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docs/Linux-Commands/.recycle/083-the-ionice-command.md

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