rt_dptbl(4) rt_dptbl(4)
NAME
rtdptbl - real-time dispatcher parameter table
DESCRIPTION
The process scheduler (or dispatcher) is the portion of the kernel
that controls allocation of the CPU to processes. The scheduler sup-
ports the notion of scheduling classes where each class defines a
scheduling policy, used to schedule processes within that class. Asso-
ciated with each scheduling class is a set of priority queues on which
ready to run processes are linked. These priority queues are mapped by
the system configuration into a set of global scheduling priorities
which are available to processes within the class. (The dispatcher
always selects for execution the process with the highest global
scheduling priority that is eligible to run on a given CPU. See PRO-
CESSOR AFFINITY AND BINDING CONSIDERATIONS below.) The priority queues
associated with a given class are viewed by that class as a contiguous
set of priority levels numbered from 0 (lowest priority) to n (highest
priority - a configuration dependent value). The set of global
scheduling priorities that the queues for a given class are mapped
into might not start at zero and might not be contiguous (depending on
the configuration).
The real-time class maintains an in-core table, with an entry for each
priority level, giving the properties of that level. This table is
called the real-time dispatcher parameter table (rtdptbl). The
rtdptbl consists of an array of parameter structures (struct
rtdpent), one for each of the n priority levels. The properties of a
given priority level i are specified by the ith parameter structure in
this array (rtdptbl[i]).
A parameter structure consists of the following members. These are
also described in the /usr/include/sys/rt.h header file.
rtglobpri The global scheduling priority associated with this
priority level. The mapping between real-time priority
levels and global scheduling priorities is determined
at boot time by the system configuration. The
rtglobpri values cannot be changed with dispadmin(1M).
rtquantum The length of the time quantum allocated to processes
at this level in ticks (HZ). The time quantum value is
only a default or starting value for processes at a
particular level as the time quantum of a real-time
process can be changed by the user with the priocntl(1)
command or the priocntl(2) system call.
An administrator can affect the behavior of the real-time portion of
the scheduler by reconfiguring the rtdptbl. There are two methods
available for doing this.
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KERNEL LINK TIME CONFIGURATION
The rtdptbl can be reconfigured at kernel link time time by specify-
ing the desired values in the /etc/conf/pack.d/rt/space.c file and
reconfiguring the system using the auto-configuration procedure; see
idbuild(1M). This is the only method that can be used to change the
number of real-time priority levels or the set of global scheduling
priorities used by the real-time class.
DISPADMIN CONFIGURATION FILE
The rtquantum values in the rtdptbl can be examined and modified on
a running system using the dispadmin(1M) command. Invoking dispadmin
for the real-time class allows the administrator to retrieve the
current rtdptbl configuration from the kernel's in-core table, or
overwrite the in-core table with values from a configuration file. The
configuration file used for input to dispadmin must conform to the
specific format described below.
Blank lines are ignored and any part of a line to the right of a #
symbol is treated as a comment. The first non-blank, non-comment line
must indicate the resolution to be used for interpreting the time
quantum values. The resolution is specified as
RES=res
where res is a positive integer between 1 and 1,000,000,000 inclusive
and the resolution used is the reciprocal of res in seconds. (For
example, RES=1000 specifies millisecond resolution.) Although very
fine (nanosecond) resolution may be specified, the time quantum
lengths are rounded up to the next integral multiple of the system
clock's resolution.
The remaining lines in the file are used to specify the rtquantum
values for each of the real-time priority levels. The first line
specifies the quantum for real-time level 0, the second line specifies
the quantum for real-time level 1, etc. There must be exactly one line
for each configured real-time priority level. Each rtquantum entry
must be either a positive integer specifying the desired time quantum
(in the resolution given by res), or the symbol RTTQINF indicating an
infinite time quantum for that level.
EXAMPLE
The following excerpt from a dispadmin configuration file illustrates
the format. Note that for each line specifying a time quantum there is
a comment indicating the corresponding priority level. These level
numbers indicate priority within the real-time class, and the mapping
between these real-time priorities and the corresponding global
scheduling priorities is determined by the configuration specified in
the rt/space.c file. The level numbers are strictly for the conveni-
ence of the administrator reading the file and, as with any comment,
they are ignored by dispadmin on input. dispadmin assumes that the
lines in the file are ordered by consecutive, increasing priority
level (from 0 to the maximum configured real-time priority). The level
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rt_dptbl(4) rt_dptbl(4)
numbers in the comments should normally agree with this ordering; if
for some reason they don't, however, dispadmin is unaffected.
# Real Time Dispatcher Configuration
RES=1000
# TIME QUANTUM PRIORITY
# (rtquantum) LEVEL
1000 # 0
1000 # 1
1000 # 2
1000 # 3
1000 # 4
1000 # 5
1000 # 6
1000 # 7
1000 # 8
1000 # 9
800 # 10
800 # 11
. . .
. . .
. . .
100 # 58
100 # 59
PROCESSOR AFFINITY AND BINDING CONSIDERATIONS
In order to increase the cache hit rates of a multiprocessor, a pro-
cess should generally execute on a particular processor. This affinity
for a particular processor will reduce the required memory bandwidth
for the process (and for the system as a whole). If a process has not
run "recently" then it probably only has a small cache state in any
given processor - and thus should loose its affinity for any particu-
lar processor.
This affinity mechanism can be enabled or disabled on a per-process
basis by the super user [see mpcntl(1M), mpcntl(3X)]. Since real time
tasks are time-critical, the affinity mechanism defaults to off - so
that the RT task will run on the first available processor (assuming
that there is no other process with a higher priority).
A process can also be bound to a set of processors. If a processor has
been so bound, then it will only run on one of the processors of that
set. If there is a idle processor that is not in the processes binding
set - then the process will still wait until there is a processor in
its set that is not running a higher priority process.
Process bindings can be established by the process [see mpcntl(3X)],
or can be temporarily established by the kernel (if the kernel is exe-
cuting driver code that must run on a particular CPU).
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FILES
/usr/include/sys/rt.h
/etc/conf/pack.d/rt/space.c
SEE ALSO
priocntl(1), dispadmin(1M), idbuild(1M), priocntl(2), mpcntl(3X).
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