1================ 2Delay accounting 3================ 4 5Tasks encounter delays in execution when they wait 6for some kernel resource to become available e.g. a 7runnable task may wait for a free CPU to run on. 8 9The per-task delay accounting functionality measures 10the delays experienced by a task while 11 12a) waiting for a CPU (while being runnable) 13b) completion of synchronous block I/O initiated by the task 14c) swapping in pages 15d) memory reclaim 16e) thrashing 17f) direct compact 18g) write-protect copy 19h) IRQ/SOFTIRQ 20 21and makes these statistics available to userspace through 22the taskstats interface. 23 24Such delays provide feedback for setting a task's cpu priority, 25io priority and rss limit values appropriately. Long delays for 26important tasks could be a trigger for raising its corresponding priority. 27 28The functionality, through its use of the taskstats interface, also provides 29delay statistics aggregated for all tasks (or threads) belonging to a 30thread group (corresponding to a traditional Unix process). This is a commonly 31needed aggregation that is more efficiently done by the kernel. 32 33Userspace utilities, particularly resource management applications, can also 34aggregate delay statistics into arbitrary groups. To enable this, delay 35statistics of a task are available both during its lifetime as well as on its 36exit, ensuring continuous and complete monitoring can be done. 37 38 39Interface 40--------- 41 42Delay accounting uses the taskstats interface which is described 43in detail in a separate document in this directory. Taskstats returns a 44generic data structure to userspace corresponding to per-pid and per-tgid 45statistics. The delay accounting functionality populates specific fields of 46this structure. See 47 48 include/uapi/linux/taskstats.h 49 50for a description of the fields pertaining to delay accounting. 51It will generally be in the form of counters returning the cumulative 52delay seen for cpu, sync block I/O, swapin, memory reclaim, thrash page 53cache, direct compact, write-protect copy, IRQ/SOFTIRQ etc. 54 55Taking the difference of two successive readings of a given 56counter (say cpu_delay_total) for a task will give the delay 57experienced by the task waiting for the corresponding resource 58in that interval. 59 60When a task exits, records containing the per-task statistics 61are sent to userspace without requiring a command. If it is the last exiting 62task of a thread group, the per-tgid statistics are also sent. More details 63are given in the taskstats interface description. 64 65The getdelays.c userspace utility in tools/accounting directory allows simple 66commands to be run and the corresponding delay statistics to be displayed. It 67also serves as an example of using the taskstats interface. 68 69Usage 70----- 71 72Compile the kernel with:: 73 74 CONFIG_TASK_DELAY_ACCT=y 75 CONFIG_TASKSTATS=y 76 77Delay accounting is disabled by default at boot up. 78To enable, add:: 79 80 delayacct 81 82to the kernel boot options. The rest of the instructions below assume this has 83been done. Alternatively, use sysctl kernel.task_delayacct to switch the state 84at runtime. Note however that only tasks started after enabling it will have 85delayacct information. 86 87After the system has booted up, use a utility 88similar to getdelays.c to access the delays 89seen by a given task or a task group (tgid). 90The utility also allows a given command to be 91executed and the corresponding delays to be 92seen. 93 94General format of the getdelays command:: 95 96 getdelays [-dilv] [-t tgid] [-p pid] 97 98Get delays, since system boot, for pid 10:: 99 100 # ./getdelays -d -p 10 101 (output similar to next case) 102 103Get sum of delays, since system boot, for all pids with tgid 5:: 104 105 # ./getdelays -d -t 5 106 print delayacct stats ON 107 TGID 5 108 109 110 CPU count real total virtual total delay total delay average 111 8 7000000 6872122 3382277 0.423ms 112 IO count delay total delay average 113 0 0 0.000ms 114 SWAP count delay total delay average 115 0 0 0.000ms 116 RECLAIM count delay total delay average 117 0 0 0.000ms 118 THRASHING count delay total delay average 119 0 0 0.000ms 120 COMPACT count delay total delay average 121 0 0 0.000ms 122 WPCOPY count delay total delay average 123 0 0 0.000ms 124 IRQ count delay total delay average 125 0 0 0.000ms 126 127Get IO accounting for pid 1, it works only with -p:: 128 129 # ./getdelays -i -p 1 130 printing IO accounting 131 linuxrc: read=65536, write=0, cancelled_write=0 132 133The above command can be used with -v to get more debug information. 134