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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 this 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