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a3baf649 SN |
1 | Delay accounting |
2 | ---------------- | |
3 | ||
4 | Tasks encounter delays in execution when they wait | |
5 | for some kernel resource to become available e.g. a | |
6 | runnable task may wait for a free CPU to run on. | |
7 | ||
8 | The per-task delay accounting functionality measures | |
9 | the delays experienced by a task while | |
10 | ||
11 | a) waiting for a CPU (while being runnable) | |
12 | b) completion of synchronous block I/O initiated by the task | |
13 | c) swapping in pages | |
14 | ||
15 | and makes these statistics available to userspace through | |
16 | the taskstats interface. | |
17 | ||
18 | Such delays provide feedback for setting a task's cpu priority, | |
19 | io priority and rss limit values appropriately. Long delays for | |
20 | important tasks could be a trigger for raising its corresponding priority. | |
21 | ||
22 | The functionality, through its use of the taskstats interface, also provides | |
23 | delay statistics aggregated for all tasks (or threads) belonging to a | |
24 | thread group (corresponding to a traditional Unix process). This is a commonly | |
25 | needed aggregation that is more efficiently done by the kernel. | |
26 | ||
27 | Userspace utilities, particularly resource management applications, can also | |
28 | aggregate delay statistics into arbitrary groups. To enable this, delay | |
29 | statistics of a task are available both during its lifetime as well as on its | |
30 | exit, ensuring continuous and complete monitoring can be done. | |
31 | ||
32 | ||
33 | Interface | |
34 | --------- | |
35 | ||
36 | Delay accounting uses the taskstats interface which is described | |
37 | in detail in a separate document in this directory. Taskstats returns a | |
38 | generic data structure to userspace corresponding to per-pid and per-tgid | |
39 | statistics. The delay accounting functionality populates specific fields of | |
40 | this structure. See | |
41 | include/linux/taskstats.h | |
42 | for a description of the fields pertaining to delay accounting. | |
43 | It will generally be in the form of counters returning the cumulative | |
44 | delay seen for cpu, sync block I/O, swapin etc. | |
45 | ||
46 | Taking the difference of two successive readings of a given | |
47 | counter (say cpu_delay_total) for a task will give the delay | |
48 | experienced by the task waiting for the corresponding resource | |
49 | in that interval. | |
50 | ||
ad4ecbcb SN |
51 | When a task exits, records containing the per-task statistics |
52 | are sent to userspace without requiring a command. If it is the last exiting | |
53 | task of a thread group, the per-tgid statistics are also sent. More details | |
54 | are given in the taskstats interface description. | |
a3baf649 SN |
55 | |
56 | The getdelays.c userspace utility in this directory allows simple commands to | |
57 | be run and the corresponding delay statistics to be displayed. It also serves | |
58 | as an example of using the taskstats interface. | |
59 | ||
60 | Usage | |
61 | ----- | |
62 | ||
63 | Compile the kernel with | |
64 | CONFIG_TASK_DELAY_ACCT=y | |
65 | CONFIG_TASKSTATS=y | |
66 | ||
163ecdff SN |
67 | Delay accounting is enabled by default at boot up. |
68 | To disable, add | |
69 | nodelayacct | |
70 | to the kernel boot options. The rest of the instructions | |
71 | below assume this has not been done. | |
a3baf649 | 72 | |
163ecdff | 73 | After the system has booted up, use a utility |
a3baf649 SN |
74 | similar to getdelays.c to access the delays |
75 | seen by a given task or a task group (tgid). | |
76 | The utility also allows a given command to be | |
77 | executed and the corresponding delays to be | |
78 | seen. | |
79 | ||
80 | General format of the getdelays command | |
81 | ||
82 | getdelays [-t tgid] [-p pid] [-c cmd...] | |
83 | ||
84 | ||
85 | Get delays, since system boot, for pid 10 | |
86 | # ./getdelays -p 10 | |
87 | (output similar to next case) | |
88 | ||
89 | Get sum of delays, since system boot, for all pids with tgid 5 | |
90 | # ./getdelays -t 5 | |
91 | ||
92 | ||
93 | CPU count real total virtual total delay total | |
94 | 7876 92005750 100000000 24001500 | |
95 | IO count delay total | |
96 | 0 0 | |
97 | MEM count delay total | |
98 | 0 0 | |
99 | ||
100 | Get delays seen in executing a given simple command | |
101 | # ./getdelays -c ls / | |
102 | ||
103 | bin data1 data3 data5 dev home media opt root srv sys usr | |
104 | boot data2 data4 data6 etc lib mnt proc sbin subdomain tmp var | |
105 | ||
106 | ||
107 | CPU count real total virtual total delay total | |
108 | 6 4000250 4000000 0 | |
109 | IO count delay total | |
110 | 0 0 | |
111 | MEM count delay total | |
112 | 0 0 |