| 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 |