As presented in Codemotion Tel Aviv 2017 http://telaviv2017.codemotionworld.com

1. Facts about multithreading
that'll keep you up at night
Guy Baron, Director Mobile @ Vonage, Codemotion Tel Aviv 2017

2. Wikipedia
multithreading is the ability of a central processing unit (CPU) to
execute multiple processes or threads concurrently

3. Advantages
• Utilize more CPU power by reducing the impact of delaying
operations such as IO operations
• If a thread cannot use all the computing resources of the CPU
(because instructions depend on each other's result), running
another thread may prevent those resources from becoming idle

4. Common Issues

5. T1 IO IO
Time
PROCESSING
Too Many Threads

6. T2
T1 IO IO
Time
PROCESSING CONTEXT SWITCH
Too Many Threads

7. T3
T2
T1 IO IO
Time
PROCESSING CONTEXT SWITCH
Too Many Threads

8. T4
T3
T2
T1 IO
Time
PROCESSING CONTEXT SWITCH
Too Many Threads

9. T5
T4
T3
T2
T1 IO
Time
PROCESSING CONTEXT SWITCH
Too Many Threads

10. Race condition
Race conditions arise in software when an application depends on the
sequence or timing of processes or threads for it to operate properly

11. T1 Memory T2
i++; int i = 0; i++;
0 0
1 0
1 1
1 1
1 2
2 2
read
Increment
write
read
write
Increment

12. T1 Memory T2
i++; int i = 0; i++;
0 0
1 0
1 0 0
1 1 0
1 1
1 1
read
Increment
write
read
write
Increment

13. Deadlock
A deadlock is a state in which each member of a group of actions, is
waiting for some other member to release a lock.

14. T1
T2
A B

15. T1
A
T2
B

16. T1
T2
A B

17. T1
T2
A B

18. T1
T2
A B

20. Priority Inversion
A problematic scenario in scheduling in which a high priority task is indirectly
preempted by a lower priority task effectively "inverting" the relative priorities of the two
tasks.

21. A
High priority
Med priority
Low priority

22. A
High priority
Med priority
Low priority

23. A
High priority
Med priority
Low priority

25. Uncommon issues

26. 64-bit values
(Java) single write to a non-volatile long or double value is treated as
two separate writes: one to each 32-bit half.
This can result in a situation where a thread sees the first 32 bits of a
64-bit value from one write, and the second 32 bits from another write.

27. 64-bit values
Memory
1 1 1 1 1 1 1 1
value1
2 2 2 2 2 2 2 2
value2

28. 64-bit values
1 1 1 1
Memory
1 1 1 1 1 1 1 1
value1
2 2 2 2 2 2 2 2
value2

29. 64-bit values
2 2 2 2
Memory
1 1 1 1 1 1 1 1
value1
2 2 2 2 2 2 2 2
value2

30. 64-bit values
2 2 2 2 2 2 2 2
Memory
1 1 1 1 1 1 1 1
value1
2 2 2 2 2 2 2 2
value2

31. 64-bit values
1 1 1 1 2 2 2 2
Memory
1 1 1 1 1 1 1 1
value1
2 2 2 2 2 2 2 2
value2

32. int x = 0;
boolean finished = false;
void executeOnCpu1() {
x = 10;
finished = true;
}
void executeOnCpu2() {
while (!finished) {}
System.out.print(x);
}
Take a moment to read the code… All modern CPUs have multiple
levels of CPU caches

33. Cache 1
Memory
int x = 0;
boolean finished = false;
void executeOnCpu1() {
x = 10;
finished = true;
}
void executeOnCpu2() {
while (!finished) {}
System.out.print(x);
}
x = 0;
finished = false;
Cache 2
x = 0;
finished = false;

34. Cache 1
Memory
int x = 0;
boolean finished = false;
void executeOnCpu1() {
x = 10;
finished = true;
}
void executeOnCpu2() {
while (!finished) {}
System.out.print(x);
}
x = 10;
finished = false;
Cache 2
x = 0;
finished = false;

35. Cache 1
Memory
int x = 10;
boolean finished = false;
void executeOnCpu1() {
x = 10;
finished = true;
}
void executeOnCpu2() {
while (!finished) {}
System.out.print(x);
}
x = 10;
finished = false;
Cache 2
x = 0;
finished = false;
X

36. Cache 1
Memory
int x = 10;
boolean finished = false;
void executeOnCpu1() {
x = 10;
finished = true;
}
void executeOnCpu2() {
while (!finished) {}
System.out.print(x);
}
x = 10;
finished = true;
Cache 2
x = 0;
finished = false;

37. Cache 1
Memory
int x = 10;
boolean finished = true;
void executeOnCpu1() {
x = 10;
finished = true;
}
void executeOnCpu2() {
while (!finished) {}
System.out.print(x);
}
x = 10;
finished = true;
Cache 2
x = 0;
finished = false;
finished

38. Cache 1
Memory
int x = 10;
boolean finished = true;
void executeOnCpu1() {
x = 10;
finished = true;
}
void executeOnCpu2() {
while (!finished) {}
System.out.print(x);
}
x = 10;
finished = true;
Cache 2
x = 0;
finished = false;
?

39. Cache 1
Memory
int x = 10;
boolean finished = true;
void executeOnCpu1() {
x = 10;
finished = true;
}
void executeOnCpu2() {
while (!finished) {}
System.out.print(x);
}
x = 10;
finished = true;
Cache 2
x = 0;
finished = true;
finished

40. Cache 1
Memory
int x = 10;
boolean finished = true;
void executeOnCpu1() {
x = 10;
finished = true;
}
void executeOnCpu2() {
while (!finished) {}
System.out.print(0);
}
x = 10;
finished = true;
Cache 2
x = 0;
finished = true;

41. Take a moment to read the code…
int orderOfEvaluation() {
return f1() + f2() + f3();
}
int orderOfEvaluationExplicit?() {
return (f1() + f2()) + f3();
}

42. int orderOfEvaluationExplicit() {
return f1() + f2() + f3();
}
class NoOrder {
int value = 1;
int f1() { return this.value; }
int f2() { return this.value; }
int f3() {
//...
this.value = 10;
//...
return this.value;
}
}

43. int orderOfEvaluationExplicit() {
return 1 + f2() + f3();
}
class NoOrder {
int value = 1;
int f1() { return this.value; }
int f2() { return this.value; }
int f3() {
//...
this.value = 10;
//...
return this.value;
}
}

44. int orderOfEvaluationExplicit() {
return 1 + 1 + f3();
}
class NoOrder {
int value = 1;
int f1() { return this.value; }
int f2() { return this.value; }
int f3() {
//...
this.value = 10;
//...
return this.value;
}
}

45. int orderOfEvaluationExplicit() {
return 1 + 1 + 10;
}
class NoOrder {
int value = 1;
int f1() { return this.value; }
int f2() { return this.value; }
int f3() {
//...
this.value = 10;
//...
return this.value;
}
}
The compiler can
evaluate operands in
any order (C++)

46. int orderOfEvaluationExplicit() {
return f1() + f2() + f3();
}
class NoOrder {
int value = 1;
int f1() { return this.value; }
int f2() { return this.value; }
int f3() {
//...
this.value = 10;
//...
return this.value;
}
}

47. int orderOfEvaluationExplicit() {
return f1() + f2() + 10;
}
class NoOrder {
int value = 1;
int f1() { return this.value; }
int f2() { return this.value; }
int f3() {
//...
this.value = 10;
//...
return this.value;
}
}

48. int orderOfEvaluationExplicit() {
return f1() + 10 + 10;
}
class NoOrder {
int value = 1;
int f1() { return this.value; }
int f2() { return this.value; }
int f3() {
//...
this.value = 10;
//...
return this.value;
}
}

49. int orderOfEvaluationExplicit() {
return 10 + 10 + 10;
}
class NoOrder {
int value = 1;
int f1() { return this.value; }
int f2() { return this.value; }
int f3() {
//...
this.value = 10;
//...
return this.value;
}
}

50. Thank you
We are hiring…