94 lines
4.2 KiB
Markdown
94 lines
4.2 KiB
Markdown
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# The `ionice` command
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The `ionice` command is used to set or get process I/O scheduling class and priority.
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If no arguments are given , `ionice` will query the current I/O scheduling class and priority for that process.
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## Usage
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```
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ionice [options] -p <pid>
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```
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```
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ionice [options] -P <pgid>
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```
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```
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ionice [options] -u <uid>
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```
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```
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ionice [options] <command>
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```
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## A process can be of three scheduling classes:
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- ### Idle
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A program with idle I/O priority will only get disk time when `no other program has asked for disk I/O for a defined grace period`.
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The impact of idle processes on normal system actively should be `zero`.
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This scheduling class `doesn’t take priority` argument.
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Presently this scheduling class is permitted for an `ordinary user (since kernel 2.6.25)`.
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- ### Best Effort
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This is `effective` scheduling class for any process that has `not asked for a specific I/O priority`.
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This class `takes priority argument from 0-7`, with `lower` number being `higher priority`.
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Programs running at the same best effort priority are served in `round- robbin fashion`.
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Note that before kernel 2.6.26 a process that has not asked for an I/O priority formally uses “None” as scheduling class , but the io schedular will treat such processes as if it were in the best effort class.
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The priority within best effort class will be dynamically derived form the CPU nice level of the process : io_priority = ( cpu_nice + 20 ) / 5/
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for kernels after 2.6.26 with CFQ I/O schedular a process that has not asked for sn io priority inherits CPU scheduling class.
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`The I/O priority is derived from the CPU nice level of the process` ( smr sd before kernel 2.6.26 ).
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- ### Real Time
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The real time schedular class is `given first access to disk, regardless of what else is going on in the system`.
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Thus the real time class needs to be used with some care, as it cans tarve other processes .
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As with the best effort class, `8 priority levels are defined denoting how big a time slice a given process will receive on each scheduling window`.
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This scheduling class is `not permitted for an ordinary user(non-root)`.
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## Options
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| Options | Description |
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|---|---|
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| -c, --class <class> | name or number of scheduling class, 0: none, 1: realtime, 2: best-effort, 3: idle|
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| -n, --classdata <num> | priority (0..7) in the specified scheduling class,only for the realtime and best-effort classes|
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| -p, --pid <pid>... | act on these already running processes|
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| -P, --pgid <pgrp>... | act on already running processes in these groups|
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| -t, --ignore | ignore failures|
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| -u, --uid <uid>... | act on already running processes owned by these users|
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| -h, --help | display this help|
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| -V, --version | display version|
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For more details see ionice(1).
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## Examples
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| Command | O/P |Explanation|
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|---|---|---|
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|`$ ionice` |*none: prio 4*|Running alone `ionice` will give the class and priority of current process |
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|`$ ionice -p 101`|*none : prio 4*|Give the details(*class : priority*) of the process specified by given process id|
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|`$ ionice -p 2` |*none: prio 4*| Check the class and priority of process with pid 2 it is none and 4 resp.|
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|`$ ionice -c2 -n0 -p2`|2 ( best-effort ) priority 0 process 2 | Now lets set process(pid) 2 as a best-effort program with highest priority|
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|$ `ionice` -p 2|best-effort : prio 0| Now if I check details of Process 2 you can see the updated one|
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|$ `ionice` /bin/ls||get priority and class info of bin/ls |
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|$ `ionice` -n4 -p2||set priority 4 of process with pid 2 |
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|$ `ionice` -p 2| best-effort: prio 4| Now observe the difference between the command ran above and this one we have changed priority from 0 to 4|
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|$ `ionice` -c0 -n4 -p2|ionice: ignoring given class data for none class|(Note that before kernel 2.6.26 a process that has not asked for an I/O priority formally uses “None” as scheduling class , |
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|||but the io schedular will treat such processes as if it were in the best effort class. )|
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|||-t option : ignore failure|
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|$ `ionice` -c0 -n4 -p2 -t| | For ignoring the warning shown above we can use -t option so it will ignore failure |
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## Conclusion
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Thus we have successfully learnt about `ionice` command.
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