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c3123552 | 1 | ================ |
a3baf649 | 2 | Delay accounting |
c3123552 | 3 | ================ |
a3baf649 SN |
4 | |
5 | Tasks encounter delays in execution when they wait | |
6 | for some kernel resource to become available e.g. a | |
7 | runnable task may wait for a free CPU to run on. | |
8 | ||
9 | The per-task delay accounting functionality measures | |
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 initiated by the task | |
14 | c) swapping in pages | |
9b0975a2 | 15 | d) memory reclaim |
f347c9d2 | 16 | e) thrashing |
ec710aa8 | 17 | f) direct compact |
662ce1dc | 18 | g) write-protect copy |
a3b2aeac | 19 | h) IRQ/SOFTIRQ |
a3baf649 SN |
20 | |
21 | and makes these statistics available to userspace through | |
22 | the taskstats interface. | |
23 | ||
24 | Such delays provide feedback for setting a task's cpu priority, | |
25 | io priority and rss limit values appropriately. Long delays for | |
26 | important tasks could be a trigger for raising its corresponding priority. | |
27 | ||
28 | The functionality, through its use of the taskstats interface, also provides | |
29 | delay statistics aggregated for all tasks (or threads) belonging to a | |
30 | thread group (corresponding to a traditional Unix process). This is a commonly | |
31 | needed aggregation that is more efficiently done by the kernel. | |
32 | ||
33 | Userspace utilities, particularly resource management applications, can also | |
34 | aggregate delay statistics into arbitrary groups. To enable this, delay | |
35 | statistics of a task are available both during its lifetime as well as on its | |
36 | exit, ensuring continuous and complete monitoring can be done. | |
37 | ||
38 | ||
39 | Interface | |
40 | --------- | |
41 | ||
42 | Delay accounting uses the taskstats interface which is described | |
43 | in detail in a separate document in this directory. Taskstats returns a | |
44 | generic data structure to userspace corresponding to per-pid and per-tgid | |
45 | statistics. The delay accounting functionality populates specific fields of | |
46 | this structure. See | |
c3123552 | 47 | |
ec710aa8 | 48 | include/uapi/linux/taskstats.h |
c3123552 | 49 | |
a3baf649 SN |
50 | for a description of the fields pertaining to delay accounting. |
51 | It will generally be in the form of counters returning the cumulative | |
ec710aa8 | 52 | delay seen for cpu, sync block I/O, swapin, memory reclaim, thrash page |
a3b2aeac | 53 | cache, direct compact, write-protect copy, IRQ/SOFTIRQ etc. |
a3baf649 SN |
54 | |
55 | Taking the difference of two successive readings of a given | |
56 | counter (say cpu_delay_total) for a task will give the delay | |
57 | experienced by the task waiting for the corresponding resource | |
58 | in that interval. | |
59 | ||
ad4ecbcb SN |
60 | When a task exits, records containing the per-task statistics |
61 | are sent to userspace without requiring a command. If it is the last exiting | |
62 | task of a thread group, the per-tgid statistics are also sent. More details | |
63 | are given in the taskstats interface description. | |
a3baf649 | 64 | |
d522b2cd SK |
65 | The getdelays.c userspace utility in tools/accounting directory allows simple |
66 | commands to be run and the corresponding delay statistics to be displayed. It | |
67 | also serves as an example of using the taskstats interface. | |
a3baf649 SN |
68 | |
69 | Usage | |
70 | ----- | |
71 | ||
c3123552 MCC |
72 | Compile the kernel with:: |
73 | ||
a3baf649 SN |
74 | CONFIG_TASK_DELAY_ACCT=y |
75 | CONFIG_TASKSTATS=y | |
76 | ||
e4042ad4 PZ |
77 | Delay accounting is disabled by default at boot up. |
78 | To enable, add:: | |
c3123552 | 79 | |
e4042ad4 | 80 | delayacct |
c3123552 | 81 | |
0cd7c741 PZ |
82 | to the kernel boot options. The rest of the instructions below assume this has |
83 | been done. Alternatively, use sysctl kernel.task_delayacct to switch the state | |
84 | at runtime. Note however that only tasks started after enabling it will have | |
85 | delayacct information. | |
a3baf649 | 86 | |
163ecdff | 87 | After the system has booted up, use a utility |
a3baf649 SN |
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 | ||
c3123552 | 94 | General format of the getdelays command:: |
a3baf649 | 95 | |
ec710aa8 | 96 | getdelays [-dilv] [-t tgid] [-p pid] |
a3baf649 | 97 | |
c3123552 | 98 | Get delays, since system boot, for pid 10:: |
a3baf649 | 99 | |
ec710aa8 | 100 | # ./getdelays -d -p 10 |
c3123552 | 101 | (output similar to next case) |
a3baf649 | 102 | |
c3123552 | 103 | Get sum of delays, since system boot, for all pids with tgid 5:: |
a3baf649 | 104 | |
ec710aa8 | 105 | # ./getdelays -d -t 5 |
106 | print delayacct stats ON | |
107 | TGID 5 | |
c3123552 | 108 | |
a3baf649 | 109 | |
ec710aa8 | 110 | CPU count real total virtual total delay total delay average |
111 | 8 7000000 6872122 3382277 0.423ms | |
112 | IO count delay total delay average | |
eca7de7c | 113 | 0 0 0.000ms |
ec710aa8 | 114 | SWAP count delay total delay average |
eca7de7c | 115 | 0 0 0.000ms |
ec710aa8 | 116 | RECLAIM count delay total delay average |
eca7de7c | 117 | 0 0 0.000ms |
ec710aa8 | 118 | THRASHING count delay total delay average |
eca7de7c | 119 | 0 0 0.000ms |
ec710aa8 | 120 | COMPACT count delay total delay average |
eca7de7c | 121 | 0 0 0.000ms |
a3b2aeac YY |
122 | WPCOPY count delay total delay average |
123 | 0 0 0.000ms | |
124 | IRQ count delay total delay average | |
eca7de7c | 125 | 0 0 0.000ms |
a3baf649 | 126 | |
ec710aa8 | 127 | Get IO accounting for pid 1, it works only with -p:: |
a3baf649 | 128 | |
ec710aa8 | 129 | # ./getdelays -i -p 1 |
130 | printing IO accounting | |
131 | linuxrc: read=65536, write=0, cancelled_write=0 | |
a3baf649 | 132 | |
ec710aa8 | 133 | The above command can be used with -v to get more debug information. |