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