1Delay accounting
2----------------
3
4Tasks encounter delays in execution when they wait
5for some kernel resource to become available e.g. a
6runnable task may wait for a free CPU to run on.
7
8The per-task delay accounting functionality measures
9the delays experienced by a task while
10
11a) waiting for a CPU (while being runnable)
12b) completion of synchronous block I/O initiated by the task
13c) swapping in pages
14
15and makes these statistics available to userspace through
16the taskstats interface.
17
18Such delays provide feedback for setting a task's cpu priority,
19io priority and rss limit values appropriately. Long delays for
20important tasks could be a trigger for raising its corresponding priority.
21
22The functionality, through its use of the taskstats interface, also provides
23delay statistics aggregated for all tasks (or threads) belonging to a
24thread group (corresponding to a traditional Unix process). This is a commonly
25needed aggregation that is more efficiently done by the kernel.
26
27Userspace utilities, particularly resource management applications, can also
28aggregate delay statistics into arbitrary groups. To enable this, delay
29statistics of a task are available both during its lifetime as well as on its
30exit, ensuring continuous and complete monitoring can be done.
31
32
33Interface
34---------
35
36Delay accounting uses the taskstats interface which is described
37in detail in a separate document in this directory. Taskstats returns a
38generic data structure to userspace corresponding to per-pid and per-tgid
39statistics. The delay accounting functionality populates specific fields of
40this structure. See
41     include/linux/taskstats.h
42for a description of the fields pertaining to delay accounting.
43It will generally be in the form of counters returning the cumulative
44delay seen for cpu, sync block I/O, swapin etc.
45
46Taking the difference of two successive readings of a given
47counter (say cpu_delay_total) for a task will give the delay
48experienced by the task waiting for the corresponding resource
49in that interval.
50
51When a task exits, records containing the per-task statistics
52are sent to userspace without requiring a command. If it is the last exiting
53task of a thread group, the per-tgid statistics are also sent. More details
54are given in the taskstats interface description.
55
56The getdelays.c userspace utility in this directory allows simple commands to
57be run and the corresponding delay statistics to be displayed. It also serves
58as an example of using the taskstats interface.
59
60Usage
61-----
62
63Compile the kernel with
64	CONFIG_TASK_DELAY_ACCT=y
65	CONFIG_TASKSTATS=y
66
67Delay accounting is enabled by default at boot up.
68To disable, add
69   nodelayacct
70to the kernel boot options. The rest of the instructions
71below assume this has not been done.
72
73After the system has booted up, use a utility
74similar to  getdelays.c to access the delays
75seen by a given task or a task group (tgid).
76The utility also allows a given command to be
77executed and the corresponding delays to be
78seen.
79
80General format of the getdelays command
81
82getdelays [-t tgid] [-p pid] [-c cmd...]
83
84
85Get delays, since system boot, for pid 10
86# ./getdelays -p 10
87(output similar to next case)
88
89Get sum of delays, since system boot, for all pids with tgid 5
90# ./getdelays -t 5
91
92
93CPU	count	real total	virtual total	delay total
94	7876	92005750	100000000	24001500
95IO	count	delay total
96	0	0
97MEM	count	delay total
98	0	0
99
100Get delays seen in executing a given simple command
101# ./getdelays -c ls /
102
103bin   data1  data3  data5  dev  home  media  opt   root  srv        sys  usr
104boot  data2  data4  data6  etc  lib   mnt    proc  sbin  subdomain  tmp  var
105
106
107CPU	count	real total	virtual total	delay total
108	6	4000250		4000000		0
109IO	count	delay total
110	0	0
111MEM	count	delay total
112	0	0
113