1 ================
2 Delay accounting
3 ================
4
5 Tasks encounter delays in execution when they
6 for some kernel resource to become available e
7 runnable task may wait for a free CPU to run o
8
9 The per-task delay accounting functionality me
10 the delays experienced by a task while
11
12 a) waiting for a CPU (while being runnable)
13 b) completion of synchronous block I/O initiat
14 c) swapping in pages
15 d) memory reclaim
16 e) thrashing
17 f) direct compact
18 g) write-protect copy
19 h) IRQ/SOFTIRQ
20
21 and makes these statistics available to usersp
22 the taskstats interface.
23
24 Such delays provide feedback for setting a tas
25 io priority and rss limit values appropriately
26 important tasks could be a trigger for raising
27
28 The functionality, through its use of the task
29 delay statistics aggregated for all tasks (or
30 thread group (corresponding to a traditional U
31 needed aggregation that is more efficiently do
32
33 Userspace utilities, particularly resource man
34 aggregate delay statistics into arbitrary grou
35 statistics of a task are available both during
36 exit, ensuring continuous and complete monitor
37
38
39 Interface
40 ---------
41
42 Delay accounting uses the taskstats interface
43 in detail in a separate document in this direc
44 generic data structure to userspace correspond
45 statistics. The delay accounting functionality
46 this structure. See
47
48 include/uapi/linux/taskstats.h
49
50 for a description of the fields pertaining to
51 It will generally be in the form of counters r
52 delay seen for cpu, sync block I/O, swapin, me
53 cache, direct compact, write-protect copy, IRQ
54
55 Taking the difference of two successive readin
56 counter (say cpu_delay_total) for a task will
57 experienced by the task waiting for the corres
58 in that interval.
59
60 When a task exits, records containing the per-
61 are sent to userspace without requiring a comm
62 task of a thread group, the per-tgid statistic
63 are given in the taskstats interface descripti
64
65 The getdelays.c userspace utility in tools/acc
66 commands to be run and the corresponding delay
67 also serves as an example of using the tasksta
68
69 Usage
70 -----
71
72 Compile the kernel with::
73
74 CONFIG_TASK_DELAY_ACCT=y
75 CONFIG_TASKSTATS=y
76
77 Delay accounting is disabled by default at boo
78 To enable, add::
79
80 delayacct
81
82 to the kernel boot options. The rest of the in
83 been done. Alternatively, use sysctl kernel.ta
84 at runtime. Note however that only tasks start
85 delayacct information.
86
87 After the system has booted up, use a utility
88 similar to getdelays.c to access the delays
89 seen by a given task or a task group (tgid).
90 The utility also allows a given command to be
91 executed and the corresponding delays to be
92 seen.
93
94 General format of the getdelays command::
95
96 getdelays [-dilv] [-t tgid] [-p pid]
97
98 Get delays, since system boot, for pid 10::
99
100 # ./getdelays -d -p 10
101 (output similar to next case)
102
103 Get sum of delays, since system boot, for all
104
105 # ./getdelays -d -t 5
106 print delayacct stats ON
107 TGID 5
108
109
110 CPU count real total
111 8 7000000
112 IO count delay total
113 0 0 0
114 SWAP count delay total
115 0 0
116 RECLAIM count delay total
117 0 0 0
118 THRASHING count delay total
119 0 0
120 COMPACT count delay total
121 0 0
122 WPCOPY count delay total
123 0 0
124 IRQ count delay total
125 0 0
126
127 Get IO accounting for pid 1, it works only wit
128
129 # ./getdelays -i -p 1
130 printing IO accounting
131 linuxrc: read=65536, write=0, cancelle
132
133 The above command can be used with -v to get m
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