sys_ioprio_get [fs/ioprio.c]
Get the I/O scheduling class and priority of one or more processes. Arguments
eax | 290
ebx | This argument determines how the argument in ecx is interpreted, and has one of the following values:
`IOPRIO_WHO_PROCESS` | `ecx contains a process ID identifying a single process.`
`IOPRIO_WHO_PGRP` | `ecx contains a process group ID identifying all the members of a process group.`
`IOPRIO_WHO_USER` | `ecx contains a user ID identifying all of the processes that have a matching real UID.`
If ebx is specified as IOPRIO_WHO_PGRP or IOPRIO_WHO_USER when calling sys_ioprio_get, and more than one process matches ecx, then the returned priority will be the highest one found among all of the matching processes. One priority is said to be higher than another one if it belongs to a higher priority class (IOPRIO_CLASS_RT is the highest priority class; IOPRIO_CLASS_IDLE is the lowest) or if it belongs to the same priority class as the other process but has a higher priority level (a lower priority number means a higher priority level).
ecx | This arguments depends on the value of ebx. See above.
Return values If the system call succeeds the return value is ioprio value of the process with highest I/O priority of any of the processes that match the criteria specified in ebx and ecx. ioprio value can be dissected to its class and data components in the following way: class component (IOPRIO_CLASS_RT, IOPRIO_CLASS_BE, or IOPRIO_CLASS_IDLE.): `ioprio shr IOPRIO_CLASS_SHIFT` priority (data) component: `ioprio and IOPRIO_PRIO_MASK` If the system call fails the return value is one of the following errno values:
`-ESRCH` | `No process(es) could be found that matched the specification in ebx and ecx.`
`-EINVAL` | `Invalid value for ebx.`
Remarks This system call only has an effect when used in conjunction with an I/O scheduler that supports I/O priorities. As at kernel 2.6.17 the only such scheduler is the Completely Fair Queuing (CFQ) I/O scheduler. Selecting an I/O Scheduler: I/O Schedulers are selected on a per-device basis via the special file /sys/block/<device>/queue/scheduler. One can view the current I/O scheduler via the /sys file system. For example, the following command displays a list of all schedulers currently loaded in the kernel: `$ cat /sys/block/hda/queue/scheduler noop anticipatory deadline [cfq]` The scheduler surrounded by brackets is the one actually in use for the device (hda in the example). Setting another scheduler is done by writing the name of the new scheduler to this file. For example, the following command will set the set the scheduler for the hda device to cfq: `$ su Password: # echo cfq > /sys/block/hda/queue/scheduler` The Completely Fair Queuing (CFQ) I/O Scheduler: Since v3 (aka CFQ Time Sliced) CFQ implements I/O nice levels similar to those of CPU scheduling. These nice levels are grouped in three scheduling classes each one containing one or more priority levels:
`IOPRIO_CLASS_RT` | `This is the real-time I/O class. This scheduling class is given higher priority than any other class: processes from this class are given first access to the disk every time. Thus this I/O class needs to be used with some care: one I/O real-time process can starve the entire system. Within the real-time class, there are 8 levels of class data (priority) that determine exactly how much time this process needs the disk for on each service. The highest real-time priority level is 0; the lowest is 7. In the future this might change to be more directly mappable to performance, by passing in a desired data rate instead.`
`IOPRIO_CLASS_BE` | `This is the best-effort scheduling class, which is the default for any process that hasn't set a specific I/O priority. The class data (priority) determines how much I/O bandwidth the process will get. Best-effort priority levels are analogous to CPU nice values (see sys_getpriority). The priority level determines a priority relative to other processes in the best-effort scheduling class. Priority levels range from 0 (highest) to 7 (lowest).`
`IOPRIO_CLASS_IDLE` | `This is the idle scheduling class. Processes running at this level only get I/O time when no one else needs the disk. The idle class has no class data. Attention is required when assigning this priority class process, since it may become starved if higher priority processes are constantly accessing the disk.`
Refer to Documentation/block/ioprio.txt for more information on the CFQ I/O Scheduler and an example program. Compatibility Available since 2.6.13.