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