Novell LINUX ENTERPRISE REAL TIME 10 SP1, SUSE LINUX ENTERPRISE REAL TIME 10 SP1 Quick Start Manual
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SUSE Linux Enterprise Real Time is an add-on to SUSE® Linux Enterprise that allows you to run tasks which require
deterministic real-time processing, in a SUSE Linux Enterprise environment. SUSE Linux Enterprise Real Time
meets this requirement by offering several different options for CPU and IO scheduling, CPU shielding and setting
CPU afnities to processes.
Installing SUSE Linux Enterprise Real
Time
There are two ways to set up SUSE Linux Enterprise Real
Time:
• Install it as an add-on product when installing the SUSE
Linux Enterprise Server 10 SP1.
• Install it on top of an already installed SUSE Linux Enterprise Server 10 SP1.
SUSE Linux Enterprise Real Time always needs a SUSE Linux
Enterprise Server SP1 base, it cannot be installed in standalone mode. Refer to the SUSE Linux Enterprise Server
Installation and Administration manual, Section “Installing
Add-On Products” at http://www.novell.com/
documentation/sles10/sles_admin/index.html
?page=/documentation/sles10/sles_admin/
data/sec_yast2_sw.html to learn more about in-
stalling add-on products.
The following sections provide a brief introduction to the
tools and possibilities of SUSE Linux Enterprise Real Time.
Using CPU Sets
In some circumstances, it is benecial to be able to run
specic tasks only on dened CPUs. For this reason, the
linux kernel provides a feature called cpuset. Cpusets provide the means to do a so called “soft partitioning” of the
system. Dedicated CPUs, together with some predened
memory, work on a number of tasks.
All systems have at least one cpuset that is called /. To retrieve the cpuset of a specic task with a certain process id
pid, use the command cat /proc/pid/cpuset. To
add, remove, or manage cpusets, a special le system with
le system type cpuset is available. Before you can use
this le system type, mount it to /dev/cpuset with the
following commands:
mkdir /dev/cpuset
mount -t cpuset none /dev/cpuset
Every cpuset has the following entries:
cpus
A list of CPUs available for the current cpuset. Ranges
of CPUs are displayed with a dash between the rst and
the last CPU, else CPUs are represented by a comma
separated list of CPU numbers.
mems
A list of memory nodes available to the current cpuset.
memory_migrate
This ag determines if memory pages should be moved
to the new conguration, in case the memory conguration of the cpuset changes.
cpu_exclusive
Denes if this cpuset becomes a scheduling domain, that
shares properties and policies.
SUSE Linux Enterprise Real Time 10 SP1 Quick Start
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mem_exclusive
Determines if userspace tasks in this cpuset can only get
their memory from the memory assigned to this cpuset.
tasks
Contains the process ids of all tasks running in this
cpuset.
notify_on_release
If this is set to 1, /sbin/cpuset_release_agent
will be called when the last process leaves this cpuset.
Note, that it is up to the administrator to create a script
or binary that matches the local needs.
memory_pressure
Provides the means to determine how often a cpuset is
running short of memory. Only calculated if memo-
ry_pressure_enabled is enabled in the top cpuset.
memory_spread_page and memory_spread_slab
Determines if le system buffers and I/O buffers are
uniformly spread across the cpuset.
In addition to these entries, the top cpuset also contains
the entry memory_pressure_enabled, which must be
set to 1 if you want to make use of the memory_pressure
entries in the different cpusets.
In order to make use of cpusets, you need detailed hardware
information for several reasons: on big machines, memory
that is local to a CPU will be much faster than memory that
is only available on a different node. If you want to create
cpusets from several nodes, you should try to combine CPUs
that are close together. Otherwise, task switches and
memory access may slow down your system noticeably.
To nd out which node a CPU belongs to, use the /sys
le system. The kernel provides information about available
CPUs to a specic node by creating links in /sys/
devices/system/node/nodeX/.
If several CPUs are to be combined to a cpuset, check the
distance of the CPUs from each other with the command
numactl --hardware. This command is available after
installing the package numactl.
The actual conguration and manipulation of cpusets is
done by modifying the le system below /dev/cpuset.
Tasks are performed in the following way:
Create a Cpuset
To create a cpuset with the name exampleset, just run
mkdir /dev/cpuset/exampleset to create the
respective directory. The newly created set will contain
several entries that reect the current status of the set.
Remove a Cpuset
To remove a cpuset, you only need to remove the cpuset
directory. For example, use rmdir
/dev/cpuset/exampleset to remove the previously
generated cpuset named exampleset. In contrast to
normal le systems, this works even if there are still entries in the directory.
Note that you will get an error like rmdir: example-
set: Device or resource busy, if there are still
tasks active in that set. To remove these tasks from the
set, just move them to another set.
Add CPUs to a Cpuset
To add CPUs to a set, you may either specify a comma
separated list of CPU numbers, or give a range of CPUs.
For example, to add CPUs with the numbers 2,3 and 7
to exampleset, you can use one of the following
commands: /bin/echo 2,3,7 >
/dev/cpuset/exampleset/cpus or /bin/echo
2-3,7 > /dev/cpuset/exampleset/cpus.
Add Memory to a Cpuset
You cannot move tasks to a cpuset without giving the
cpuset access to some system memory. To do so, echo
a node number into /dev/cpuset/exampleset/
mems. If possible, use a node that is close to the used
CPUs in this set.
Moving Tasks to Cpusets
A cpuset is just a useless structure, unless it handles some
tasks. To add a task to /dev/cpuset/exampleset/,
simply echo the task number into /dev/cpuset/
exampleset/. The following script moves all user space
processes to /dev/cpuset/exampleset/ and leaves
all kernel threads untouched:
cd /dev/cpuset/exampleset; \
for pid in $(cat ../tasks); do \
test -e /proc/$pid/exe && \
echo $pid > tasks; done
Note, that for a clean solution, you would have to stop
all processes, move them to the new cpuset, and let them
continue afterward. Otherwise, the process may nish
before the for loop nishes, or other processes may start
during moving.
This loop liberates all CPUs not contained in the exampleset from all processes. Check the result with the
command cat /dev/cpuset/tasks, which then
should not have any entries.
Of course, you can move all tasks from a special cpuset
to the top level set, if you intend to remove this special
cpuset.
Automatically Remove Unused Cpusets
In case a cpuset is not used any longer by any process,
you might want to clean up such unused cpusets automatically. To initialize the removal, you can use the no-
tify_on_release ag. If this is set to 1, the kernel
will run /sbin/cpuset_release_agent when the
last process exits. To remove an unused script, you may,
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for example, add the following script in /sbin/cpuset
_release_agent:
#!/bin/sh
logger cpuset: releasing $1
rmdir /dev/cpuset/$1
After adding the script to your system, run chmod 755
/sbin/cpuset_release_agent to make the script
executable.
Determine the Cpuset of a Specic Process
All processes with the process id PID have an entry in
/proc/PID/cpuset. If you run the command cat
/proc/PID/cpuset on a PID that runs in the cpuset
exampleset, you will nd the results in /exampleset.
Specifying a CPU Afnity with
taskset
The default behavior of the kernel, is to keep a process
running on the same CPU, if the system load is balanced
over the available CPUs. Otherwise, the kernel tries to improve the load balancing by moving processes to an idling
CPU. In some situations, however, it is desirable to set a
CPU afnity for a given process. In this case, the kernel will
not move the process away from the selected CPUs. For
example, if you use shielding, the shielded CPUs will not
run any process that does not have an afnity to the
shielded CPUs. Another possibility is to run all low priority
tasks on a selected CPU to remove load from the other
CPUs.
Note, that if a task is running inside a specic cpuset, the
afnity mask must match at least one of the CPUs available
in this set. The taskset command will not move a process
outside the cpuset it is running in.
To set or retrieve the CPU afnity of a task, a bitmask is
used, that is represented by a hexadecimal number. If you
count the bits of this bitmask, the lowest bit represents the
rst logical CPU as they are found in /proc/cpuinfo. For
example:
0x00000001
is processor #0.
0x00000002
is processor #1.
0x00000003
is processor #0 and processor #1.
0xFFFFFFFE
all but the rst CPU.
If a given mask does not contain any valid CPU on the system, an error is returned. If taskset returns without an error,
the given program has been scheduled to the specied list
of CPUs.
The command taskset can either be used to start a new
process with a given CPU afnity, or to redene the CPU
afnity of a already running process.
Examples
taskset -p pid
Retrieves the current CPU afnity of the process with
PID pid.
taskset -p mask pid
Sets the CPU afnity of the process with PID pid to
mask.
taskset mask command
Runs command with a CPU afnity of mask.
Changing I/O Priorities with ionice
Handling I/O is one of the critical issues for all high-performance systems. If a task has lots of CPU power available,
but must wait for the disk, it will not work as efcient as it
could. The Linux kernel provides three different scheduling
classes to determine the I/O handling for a process. All of
these classes can be ne-tuned with a nice level.
The Best Effort Scheduler
The Best Effort scheduler is the default I/O scheduler,
and is used for all processes that do not specify a different I/O scheduler class. By default, this scheduler sets its
niceness according to the nice value of the running
process.
There are eight different nice levels available for this
scheduler. The lowest priority is represented by a nice
level of seven, the highest priority is zero.
This scheduler has the scheduling class number 2.
The Real Time Scheduler
The real-time I/O class always gets the highest priority
for disk access. The other schedulers will only be served,
if no real-time request is present. This scheduling class
may easily lock up the system if not implemented with
care.
The real-time scheduler denes nice levels just like the
Best Effort scheduler.
This scheduler has the scheduling class number 1.
The Idle Scheduler
The Idle scheduler does not dene any nice levels. I/O
is only done in this class, if no other scheduler runs an
I/O request. This scheduler has the lowest available priority and can be used for processes that are not timecritical at all.
This scheduler has the scheduling class number 3.
To change I/O schedulers and nice values, use the ionice
command. This provides a means to tune the scheduler of
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already running processes, or to start new processes with
specic I/O settings.
Examples
ionice -c3 -p$$
Sets the scheduler of the current shell to Idle.
ionice
Without additional parameters, this prints the I/O
scheduler settings of the current shell.
ionice -c1 -p42 -n2
Sets the scheduler of the process with process id 42 to
Real Time, and its nice value to 2.
ionice -c3 /bin/bash
Starts the Bash shell with the Idle I/O scheduler.
Changing the I/O Scheduler for Block
Devices
The Linux kernel provides several block device schedulers
that can be selected individually for each block device. All
but the noop scheduler perform a kind of ordering of requested blocks to reduce head movements on the hard
disk. If you use an external storage system that has its own
scheduler, you may want to disable the Linux internal reordering by selecting the noop scheduler.
The Linux I/O Schedulers
noop
The noop scheduler is a very simple scheduler, that performs basic merging and sorting on I/O requests. This
scheduler is mainly used for specialized environments
that run their own schedulers optimized for the used
hardware, such as storage systems or hardware RAID
controllers.
anticipatory
The main principle of anticipatory scheduling is, that after a read, the scheduler simply expects further reads
from userspace. For this reason, after a read completes,
the anticipatory scheduler will do nothing at all for a few
milliseconds, giving userspace the possibility to ask for
another read. If such a read is requested, it will be performed immediately. Otherwise the scheduler continues
with doing writes after a short time-out.
The advantage of this procedure is a major reduction of
seeks and thus a decreased read latency. This also increases read and write bandwidth.
deadline
The main point of deadline scheduling is to try hard to
answer a request before a given deadline. This results in
very good I/O for a random single I/O in real-time environments.
In principle, the deadline uses two lists with all requests.
One is sorted by block sequences to reduce seeking la-
tencies, the other is sorted by expire times for each request. Normally, requests are served according to the
block sequence, but if a request reaches its deadline, the
scheduler starts to work on this request.
cfq
The Completely Fair Queuing scheduler uses a separate
I/O queue for each process. All of these queues get a
similar time slice for disk access. With this procedure,
the CFQ tries to divide the bandwidth evenly between
all requesting processes. This scheduler has a similar
throughput as the anticipatory scheduler, but the maximum latency is much shorter.
For the average system, this scheduler yields the best
results, and thus is the default I/O scheduler on SUSE
Linux Enterprise systems.
To print the current scheduler of a block device like /dev/
sda, use the following command:
cat /sys/block/sda/queue/scheduler
noop anticipatory deadline [cfq]
In this case, the scheduler for /dev/sda is set to cfq, the
Completely Fair Queuing scheduler. This is the de-
fault scheduler on SUSE Linux Enterprise Real Time.
To change the schedulers, echo one of the names noop,
anticipatory, deadline, or cfq into /sys/block/
<device>/scheduler. For example, if you want to set
the I/O scheduler of the device /dev/sda to noop, use
the command echo "noop" >
/sys/block/sda/scheduler. To set other variables in
the /sys le system, use a similar approach.
Tuning the Block Device I/O Scheduler
All schedulers, except for the noop scheduler, have several
common parameters that may be tuned for each block device. You can access these parameters with sysfs in the
/sys/block/<device>/queue/iosched/ directory.
The following parameters are tuneable for the respective
scheduler:
Anticipatory Scheduler
antic_expire
Time in milliseconds that the anticipatory scheduler
waits for another read request close to the last read
request performed. The anticipatory scheduler will
not wait for upcoming read requests, if this value is
set to zero.
read_expire
Deadline of a read request in milliseconds. This
scheduler also controls the interval between expired
requests. By default, read_expire is set to 125 milliseconds. Until a read request is served which is next on
the list, it can thus take up to 250 milliseconds.
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write_expire
Similar to read_expire for write requests.
read_batch_expire
If write requests are scheduled, this is the time in
milliseconds that reads are served before pending
writes get a time slice. If writes are more important
than reads, set this value lower than read_expire.
write_batch_expire
Similar to read_batch_expire for write requests.
Deadline Scheduler
read_expire
The main focus of this scheduler is to limit the start
latency for a request to a given time. Therefore, for
each request, a deadline is calculated from the current
time plus the value of read_expire in milliseconds.
write_expire
Similar to read_expire for write requests.
fifo_batch
If a request hits its deadline, it is necessary to move
the request from the sorted I/O scheduler list to the
dispatch queue. The variable fifo_batch controls
how many requests are moved, depending on the
cost of each request.
front_merges
The scheduler normally tries to nd contiguous I/O
requests and merges them. There are two kinds of
merges: The new I/O request may be in front of the
existing I/O request (front merge), or it may follow
behind the existing request (back merge). Because
most merges are back merges, you can disable the
front merge functionality by setting front_merges
to 0.
write_starved
In case some read or write requests hit their deadline,
the scheduler prefers the read requests by default.
To prevent write requests from being postponed forever, the variable write_starved controls how
often read requests are preferred until write requests
are preferred over read requests.
CFQ Scheduler
back_seek_max and back_seek_penalty
The CFQ scheduler normally uses a strict ascending
elevator. When needed, it also allows small backward
seeks, but it puts some penalty on them. The maximum backward sector seek is dened with
back_seek_max, and the multiplier for the penalty
is set by back_seek_penalty.
fifo_expire_async and fifo_expire_sync
The fifo_expire_* variables dene the timeout
in milliseconds for asynchronous and synchronous
I/O requests. Typically, fifo_expire_async affects
write and fifo_expire_sync affects both read
and write operations.
quantum
Denes the number of I/O requests to dispatch when
the block device is idle.
slice_async, slice_async_rq, slice_sync, and
slice_idle
These variables dene the time slices a block device
gets for synchronous or asynchronous operations.
• slice_async and slice_sync represent the
length of an asynchronous or synchronous disk slice
in milliseconds.
• slice_async_rq denes for how many requests
an asynchronous disk slice lasts.
• slice_idle denes how long a sync slice may
idle.
For More Information
A lot of information about real-time implementations and
administration can be found on the Internet. The following
list contains a number of selected links:
• The cpuset feature of the kernel is explained in /usr/
src/linux/Documentation/cpusets.txt. More
detailed documentation is available from http://
techpubs.sgi.com/library/tpl/cgi-bin/
getdoc.cgi/linux/bks/SGI_Admin/books/LX
_Resource_AG/sgi_html/ch04.html, http://
www.bullopensource.org/cpuset/, and http://
lwn.net/Articles/127936/.
• An overview of CPU and I/O schedulers available in Linux
can be found at http://aplawrence.com/Linux/
linux26_features.html.
• Detailed information about the anticipatory I/O scheduler
is available at http://www.cs.rice.edu/~ssiyer/
r/antsched/antio.html and http://www.cs
.rice.edu/~ssiyer/r/antsched/.
• For more information about the deadline I/O scheduler,
refer to http://lwn.net/2002/0110/a/
io-scheduler.php3, or http://kerneltrap.org/
node/431. In your installed system, nd further informa-
tion in /usr/src/linux/Documentation/block/
deadline-iosched.txt.
• The CFQ I/O scheduler is covered in detail in http://
en.wikipedia.org/wiki/CFQ.
• General information about I/O scheduling in Linux is
available at http://lwn.net/Articles/101029/,
http://lwn.net/Articles/114273/, and http://
donami.com/118.
• A lot of information about real-time can be found at
http://linuxdevices.com/articles/
AT6476691775.html.
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