linux/Documentation/accounting/delay-accounting.txt
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   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
  14d) memory reclaim
  15
  16and makes these statistics available to userspace through
  17the taskstats interface.
  18
  19Such delays provide feedback for setting a task's cpu priority,
  20io priority and rss limit values appropriately. Long delays for
  21important tasks could be a trigger for raising its corresponding priority.
  22
  23The functionality, through its use of the taskstats interface, also provides
  24delay statistics aggregated for all tasks (or threads) belonging to a
  25thread group (corresponding to a traditional Unix process). This is a commonly
  26needed aggregation that is more efficiently done by the kernel.
  27
  28Userspace utilities, particularly resource management applications, can also
  29aggregate delay statistics into arbitrary groups. To enable this, delay
  30statistics of a task are available both during its lifetime as well as on its
  31exit, ensuring continuous and complete monitoring can be done.
  32
  33
  34Interface
  35---------
  36
  37Delay accounting uses the taskstats interface which is described
  38in detail in a separate document in this directory. Taskstats returns a
  39generic data structure to userspace corresponding to per-pid and per-tgid
  40statistics. The delay accounting functionality populates specific fields of
  41this structure. See
  42     include/linux/taskstats.h
  43for a description of the fields pertaining to delay accounting.
  44It will generally be in the form of counters returning the cumulative
  45delay seen for cpu, sync block I/O, swapin, memory reclaim etc.
  46
  47Taking the difference of two successive readings of a given
  48counter (say cpu_delay_total) for a task will give the delay
  49experienced by the task waiting for the corresponding resource
  50in that interval.
  51
  52When a task exits, records containing the per-task statistics
  53are sent to userspace without requiring a command. If it is the last exiting
  54task of a thread group, the per-tgid statistics are also sent. More details
  55are given in the taskstats interface description.
  56
  57The getdelays.c userspace utility in this directory allows simple commands to
  58be run and the corresponding delay statistics to be displayed. It also serves
  59as an example of using the taskstats interface.
  60
  61Usage
  62-----
  63
  64Compile the kernel with
  65        CONFIG_TASK_DELAY_ACCT=y
  66        CONFIG_TASKSTATS=y
  67
  68Delay accounting is enabled by default at boot up.
  69To disable, add
  70   nodelayacct
  71to the kernel boot options. The rest of the instructions
  72below assume this has not been done.
  73
  74After the system has booted up, use a utility
  75similar to  getdelays.c to access the delays
  76seen by a given task or a task group (tgid).
  77The utility also allows a given command to be
  78executed and the corresponding delays to be
  79seen.
  80
  81General format of the getdelays command
  82
  83getdelays [-t tgid] [-p pid] [-c cmd...]
  84
  85
  86Get delays, since system boot, for pid 10
  87# ./getdelays -p 10
  88(output similar to next case)
  89
  90Get sum of delays, since system boot, for all pids with tgid 5
  91# ./getdelays -t 5
  92
  93
  94CPU     count   real total      virtual total   delay total
  95        7876    92005750        100000000       24001500
  96IO      count   delay total
  97        0       0
  98SWAP    count   delay total
  99        0       0
 100RECLAIM count   delay total
 101        0       0
 102
 103Get delays seen in executing a given simple command
 104# ./getdelays -c ls /
 105
 106bin   data1  data3  data5  dev  home  media  opt   root  srv        sys  usr
 107boot  data2  data4  data6  etc  lib   mnt    proc  sbin  subdomain  tmp  var
 108
 109
 110CPU     count   real total      virtual total   delay total
 111        6       4000250         4000000         0
 112IO      count   delay total
 113        0       0
 114SWAP    count   delay total
 115        0       0
 116RECLAIM count   delay total
 117        0       0
 118
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