1.
Martin is getting
the projector
to work with
his laptop.

2.
Threads,
Critical Sections,
and Termination
Martin McClure

3.
Thread
vs.
Process

4.
Shared
State

5.
...sigh

6.
GemBuilder®
for
Smalltalk

7.
GBS

8.
Mapping
Dictionaries

9.
ClientMap
• Identity Dictionary
•~2M keys
•~100K updates/s
•~(1-10)×100K lookups/s

10.
32-bit VW

11.
32-bit VW
214
hash values

12.
Safe sharing of
this structure?

13.
Semaphore

14.
Semaphore
...performance?

15.
Performance

16.
"Premature
optimization is the
root of all evil."
--Donald Knuth

17.
Simplicity
Clarity

18.
Performance

19.
Performance
5ms

20.
Semaphore
...performance?

21.
Time microsecondsToRun:
[1000000 timesRepeat: []]

22.
Time microsecondsToRun:
[1000000 timesRepeat: []]
3ns-4ns

23.
[]
3ns-4ns

24.
[sem critical: []]

25.
[sem critical: []]
~70ns

26.
[dict
delegateAtClient: key]

27.
[dict
delegateAtClient: key]
~40ns

28.
[sem critical:
[dict
delegateAtClient: key]]

29.
[sem critical:
[dict
delegateAtClient: key]]
~130ns

30.
Batched
semaphore
for modifying

31.
No
semaphore
for lookup

32.
Hash Bucket Lookup
index := linear search for key.
^index == 0
ifTrue: [nil]
ifFalse:
[value := self basicAt: index.
(keys at: index) == key
ifTrue: [value] ifFalse: [nil]]

33.
Hash Bucket Add
index := freeHead.
[] ensure:
[freeHead := keys at: freeHead.
self basicAt: index put: value.
keys at: index put: key.
size := size + 1].

34.
...but there’s
a hole

35.
Hash Bucket Lookup
index := linear search for key.
^index == 0
ifTrue: [nil]
ifFalse:
[value := self basicAt: index.
(keys at: index) == key
ifTrue: [value] ifFalse: [nil]]

36.
Hash Bucket Add
index := freeHead.
[] ensure:
[freeHead := keys at: freeHead.
self basicAt: index put: value.
keys at: index put: key.
size := size + 1].

37.
[“empty block”] ensure:
[“non-empty block”].

38.
Hash Bucket Add
index := freeHead.
[] ensure:
[freeHead := keys at: freeHead.
self basicAt: index put: value.
keys at: index put: key.
size := size + 1].

39.
Semaphore
critical: mutuallyExcludedBlock
self waitIfCurtailedSignal.
^mutuallyExcludedBlock
ensure: [self signal]

40.
testTerminateInEnsure
| process count random delay |
random := Random new.
10 timesRepeat:
[process :=
[count := 0.
[] ensure:
[10 timesRepeat:
[count := count + 1.
1000000 timesRepeat: [12 factorial]].
count := count + 1]]
forkAt: Processor activeProcess priority - 1.
delay := (random next * 100) asInteger + 10. "avoid 0-ms delay"
(Delay forMilliseconds: delay) wait.
self assert: process isTerminated not.
process terminate.
process priority: Processor activeProcess priority + 1.
self
assert: process isTerminated;
assert: count equals: 11]

41.
Terminating
in a
Critical Section

42.
Hash Bucket Add
index := freeHead.
[] ensure:
[freeHead := keys at: freeHead.
self basicAt: index put: value.
keys at: index put: key.
size := size + 1].

43.
Is this
the
best way?

44.
Other Ways
• Semaphore
•#valueUninterruptably
•#valueUnpreemptively

45.
Semaphore

46.
Semaphore
•No termination
protection

47.
Semaphore
•No termination
protection
•70ns

48.
#valueUninterruptably

49.
~87ns
[]valueUninterruptably

50.
#valueUnpreemptively

51.
[]valueUnpreemptively
~175ns

52.
[] ensure: []

53.
[] ensure: []
~15ns

54.
Final
Thoughts

55.
?

56.
Threads,
Critical Sections,
and Termination
Martin McClure