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 16 17and makes these statistics available to userspace through 18the taskstats interface. 19 20Such delays provide feedback for setting a task's cpu priority, 21io priority and rss limit values appropriately. Long delays for 22important tasks could be a trigger for raising its corresponding priority. 23 24The functionality, through its use of the taskstats interface, also provides 25delay statistics aggregated for all tasks (or threads) belonging to a 26thread group (corresponding to a traditional Unix process). This is a commonly 27needed aggregation that is more efficiently done by the kernel. 28 29Userspace utilities, particularly resource management applications, can also 30aggregate delay statistics into arbitrary groups. To enable this, delay 31statistics of a task are available both during its lifetime as well as on its 32exit, ensuring continuous and complete monitoring can be done. 33 34 35Interface 36--------- 37 38Delay accounting uses the taskstats interface which is described 39in detail in a separate document in this directory. Taskstats returns a 40generic data structure to userspace corresponding to per-pid and per-tgid 41statistics. The delay accounting functionality populates specific fields of 42this structure. See 43 44 include/linux/taskstats.h 45 46for a description of the fields pertaining to delay accounting. 47It will generally be in the form of counters returning the cumulative 48delay seen for cpu, sync block I/O, swapin, memory reclaim etc. 49 50Taking the difference of two successive readings of a given 51counter (say cpu_delay_total) for a task will give the delay 52experienced by the task waiting for the corresponding resource 53in that interval. 54 55When a task exits, records containing the per-task statistics 56are sent to userspace without requiring a command. If it is the last exiting 57task of a thread group, the per-tgid statistics are also sent. More details 58are given in the taskstats interface description. 59 60The getdelays.c userspace utility in tools/accounting directory allows simple 61commands to be run and the corresponding delay statistics to be displayed. It 62also serves as an example of using the taskstats interface. 63 64Usage 65----- 66 67Compile the kernel with:: 68 69 CONFIG_TASK_DELAY_ACCT=y 70 CONFIG_TASKSTATS=y 71 72Delay accounting is disabled by default at boot up. 73To enable, add:: 74 75 delayacct 76 77to the kernel boot options. The rest of the instructions below assume this has 78been done. Alternatively, use sysctl kernel.task_delayacct to switch the state 79at runtime. Note however that only tasks started after enabling it will have 80delayacct information. 81 82After the system has booted up, use a utility 83similar to getdelays.c to access the delays 84seen by a given task or a task group (tgid). 85The utility also allows a given command to be 86executed and the corresponding delays to be 87seen. 88 89General format of the getdelays command:: 90 91 getdelays [-t tgid] [-p pid] [-c cmd...] 92 93 94Get delays, since system boot, for pid 10:: 95 96 # ./getdelays -p 10 97 (output similar to next case) 98 99Get sum of delays, since system boot, for all pids with tgid 5:: 100 101 # ./getdelays -t 5 102 103 104 CPU count real total virtual total delay total 105 7876 92005750 100000000 24001500 106 IO count delay total 107 0 0 108 SWAP count delay total 109 0 0 110 RECLAIM count delay total 111 0 0 112 113Get delays seen in executing a given simple command:: 114 115 # ./getdelays -c ls / 116 117 bin data1 data3 data5 dev home media opt root srv sys usr 118 boot data2 data4 data6 etc lib mnt proc sbin subdomain tmp var 119 120 121 CPU count real total virtual total delay total 122 6 4000250 4000000 0 123 IO count delay total 124 0 0 125 SWAP count delay total 126 0 0 127 RECLAIM count delay total 128 0 0 129