GNU/Linux man pages
Livre :
Expressions régulières,
Syntaxe et mise en oeuvre :
GNU/Linux |
CentOS 5.4 |
|
 |
sched_getscheduler(2) |
 |
sched_setscheduler, sched_getscheduler − set and get scheduling algorithm/parameters
#include <sched.h>
int
sched_setscheduler(pid_t pid, int
policy,
const struct sched_param *param);
int sched_getscheduler(pid_t pid);
struct
sched_param {
...
int sched_priority;
...
};
sched_setscheduler() sets both the scheduling policy and the associated parameters for the process identified by pid. If pid equals zero, the scheduler of the calling process will be set. The interpretation of the parameter param depends on the selected policy. Currently, the following three scheduling policies are supported under Linux: SCHED_FIFO, SCHED_RR, SCHED_OTHER, and SCHED_BATCH; their respective semantics are described below.
sched_getscheduler() queries the scheduling policy currently applied to the process identified by pid. If pid equals zero, the policy of the calling process will be retrieved.
Scheduling
Policies
The scheduler is the kernel part that decides which runnable
process will be executed by the CPU next. The Linux
scheduler offers three different scheduling policies, one
for normal processes and two for real-time applications. A
static priority value sched_priority is assigned to
each process and this value can be changed only via system
calls. Conceptually, the scheduler maintains a list of
runnable processes for each possible sched_priority
value, and sched_priority can have a value in the
range 0 to 99. In order to determine the process that runs
next, the Linux scheduler looks for the non-empty list with
the highest static priority and takes the process at the
head of this list. The scheduling policy determines for each
process, where it will be inserted into the list of
processes with equal static priority and how it will move
inside this list.
SCHED_OTHER is the default universal time-sharing scheduler policy used by most processes. SCHED_BATCH is intended for "batch" style execution of processes. SCHED_FIFO and SCHED_RR are intended for special time-critical applications that need precise control over the way in which runnable processes are selected for execution.
Processes scheduled with SCHED_OTHER or SCHED_BATCH must be assigned the static priority 0. Processes scheduled under SCHED_FIFO or SCHED_RR can have a static priority in the range 1 to 99. The system calls sched_get_priority_min() and sched_get_priority_max() can be used to find out the valid priority range for a scheduling policy in a portable way on all POSIX.1-2001 conforming systems.
All scheduling is preemptive: If a process with a higher static priority gets ready to run, the current process will be preempted and returned into its wait list. The scheduling policy only determines the ordering within the list of runnable processes with equal static priority.
SCHED_FIFO:
First In-First Out scheduling
SCHED_FIFO can only be used with static priorities
higher than 0, which means that when a SCHED_FIFO
processes becomes runnable, it will always immediately
preempt any currently running SCHED_OTHER or
SCHED_BATCH process. SCHED_FIFO is a simple
scheduling algorithm without time slicing. For processes
scheduled under the SCHED_FIFO policy, the following
rules are applied: A SCHED_FIFO process that has been
preempted by another process of higher priority will stay at
the head of the list for its priority and will resume
execution as soon as all processes of higher priority are
blocked again. When a SCHED_FIFO process becomes
runnable, it will be inserted at the end of the list for its
priority. A call to sched_setscheduler() or
sched_setparam() will put the SCHED_FIFO (or
SCHED_RR) process identified by pid at the
start of the list if it was runnable. As a consequence, it
may preempt the currently running process if it has the same
priority. (POSIX.1-2001 specifies that the process should go
to the end of the list.) A process calling
sched_yield() will be put at the end of the list. No
other events will move a process scheduled under the
SCHED_FIFO policy in the wait list of runnable
processes with equal static priority. A SCHED_FIFO
process runs until either it is blocked by an I/O request,
it is preempted by a higher priority process, or it calls
sched_yield().
SCHED_RR:
Round Robin scheduling
SCHED_RR is a simple enhancement of SCHED_FIFO.
Everything described above for SCHED_FIFO also
applies to SCHED_RR, except that each process is only
allowed to run for a maximum time quantum. If a
SCHED_RR process has been running for a time period
equal to or longer than the time quantum, it will be put at
the end of the list for its priority. A SCHED_RR
process that has been preempted by a higher priority process
and subsequently resumes execution as a running process will
complete the unexpired portion of its round robin time
quantum. The length of the time quantum can be retrieved
using sched_rr_get_interval(2).
SCHED_OTHER:
Default Linux time-sharing scheduling
SCHED_OTHER can only be used at static priority 0.
SCHED_OTHER is the standard Linux time-sharing
scheduler that is intended for all processes that do not
require special static priority real-time mechanisms. The
process to run is chosen from the static priority 0 list
based on a dynamic priority that is determined only inside
this list. The dynamic priority is based on the nice level
(set by nice(2) or setpriority(2)) and
increased for each time quantum the process is ready to run,
but denied to run by the scheduler. This ensures fair
progress among all SCHED_OTHER processes.
SCHED_BATCH:
Scheduling batch processes
(Since Linux 2.6.16.) SCHED_BATCH can only be used at
static priority 0. This policy is similar to
SCHED_OTHER, except that this policy will cause the
scheduler to always assume that the process is
CPU-intensive. Consequently, the scheduler will apply a
small scheduling penalty so that this process is mildly
disfavoured in scheduling decisions. This policy is useful
for workloads that are non-interactive, but do not want to
lower their nice value, and for workloads that want a
deterministic scheduling policy without interactivity
causing extra preemptions (between the workload’s
tasks).
Privileges
and resource limits
In Linux kernels before 2.6.12, only privileged
(CAP_SYS_NICE) processes can set a non-zero static
priority. The only change that an unprivileged process can
make is to set the SCHED_OTHER policy, and this can
only be done if the effective user ID of the caller of
sched_setscheduler() matches the real or effective
user ID of the target process (i.e., the process specified
by pid) whose policy is being changed.
Since Linux 2.6.12, the RLIMIT_RTPRIO resource limit defines a ceiling on an unprivileged process’s priority for the SCHED_RR and SCHED_FIFO policies. If an unprivileged process has a non-zero RLIMIT_RTPRIO soft limit, then it can change its scheduling policy and priority, subject to the restriction that the priority cannot be set to a value higher than the RLIMIT_RTPRIO soft limit. If the RLIMIT_RTPRIO soft limit is 0, then the only permitted change is to lower the priority. Subject to the same rules, another unprivileged process can also make these changes, as long as the effective user ID of the process making the change matches the real or effective user ID of the target process. See getrlimit(2) for further information on RLIMIT_RTPRIO. Privileged (CAP_SYS_NICE) processes ignore this limit; as with older older kernels, they can make arbitrary changes to scheduling policy and priority.
Response
time
A blocked high priority process waiting for the I/O has a
certain response time before it is scheduled again. The
device driver writer can greatly reduce this response time
by using a "slow interrupt" interrupt handler.
Miscellaneous
Child processes inherit the scheduling algorithm and
parameters across a fork(). The scheduling algorithm
and parameters are preserved across execve(2).
Memory locking is usually needed for real-time processes to avoid paging delays, this can be done with mlock() or mlockall().
As a non-blocking end-less loop in a process scheduled under SCHED_FIFO or SCHED_RR will block all processes with lower priority forever, a software developer should always keep available on the console a shell scheduled under a higher static priority than the tested application. This will allow an emergency kill of tested real-time applications that do not block or terminate as expected.
POSIX systems on which sched_setscheduler() and sched_getscheduler() are available define _POSIX_PRIORITY_SCHEDULING in <unistd.h>.
On success, sched_setscheduler() returns zero. On success, sched_getscheduler() returns the policy for the process (a non-negative integer). On error, −1 is returned, and errno is set appropriately.
|
EINVAL |
The scheduling policy is not one of the recognized policies, or the parameter param does not make sense for the policy. | ||
|
EPERM |
The calling process does not have appropriate privileges. | ||
|
ESRCH |
The process whose ID is pid could not be found. |
POSIX.1-2001. The SCHED_BATCH policy is Linux specific.
Standard Linux is a general-purpose operating system and can handle background processes, interactive applications, and soft real-time applications (applications that need to usually meet timing deadlines). This man page is directed at these kinds of applications.
Standard Linux is not designed to support hard real-time applications, that is, applications in which deadlines (often much shorter than a second) must be guaranteed or the system will fail catastrophically. Like all general-purpose operating systems, Linux is designed to maximize average case performance instead of worst case performance. Linux’s worst case performance for interrupt handling is much poorer than its average case, its various kernel locks (such as for SMP) produce long maximum wait times, and many of its performance improvement techniques decrease average time by increasing worst-case time. For most situations, that’s what you want, but if you truly are developing a hard real-time application, consider using hard real-time extensions to Linux such as RTLinux (http://www.rtlinux.org) or RTAI (http://www.rtai.org) or use a different operating system designed specifically for hard real-time applications.
getpriority(2), mlock(2), mlockall(2), munlock(2), munlockall(2), nice(2), sched_get_priority_max(2), sched_get_priority_min(2), sched_getaffinity(2), sched_getparam(2), sched_rr_get_interval(2), sched_setaffinity(2), sched_setparam(2), sched_yield(2), setpriority(2), capabilities(7)
Programming for the real world − POSIX.4 by Bill O. Gallmeister, O’Reilly & Associates, Inc., ISBN 1-56592-074-0
|
sched_getscheduler(2) | |