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+================
+Delay accounting
+================
+
+Tasks encounter delays in execution when they wait
+for some kernel resource to become available e.g. a
+runnable task may wait for a free CPU to run on.
+
+The per-task delay accounting functionality measures
+the delays experienced by a task while
+
+a) waiting for a CPU (while being runnable)
+b) completion of synchronous block I/O initiated by the task
+c) swapping in pages
+d) memory reclaim
+
+and makes these statistics available to userspace through
+the taskstats interface.
+
+Such delays provide feedback for setting a task's cpu priority,
+io priority and rss limit values appropriately. Long delays for
+important tasks could be a trigger for raising its corresponding priority.
+
+The functionality, through its use of the taskstats interface, also provides
+delay statistics aggregated for all tasks (or threads) belonging to a
+thread group (corresponding to a traditional Unix process). This is a commonly
+needed aggregation that is more efficiently done by the kernel.
+
+Userspace utilities, particularly resource management applications, can also
+aggregate delay statistics into arbitrary groups. To enable this, delay
+statistics of a task are available both during its lifetime as well as on its
+exit, ensuring continuous and complete monitoring can be done.
+
+
+Interface
+---------
+
+Delay accounting uses the taskstats interface which is described
+in detail in a separate document in this directory. Taskstats returns a
+generic data structure to userspace corresponding to per-pid and per-tgid
+statistics. The delay accounting functionality populates specific fields of
+this structure. See
+
+     include/linux/taskstats.h
+
+for a description of the fields pertaining to delay accounting.
+It will generally be in the form of counters returning the cumulative
+delay seen for cpu, sync block I/O, swapin, memory reclaim etc.
+
+Taking the difference of two successive readings of a given
+counter (say cpu_delay_total) for a task will give the delay
+experienced by the task waiting for the corresponding resource
+in that interval.
+
+When a task exits, records containing the per-task statistics
+are sent to userspace without requiring a command. If it is the last exiting
+task of a thread group, the per-tgid statistics are also sent. More details
+are given in the taskstats interface description.
+
+The getdelays.c userspace utility in tools/accounting directory allows simple
+commands to be run and the corresponding delay statistics to be displayed. It
+also serves as an example of using the taskstats interface.
+
+Usage
+-----
+
+Compile the kernel with::
+
+        CONFIG_TASK_DELAY_ACCT=y
+        CONFIG_TASKSTATS=y
+
+Delay accounting is enabled by default at boot up.
+To disable, add::
+
+   nodelayacct
+
+to the kernel boot options. The rest of the instructions
+below assume this has not been done.
+
+After the system has booted up, use a utility
+similar to  getdelays.c to access the delays
+seen by a given task or a task group (tgid).
+The utility also allows a given command to be
+executed and the corresponding delays to be
+seen.
+
+General format of the getdelays command::
+
+        getdelays [-t tgid] [-p pid] [-c cmd...]
+
+
+Get delays, since system boot, for pid 10::
+
+        # ./getdelays -p 10
+        (output similar to next case)
+
+Get sum of delays, since system boot, for all pids with tgid 5::
+
+        # ./getdelays -t 5
+
+
+        CPU     count   real total      virtual total   delay total
+                7876    92005750        100000000       24001500
+        IO      count   delay total
+                0       0
+        SWAP    count   delay total
+                0       0
+        RECLAIM count   delay total
+                0       0
+
+Get delays seen in executing a given simple command::
+
+  # ./getdelays -c ls /
+
+  bin   data1  data3  data5  dev  home  media  opt   root  srv        sys  usr
+  boot  data2  data4  data6  etc  lib   mnt    proc  sbin  subdomain  tmp  var
+
+
+  CPU   count   real total      virtual total   delay total
+        6       4000250         4000000         0
+  IO    count   delay total
+        0       0
+  SWAP  count   delay total
+        0       0
+  RECLAIM       count   delay total
+        0       0