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SubjectRe: [Jackit-devel] Re: Statistical methods for latency profiling
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On Sat, 2004-07-31 at 23:24, Lee Revell wrote:

> I will have some numbers soon.

Here is the histogram from running 500000 cycles (~5 minutes), with
max_sectors_kb -> 64, normal desktop usage. Kernel is 2.6.8-rc2-M5, set
to 1:3, with all irqs threaded except the sound card.

Delay Count
----- -----
6 2257
7 3210
8 797
9 717
10 283
11 219
12 235
13 250
14 263
15 320
16 334
17 287
18 268
19 260
20 223
21 155
22 109
23 90
24 41
25 19
26 3
27 4
28 1
29 3
30 1
31 2
32 5
33 3
34 1
35 7
36 1
37 3
38 4
39 1
40 5
41 8
42 2
43 1
44 1
47 1
48 1
52 2
62 1
79 1
100 1

The next three tests were done with iozone -a running.

Delay Count
----- -----
6 21
7 203
8 595
9 1756
10 1643
11 957
12 618
13 507
14 532
15 540
16 523
17 423
18 400
19 424
20 418
21 293
22 202
23 119
24 71
25 38
26 20
27 8
28 7
29 2
30 1
31 1
32 4
33 4
34 7
35 6
36 1
37 2
38 5
39 7
40 3
41 4
42 4
43 1
44 2
47 1
48 1
50 1
53 1
54 1
57 1
59 1
63 1
69 1
70 2
79 1
91 1
92 1
93 1
100 1
101 1
107 1
109 1
113 1
118 1
123 1
131 1
143 1
145 1
146 1
157 1
161 1

So stressing the filesystem moves the center to the right a bit, from
6-7 to 9-10, and *drastically* lengthens the 'tail'.

These numbers suggest to me that a lot of the latencies from 47 usecs
and up are caused by one code path, because they are so uniformly
distributed over the upper part of the histogram. The prime suspect of
course being the ide io completions. I tested this theory by lowering
max_sectors_kb from 64 to 32:

Delay Count
----- -----
6 3
7 427
8 742
9 1583
10 3429
11 2010
12 614
13 203
14 200
15 275
16 267
17 177
18 104
19 50
20 55
21 36
22 22
23 21
24 22
25 29
26 26
27 9
28 2
29 3
30 1
32 2
33 2
34 2
35 1
36 1
37 2
41 1
42 1
43 1
48 1
65 1
70 1
71 4
72 3
73 6
74 3
75 1
76 1
77 3
78 2
79 4
80 2
81 3
82 3
83 3
84 3
85 2
86 2
87 4
88 4
90 1
91 1
92 3
94 3
96 2
101 2
102 3
103 1
106 1
111 1
112 1
122 1
132 1

Finally, 16:

Delay Count
----- -----
6 420
7 1463
8 652
9 1444
10 2365
11 1111
12 537
13 265
14 232
15 241
16 253
17 218
18 173
19 181
20 233
21 188
22 125
23 76
24 55
25 23
26 9
27 15
28 6
29 2
30 2
31 6
32 5
33 9
34 8
35 4
36 2
37 5
38 2
39 1
40 2
41 3
42 3
43 2
45 1
47 1
49 1
51 3
52 1
53 1
54 1
55 1
56 1
57 1
59 2
60 2
61 2
62 2
63 1
64 1
66 2
67 2
68 3
69 1
71 1
72 3
74 2
75 1
76 2
77 1
78 2
79 1
80 1
81 2
82 1
84 1
85 3
86 1
87 1
89 1
90 2
92 1
100 1
101 1

These numbers all point to the ide sg completion code as the only
thing on the system generating latencies over ~42 usecs.

I have sent the patch separately to jackit-devel so as not to clutter
LKML with unrelated patches.

Lee

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