On Tue, May 10, 2016 at 6:31 PM, Vince Refiti <vin...@gmail.com> wrote:
> Hello
>
> I am playing around with Semaphore, and wrote the following:
>
> | coll count sem |
> coll := Array withAll: (1 to: 100).
> count := 0.
> sem := Semaphore new.
> coll do: [ :each |
> count := count + 1.
> (count >= 5)
> ifTrue: [
> Transcript show: 'waiting...'; cr.
> sem wait ].
>
> [ [ 2 seconds asDelay wait.
> Transcript show: each printString, ' ', count printString; cr ]
> ensure: [
> count := count -1.
> sem signal ] ] fork ]
>
> The output is:
[snipped]
> I was expecting 'waiting...' to alternate with the 'each-count' lines. Also
> the entire statement paused for 2 seconds, spat out some lines, then waited
> a little longer, and then finished.
>
> Can someone please explain this pattern?
>
> Thanks, Vince
Hi Vince,
General things to be aware of...
* Transcript is not perturbing your result.
* Playground code is executed in the context of the main UI-process. Thus...
* An infinite loop will lock the UI.
* A sem wait will lock the UI.
* Processes at the same priority are scheduled cooperatively.
* Processes at different priority are pre-emptively scheduled.
I refactored your example to better highlight program flow...
| coll count sem |
Transcript clear.
coll := Array withAll: (1 to: 99).
count := 0.
sem := Semaphore new.
coll do:
[ :each |
count := count + 1.
Transcript crShow: each printString; tab; tab; show: count printString, '+'.
(count >= 5) ifTrue:
[ sem isSignaled
ifTrue: [ Transcript tab; show: ' nowait'.
sem wait ]
ifFalse: [ Transcript tab; show: ' wait'.
sem wait.
Transcript crShow: each printString; tab;
tab; show: count printString; tab; show: ' postwait, excessSignals ' ,
sem excessSignals printString. ]
].
[ Transcript crShow: ' ', each printString; tab; tab; show: count
printString; tab; show: ' delay'.
2 seconds asDelay wait.
count := count -1.
Transcript crShow: ' ', each printString; tab ; tab; show: count
printString, '-'; tab; show: ' signal'.
sem signal.
] fork.
]
You will need to add...
Semaphore>>excessSignals
^excessSignals
For counts 1, 2, 3 & 4, forked processes -1 to -4 are queued to run,
but don't run until the UI-process suspended by the wait at each=5,
count=5, which is why the "waiting..." tag appears first.
When the UI-process waits, processes -1 to -4 run in turn and
immediately delay for two seconds. After this, they queued to run.
Process-1 signals the UI-process to be queued to run. But processes
-2 to -4 run first, signalling sem an extra three times.
When the UI-process resumes, each=5, count=1 and sem has three
excessSignals. Completing the do: adds process -5 to the run queue.
Then counts 2, 3, & 4 queues processes -6, -7 & -8 to run. Counts 5,
6 & 7 consume the excessSignals without suspending, queing processes
-9, -10 & -11 to run.
At count=8, the UI-process suspends at the wait, and process -5 runs.
This signals the UI-process to be queued to run. Processes -6 to -11
run, signalling sem an extra six times.
When the UI-process resumes, each=12, count=1 and sem has six
excessSignals. Completing the do: adds process -12 to the run queue.
Then counts 2, 3, & 4 queues processes -13, -14 & -15 to run. Counts
5, 6, 7, 8, 9, 10 consume the excessSignals without suspending, queing
processes -16, -17, -18, -19, -20, -21 to run.
At count=11, the UI-process suspends at the wait, and process -12
runs. This signals the UI-process to be queued to run. Processes -13
to -21 run, signalling sem an extra nine times.
ecetera...
btw, The delay doesn't make much difference to how the forked
processes interact with the main UI-process, since all forked
processes execute before the UI-process resumed. It just interleaves
the delay/signal tags rather than grouping them.
1 1+
2 2+
3 3+
4 4+
5 5+ wait
1 5 delay
2 5 delay
3 5 delay
4 5 delay
1 4- signal
2 3- signal
3 2- signal
4 1- signal
5 1 postwait, excessSignals 3
6 2+
7 3+
8 4+
9 5+ nowait
10 6+ nowait
11 7+ nowait
12 8+ wait
5 8 delay
6 8 delay
7 8 delay
8 8 delay
9 8 delay
10 8 delay
11 8 delay
5 7- signal
6 6- signal
7 5- signal
8 4- signal
9 3- signal
10 2- signal
11 1- signal
12 1 postwait, excessSignals 6
13 2+
14 3+
15 4+
16 5+ nowait
17 6+ nowait
18 7+ nowait
19 8+ nowait
20 9+ nowait
21 10+ nowait
22 11+ wait
12 11 delay
13 11 delay
14 11 delay
15 11 delay
16 11 delay
17 11 delay
18 11 delay
19 11 delay
20 11 delay
21 11 delay
12 10- signal
13 9- signal
14 8- signal
15 7- signal
16 6- signal
17 5- signal
18 4- signal
19 3- signal
20 2- signal
21 1- signal
22 1 postwait, excessSignals 9
23 2+
24 3+
25 4+
26 5+ nowait
27 6+ nowait
28 7+ nowait
29 8+ nowait
30 9+ nowait
31 10+ nowait
32 11+ nowait
33 12+ nowait
34 13+ nowait
35 14+ wait
22 14 delay
23 14 delay
24 14 delay
25 14 delay
26 14 delay
27 14 delay
28 14 delay
29 14 delay
30 14 delay
31 14 delay
32 14 delay
33 14 delay
34 14 delay
22 13- signal
23 12- signal
24 11- signal
25 10- signal
26 9- signal
27 8- signal
28 7- signal
29 6- signal
30 5- signal
31 4- signal
32 3- signal
33 2- signal
34 1- signal
35 1 postwait, excessSignals 12
36 2+
37 3+
38 4+
39 5+ nowait
40 6+ nowait
41 7+ nowait
42 8+ nowait
43 9+ nowait
44 10+ nowait
45 11+ nowait
46 12+ nowait
47 13+ nowait
48 14+ nowait
49 15+ nowait
50 16+ nowait
51 17+ wait
35 17 delay
36 17 delay
37 17 delay
38 17 delay
39 17 delay
40 17 delay
41 17 delay
42 17 delay
43 17 delay
44 17 delay
45 17 delay
46 17 delay
47 17 delay
48 17 delay
49 17 delay
50 17 delay
35 16- signal
36 15- signal
37 14- signal
38 13- signal
39 12- signal
40 11- signal
41 10- signal
42 9- signal
43 8- signal
44 7- signal
45 6- signal
46 5- signal
47 4- signal
48 3- signal
49 2- signal
50 1- signal
51 1 postwait, excessSignals 15
52 2+
53 3+
54 4+
55 5+ nowait
56 6+ nowait
57 7+ nowait
58 8+ nowait
59 9+ nowait
60 10+ nowait
61 11+ nowait
62 12+ nowait
63 13+ nowait
64 14+ nowait
65 15+ nowait
66 16+ nowait
67 17+ nowait
68 18+ nowait
69 19+ nowait
70 20+ wait
51 20 delay
52 20 delay
53 20 delay
54 20 delay
55 20 delay
56 20 delay
57 20 delay
58 20 delay
59 20 delay
60 20 delay
61 20 delay
62 20 delay
63 20 delay
64 20 delay
65 20 delay
66 20 delay
67 20 delay
68 20 delay
69 20 delay
51 19- signal
52 18- signal
53 17- signal
54 16- signal
55 15- signal
56 14- signal
57 13- signal
58 12- signal
59 11- signal
60 10- signal
61 9- signal
62 8- signal
63 7- signal
64 6- signal
65 5- signal
66 4- signal
67 3- signal
68 2- signal
69 1- signal
70 1 postwait, excessSignals 18
71 2+
72 3+
73 4+
74 5+ nowait
75 6+ nowait
76 7+ nowait
77 8+ nowait
78 9+ nowait
79 10+ nowait
80 11+ nowait
81 12+ nowait
82 13+ nowait
83 14+ nowait
84 15+ nowait
85 16+ nowait
86 17+ nowait
87 18+ nowait
88 19+ nowait
89 20+ nowait
90 21+ nowait
91 22+ nowait
92 23+ wait
70 23 delay
71 23 delay
72 23 delay
73 23 delay
74 23 delay
75 23 delay
76 23 delay
77 23 delay
78 23 delay
79 23 delay
80 23 delay
81 23 delay
82 23 delay
83 23 delay
84 23 delay
85 23 delay
86 23 delay
87 23 delay
88 23 delay
89 23 delay
90 23 delay
91 23 delay
70 22- signal
71 21- signal
72 20- signal
73 19- signal
74 18- signal
75 17- signal
76 16- signal
77 15- signal
78 14- signal
79 13- signal
80 12- signal
81 11- signal
82 10- signal
83 9- signal
84 8- signal
85 7- signal
86 6- signal
87 5- signal
88 4- signal
89 3- signal
90 2- signal
91 1- signal
92 1 postwait, excessSignals 21
93 2+
94 3+
95 4+
96 5+ nowait
97 6+ nowait
98 7+ nowait
99 8+ nowait
92 8 delay
93 8 delay
94 8 delay
95 8 delay
96 8 delay
97 8 delay
98 8 delay
99 8 delay
92 7- signal
93 6- signal
94 5- signal
95 4- signal
96 3- signal
97 2- signal
98 1- signal
99 0- signal
