming

103. Real-World Scaling Process
Speedup
2.9x 2x
1.8x
User code
Multicore
This is because splitting the application up to utilize the cores is not simple, and coordination among the various code parts requires care.
Parallelization and Synchronization
will require great care…
Art of Multiprocessor Programming

104. Future: Smoother Scaling …
Speedup
1.8x 7x
3.6x
User code
Abstractions
(like TM)
Multicore

105. Multicore Programming
We are at the dawn of a new era…
Still a lot of research and development to be done
And a body of multicore programming experience to be accumelated by us all
© 2006 Herlihy & Shavit

106. Thanks for Attending! Good luck with your programming…
If you have questions or run into interesting problems, just us:
© 2006 Herlihy & Shavit

107. Art of Multiprocessor Programming
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Art of Multiprocessor Programming