IOPRIO_SET(2)                                       Linux Programmer's Manual                                      IOPRIO_SET(2)



NAME
       ioprio_get, ioprio_set - get/set I/O scheduling class and priority

SYNOPSIS
       int ioprio_get(int which, int who);
       int ioprio_set(int which, int who, int ioprio);

DESCRIPTION
       The  ioprio_get()  and ioprio_set() system calls respectively get and set the I/O scheduling class and priority of one or
       more processes.

       The which and who arguments identify the process(es) on which the system calls operate.  The  which  argument  determines
       how who is interpreted, and has one of the following values:

       IOPRIO_WHO_PROCESS
              who is a process ID identifying a single process.

       IOPRIO_WHO_PGRP
              who is a process group ID identifying all the members of a process group.

       IOPRIO_WHO_USER
              who is a user ID identifying all of the processes that have a matching real UID.

       If  which is specified as IOPRIO_WHO_PGRP or IOPRIO_WHO_USER when calling ioprio_get(), and more than one process matches
       who, 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 prior-
       ity level (a lower priority number means a higher priority level).

       The  ioprio  argument given to ioprio_set() is a bit mask that specifies both the scheduling class and the priority to be
       assigned to the target process(es).  The following macros are used for assembling and dissecting ioprio values:

       IOPRIO_PRIO_VALUE(class, data)
              Given a scheduling class and priority (data), this macro combines the two values to produce an ioprio value, which
              is returned as the result of the macro.

       IOPRIO_PRIO_CLASS(mask)
              Given  mask  (an  ioprio  value),  this  macro  returns  its  I/O  class  component,  that  is,  one of the values
              IOPRIO_CLASS_RT, IOPRIO_CLASS_BE, or IOPRIO_CLASS_IDLE.

       IOPRIO_PRIO_DATA(mask)
              Given mask (an ioprio value), this macro returns its priority (data) component.

       See the NOTES 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 VALUE
       On success, ioprio_get() returns the ioprio value of the process with highest I/O priority of any of the  processes  that
       match the criteria specified in which and who.  On error, -1 is returned, and errno is set to indicate the error.

       On success, ioprio_set() returns 0.  On error, -1 is returned, and errno is set to indicate the error.

ERRORS
       EINVAL Invalid value for which or ioprio.  Refer to the NOTES section for available scheduler classes and priority levels
              for ioprio.

       EPERM  The calling process does not have the privilege needed to assign this ioprio to the  specified  process(es).   See
              the NOTES section for more information on required privileges for ioprio_set().

       ESRCH  No process(es) could be found that matched the specification in which and who.

VERSIONS
       These system calls have been available on Linux since kernel 2.6.13.

CONFORMING TO
       These system calls are Linux-specific.

NOTES
       Glibc does not provide wrapper for these system calls; call them using syscall(2).

       These  system  calls only have 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
       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 (1)
              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  map-
              pable to performance, by passing in a desired data rate instead.

       IOPRIO_CLASS_BE (2)
              This is the best-effort scheduling class, which is the default for any process that hasn't set a specific I/O pri-
              ority.  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 getpriority(2)).  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 (3)
              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 to a 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) require the CAP_SYS_ADMIN capability.  Kernel  versions  up
              to 2.6.24 also required CAP_SYS_ADMIN to set a very low priority (IOPRIO_CLASS_IDLE), but since Linux 2.6.25, this
              is no longer required.

       A call to ioprio_set() must follow both rules, or the call will fail with the error EPERM.

BUGS
       Glibc does not yet provide a suitable header file defining the function prototypes and macros  described  on  this  page.
       Suitable definitions can be found in linux/ioprio.h.

SEE ALSO
       getpriority(2), open(2), capabilities(7)

       Documentation/block/ioprio.txt in the kernel source tree.

COLOPHON
       This  page  is  part of release 3.25 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2008-07-09                                              IOPRIO_SET(2)