Novell LINUX ENTERPRISE REAL TIME 10 SP1 Manuale utente

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 afnities 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 Enter-

prise Server 10 SP1.

SUSE Linux Enterprise Real Time always needs a SUSE Linux

Enterprise Server SP1 base, it cannot be installed in stan-

dalone 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 benecial to be able to run

specic tasks only on dened CPUs. For this reason, the

linux kernel provides a feature called cpuset. Cpusets pro-

vide the means to do a so called “soft partitioning” of the

system. Dedicated CPUs, together with some predened

memory, work on a number of tasks.

All systems have at least one cpuset that is called /. To re-

trieve the cpuset of a specic 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 conguration, in case the memory congura-

tion of the cpuset changes.

cpu_exclusive

Denes if this cpuset becomes a scheduling domain, that

shares properties and policies.

SUSE Linux Enterprise Real Time 10 SP1 Quick Start

SUSE Linux Enterprise 10 SP1

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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 1if 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 specic 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 conguration 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 reect 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 en-

tries 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 exam-

pleset 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 auto-

matically. 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,

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 Specic 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 Afnity 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 im-

prove the load balancing by moving processes to an idling

CPU. In some situations, however, it is desirable to set a

CPU afnity 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 afnity 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 specic cpuset, the

afnity 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 afnity 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 sys-

tem, an error is returned. If taskset returns without an error,

the given program has been scheduled to the specied list

of CPUs.

The command taskset can either be used to start a new

process with a given CPU afnity, or to redene the CPU

afnity of a already running process.

Examples

taskset -p pid

Retrieves the current CPU afnity of the process with

PID pid.

taskset -p mask pid

Sets the CPU afnity of the process with PID pid to

mask.

taskset mask command

Runs command with a CPU afnity of mask.

Changing I/O Priorities with ionice

Handling I/O is one of the critical issues for all high-perfor-

mance systems. If a task has lots of CPU power available,

but must wait for the disk, it will not work as efcient 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 differ-

ent 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 denes nice levels just like the

Best Effort scheduler.

This scheduler has the scheduling class number 1.

The Idle Scheduler

The Idle scheduler does not dene 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 pri-

ority and can be used for processes that are not time-

critical 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

already running processes, or to start new processes with

specic 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 re-

quested 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 re-

ordering by selecting the noop scheduler.

The Linux I/O Schedulers

noop

The noop scheduler is a very simple scheduler, that per-

forms 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 af-

ter 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 per-

formed 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 increas-

es 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 envi-

ronments.

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 re-

quest. 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 maxi-

mum 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 de-

vice. 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 millisec-

onds. Until a read request is served which is next on

the list, it can thus take up to 250 milliseconds.

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 for-

ever, 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 maxi-

mum backward sector seek is dened 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 dene 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

Denes 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 dene 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 denes for how many requests

an asynchronous disk slice lasts.

•slice_idle denes 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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