sys_ioprio_set [fs/ioprio.c]
Set the I/O scheduling class and priority of one or more processes. Arguments
eax | 289
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.`
ecx | This arguments depends on the value of ebx. See above.
edx | A bit mask that specifies both the scheduling class and the priority to be assigned to the target process(es). The ioprio value is assembled in the following way: `(class shl IOPRIO_CLASS_SHIFT) or data` Where class is the scheduling class and data is the priority. (See the Remarks section for more information on scheduling classes and priorities.) I/O priorities are supported for reads and for synchronous (O_DIRECT, O_SYNC) writes. I/O priorities are not supported for asynchronous writes because they are issued outside the context of the program dirtying the memory, and thus program-specific priorities do not apply.
Return values If the system call succeeds the return value is 0. If the system call fails the return value is one of the following errno values:
`-EPERM` | `The calling process does not have the privilege needed to assign this ioprio to the specified process(es). See the Remarks section for more information on required privileges for sys_ioprio_set.`
`-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. Required permissions to set I/O priorities: Permission to change a process's priority is granted or denied based on two assertions: Process ownership: An unprivileged process may only set the I/O priority of a process whose real UID matches the real or effective UID of the calling process. A process which has the CAP_SYS_NICE capability can change the priority of any process. What is the desired priority: Attempts to set very high priorities (IOPRIO_CLASS_RT) or very low ones (IOPRIO_CLASS_IDLE) require the CAP_SYS_ADMIN capability. A call to sys_ioprio_set must follow both rules, or the call will fail with the error -EPERM. Compatibility Available since 2.6.13