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diff --git a/Documentation/accounting/delay-accounting.rst b/Documentation/accounting/delay-accounting.rst new file mode 100644 index 000000000000..7cc7f5852da0 --- /dev/null +++ b/Documentation/accounting/delay-accounting.rst @@ -0,0 +1,126 @@ +================ +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 |