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 enabled by default at boot up. 73To disable, add:: 74 75 nodelayacct 76 77to the kernel boot options. The rest of the instructions 78below assume this has not been done. 79 80After the system has booted up, use a utility 81similar to getdelays.c to access the delays 82seen by a given task or a task group (tgid). 83The utility also allows a given command to be 84executed and the corresponding delays to be 85seen. 86 87General format of the getdelays command:: 88 89 getdelays [-t tgid] [-p pid] [-c cmd...] 90 91 92Get delays, since system boot, for pid 10:: 93 94 # ./getdelays -p 10 95 (output similar to next case) 96 97Get sum of delays, since system boot, for all pids with tgid 5:: 98 99 # ./getdelays -t 5 100 101 102 CPU count real total virtual total delay total 103 7876 92005750 100000000 24001500 104 IO count delay total 105 0 0 106 SWAP count delay total 107 0 0 108 RECLAIM count delay total 109 0 0 110 111Get delays seen in executing a given simple command:: 112 113 # ./getdelays -c ls / 114 115 bin data1 data3 data5 dev home media opt root srv sys usr 116 boot data2 data4 data6 etc lib mnt proc sbin subdomain tmp var 117 118 119 CPU count real total virtual total delay total 120 6 4000250 4000000 0 121 IO count delay total 122 0 0 123 SWAP count delay total 124 0 0 125 RECLAIM count delay total 126 0 0 127