Based on kernel version 2.6.26. Page generated on 2008-07-16 21:12 EST.
1 Delay accounting 2 ---------------- 3 4 Tasks encounter delays in execution when they wait 5 for some kernel resource to become available e.g. a 6 runnable task may wait for a free CPU to run on. 7 8 The per-task delay accounting functionality measures 9 the delays experienced by a task while 10 11 a) waiting for a CPU (while being runnable) 12 b) completion of synchronous block I/O initiated by the task 13 c) swapping in pages 14 15 and makes these statistics available to userspace through 16 the taskstats interface. 17 18 Such delays provide feedback for setting a task's cpu priority, 19 io priority and rss limit values appropriately. Long delays for 20 important tasks could be a trigger for raising its corresponding priority. 21 22 The functionality, through its use of the taskstats interface, also provides 23 delay statistics aggregated for all tasks (or threads) belonging to a 24 thread group (corresponding to a traditional Unix process). This is a commonly 25 needed aggregation that is more efficiently done by the kernel. 26 27 Userspace utilities, particularly resource management applications, can also 28 aggregate delay statistics into arbitrary groups. To enable this, delay 29 statistics of a task are available both during its lifetime as well as on its 30 exit, ensuring continuous and complete monitoring can be done. 31 32 33 Interface 34 --------- 35 36 Delay accounting uses the taskstats interface which is described 37 in detail in a separate document in this directory. Taskstats returns a 38 generic data structure to userspace corresponding to per-pid and per-tgid 39 statistics. The delay accounting functionality populates specific fields of 40 this structure. See 41 include/linux/taskstats.h 42 for a description of the fields pertaining to delay accounting. 43 It will generally be in the form of counters returning the cumulative 44 delay seen for cpu, sync block I/O, swapin etc. 45 46 Taking the difference of two successive readings of a given 47 counter (say cpu_delay_total) for a task will give the delay 48 experienced by the task waiting for the corresponding resource 49 in that interval. 50 51 When a task exits, records containing the per-task statistics 52 are sent to userspace without requiring a command. If it is the last exiting 53 task of a thread group, the per-tgid statistics are also sent. More details 54 are given in the taskstats interface description. 55 56 The getdelays.c userspace utility in this directory allows simple commands to 57 be run and the corresponding delay statistics to be displayed. It also serves 58 as an example of using the taskstats interface. 59 60 Usage 61 ----- 62 63 Compile the kernel with 64 CONFIG_TASK_DELAY_ACCT=y 65 CONFIG_TASKSTATS=y 66 67 Delay accounting is enabled by default at boot up. 68 To disable, add 69 nodelayacct 70 to the kernel boot options. The rest of the instructions 71 below assume this has not been done. 72 73 After the system has booted up, use a utility 74 similar to getdelays.c to access the delays 75 seen by a given task or a task group (tgid). 76 The utility also allows a given command to be 77 executed and the corresponding delays to be 78 seen. 79 80 General format of the getdelays command 81 82 getdelays [-t tgid] [-p pid] [-c cmd...] 83 84 85 Get delays, since system boot, for pid 10 86 # ./getdelays -p 10 87 (output similar to next case) 88 89 Get sum of delays, since system boot, for all pids with tgid 5 90 # ./getdelays -t 5 91 92 93 CPU count real total virtual total delay total 94 7876 92005750 100000000 24001500 95 IO count delay total 96 0 0 97 MEM count delay total 98 0 0 99 100 Get delays seen in executing a given simple command 101 # ./getdelays -c ls / 102 103 bin data1 data3 data5 dev home media opt root srv sys usr 104 boot data2 data4 data6 etc lib mnt proc sbin subdomain tmp var 105 106 107 CPU count real total virtual total delay total 108 6 4000250 4000000 0 109 IO count delay total 110 0 0 111 MEM count delay total 112 0 0