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                                CPUSETS

                                -------

Copyright (C) 2004 BULL SA.

Written by Simon.Derr@bull.net

Portions Copyright (c) 2004-2006 Silicon Graphics, Inc.

Modified by Paul Jackson 

Modified by Christoph Lameter 

Modified by Paul Menage 

CONTENTS:

=========

1. Cpusets

  1.1 What are cpusets ?

  1.2 Why are cpusets needed ?

  1.3 How are cpusets implemented ?

  1.4 What are exclusive cpusets ?

  1.5 What is memory_pressure ?

  1.6 What is memory spread ?

  1.7 What is sched_load_balance ?

  1.8 How do I use cpusets ?

2. Usage Examples and Syntax

  2.1 Basic Usage

  2.2 Adding/removing cpus

  2.3 Setting flags

  2.4 Attaching processes

3. Questions

4. Contact

1. Cpusets

==========

1.1 What are cpusets ?

----------------------

Cpusets provide a mechanism for assigning a set of CPUs and Memory

Nodes to a set of tasks.   In this document "Memory Node" refers to

an on-line node that contains memory.

Cpusets constrain the CPU and Memory placement of tasks to only

the resources within a tasks current cpuset.  They form a nested

hierarchy visible in a virtual file system.  These are the essential

hooks, beyond what is already present, required to manage dynamic

job placement on large systems.

Cpusets use the generic cgroup subsystem described in

Documentation/cgroup.txt.

Requests by a task, using the sched_setaffinity(2) system call to

include CPUs in its CPU affinity mask, and using the mbind(2) and

set_mempolicy(2) system calls to include Memory Nodes in its memory

policy, are both filtered through that tasks cpuset, filtering out any

CPUs or Memory Nodes not in that cpuset.  The scheduler will not

schedule a task on a CPU that is not allowed in its cpus_allowed

vector, and the kernel page allocator will not allocate a page on a

node that is not allowed in the requesting tasks mems_allowed vector.

User level code may create and destroy cpusets by name in the cgroup

virtual file system, manage the attributes and permissions of these

cpusets and which CPUs and Memory Nodes are assigned to each cpuset,

specify and query to which cpuset a task is assigned, and list the

task pids assigned to a cpuset.

1.2 Why are cpusets needed ?

----------------------------

The management of large computer systems, with many processors (CPUs),

complex memory cache hierarchies and multiple Memory Nodes having

non-uniform access times (NUMA) presents additional challenges for

the efficient scheduling and memory placement of processes.

Frequently more modest sized systems can be operated with adequate

efficiency just by letting the operating system automatically share

the available CPU and Memory resources amongst the requesting tasks.

But larger systems, which benefit more from careful processor and

memory placement to reduce memory access times and contention,

and which typically represent a larger investment for the customer,

can benefit from explicitly placing jobs on properly sized subsets of

the system.

This can be especially valuable on:

    * Web Servers running multiple instances of the same web application,

    * Servers running different applications (for instance, a web server

      and a database), or

    * NUMA systems running large HPC applications with demanding

      performance characteristics.

These subsets, or "soft partitions" must be able to be dynamically

adjusted, as the job mix changes, without impacting other concurrently

executing jobs. The location of the running jobs pages may also be moved

when the memory locations are changed.

The kernel cpuset patch provides the minimum essential kernel

mechanisms required to efficiently implement such subsets.  It

leverages existing CPU and Memory Placement facilities in the Linux

kernel to avoid any additional impact on the critical scheduler or

memory allocator code.

1.3 How are cpusets implemented ?

---------------------------------

Cpusets provide a Linux kernel mechanism to constrain which CPUs and

Memory Nodes are used by a process or set of processes.

The Linux kernel already has a pair of mechanisms to specify on which

CPUs a task may be scheduled (sched_setaffinity) and on which Memory

Nodes it may obtain memory (mbind, set_mempolicy).

Cpusets extends these two mechanisms as follows:

 - Cpusets are sets of allowed CPUs and Memory Nodes, known to the

   kernel.

 - Each task in the system is attached to a cpuset, via a pointer

   in the task structure to a reference counted cgroup structure.

 - Calls to sched_setaffinity are filtered to just those CPUs

   allowed in that tasks cpuset.

 - Calls to mbind and set_mempolicy are filtered to just

   those Memory Nodes allowed in that tasks cpuset.

 - The root cpuset contains all the systems CPUs and Memory

   Nodes.

 - For any cpuset, one can define child cpusets containing a subset

   of the parents CPU and Memory Node resources.

 - The hierarchy of cpusets can be mounted at /dev/cpuset, for

   browsing and manipulation from user space.

 - A cpuset may be marked exclusive, which ensures that no other

   cpuset (except direct ancestors and descendents) may contain

   any overlapping CPUs or Memory Nodes.

 - You can list all the tasks (by pid) attached to any cpuset.

The implementation of cpusets requires a few, simple hooks

into the rest of the kernel, none in performance critical paths:

 - in init/main.c, to initialize the root cpuset at system boot.

 - in fork and exit, to attach and detach a task from its cpuset.

 - in sched_setaffinity, to mask the requested CPUs by what's

   allowed in that tasks cpuset.

 - in sched.c migrate_all_tasks(), to keep migrating tasks within

   the CPUs allowed by their cpuset, if possible.

 - in the mbind and set_mempolicy system calls, to mask the requested

   Memory Nodes by what's allowed in that tasks cpuset.

 - in page_alloc.c, to restrict memory to allowed nodes.

 - in vmscan.c, to restrict page recovery to the current cpuset.

You should mount the "cgroup" filesystem type in order to enable

browsing and modifying the cpusets presently known to the kernel.  No

new system calls are added for cpusets - all support for querying and

modifying cpusets is via this cpuset file system.

The /proc//status file for each task has two added lines,

displaying the tasks cpus_allowed (on which CPUs it may be scheduled)

and mems_allowed (on which Memory Nodes it may obtain memory),

in the format seen in the following example:

  Cpus_allowed:   ffffffff,ffffffff,ffffffff,ffffffff

  Mems_allowed:   ffffffff,ffffffff

Each cpuset is represented by a directory in the cgroup file system

containing (on top of the standard cgroup files) the following

files describing that cpuset:

 - cpus: list of CPUs in that cpuset

 - mems: list of Memory Nodes in that cpuset

 - memory_migrate flag: if set, move pages to cpusets nodes

 - cpu_exclusive flag: is cpu placement exclusive?

 - mem_exclusive flag: is memory placement exclusive?

 - memory_pressure: measure of how much paging pressure in cpuset

In addition, the root cpuset only has the following file:

 - memory_pressure_enabled flag: compute memory_pressure?

New cpusets are created using the mkdir system call or shell

command.  The properties of a cpuset, such as its flags, allowed

CPUs and Memory Nodes, and attached tasks, are modified by writing

to the appropriate file in that cpusets directory, as listed above.

The named hierarchical structure of nested cpusets allows partitioning

a large system into nested, dynamically changeable, "soft-partitions".

The attachment of each task, automatically inherited at fork by any

children of that task, to a cpuset allows organizing the work load

on a system into related sets of tasks such that each set is constrained

to using the CPUs and Memory Nodes of a particular cpuset.  A task

may be re-attached to any other cpuset, if allowed by the permissions

on the necessary cpuset file system directories.

Such management of a system "in the large" integrates smoothly with

the detailed placement done on individual tasks and memory regions

using the sched_setaffinity, mbind and set_mempolicy system calls.

The following rules apply to each cpuset:

 - Its CPUs and Memory Nodes must be a subset of its parents.

 - It can only be marked exclusive if its parent is.

 - If its cpu or memory is exclusive, they may not overlap any sibling.

These rules, and the natural hierarchy of cpusets, enable efficient

enforcement of the exclusive guarantee, without having to scan all

cpusets every time any of them change to ensure nothing overlaps a

exclusive cpuset.  Also, the use of a Linux virtual file system (vfs)

to represent the cpuset hierarchy provides for a familiar permission

and name space for cpusets, with a minimum of additional kernel code.

The cpus and mems files in the root (top_cpuset) cpuset are

read-only.  The cpus file automatically tracks the value of

cpu_online_map using a CPU hotplug notifier, and the mems file

automatically tracks the value of node_states[N_MEMORY]--i.e.,

nodes with memory--using the cpuset_track_online_nodes() hook.

1.4 What are exclusive cpusets ?

--------------------------------

If a cpuset is cpu or mem exclusive, no other cpuset, other than

a direct ancestor or descendent, may share any of the same CPUs or

Memory Nodes.

A cpuset that is mem_exclusive restricts kernel allocations for

page, buffer and other data commonly shared by the kernel across

multiple users.  All cpusets, whether mem_exclusive or not, restrict

allocations of memory for user space.  This enables configuring a

system so that several independent jobs can share common kernel data,

such as file system pages, while isolating each jobs user allocation in

its own cpuset.  To do this, construct a large mem_exclusive cpuset to

hold all the jobs, and construct child, non-mem_exclusive cpusets for

each individual job.  Only a small amount of typical kernel memory,

such as requests from interrupt handlers, is allowed to be taken

outside even a mem_exclusive cpuset.

1.5 What is memory_pressure ?

-----------------------------

The memory_pressure of a cpuset provides a simple per-cpuset metric

of the rate that the tasks in a cpuset are attempting to free up in

use memory on the nodes of the cpuset to satisfy additional memory

requests.

This enables batch managers monitoring jobs running in dedicated

cpusets to efficiently detect what level of memory pressure that job

is causing.

This is useful both on tightly managed systems running a wide mix of

submitted jobs, which may choose to terminate or re-prioritize jobs that

are trying to use more memory than allowed on the nodes assigned them,

and with tightly coupled, long running, massively parallel scientific

computing jobs that will dramatically fail to meet required performance

goals if they start to use more memory than allowed to them.

This mechanism provides a very economical way for the batch manager

to monitor a cpuset for signs of memory pressure.  It's up to the

batch manager or other user code to decide what to do about it and

take action.

==> Unless this feature is enabled by writing "1" to the special file

    /dev/cpuset/memory_pressure_enabled, the hook in the rebalance

    code of __alloc_pages() for this metric reduces to simply noticing

    that the cpuset_memory_pressure_enabled flag is zero.  So only

    systems that enable this feature will compute the metric.

Why a per-cpuset, running average:

    Because this meter is per-cpuset, rather than per-task or mm,

    the system load imposed by a batch scheduler monitoring this

    metric is sharply reduced on large systems, because a scan of

    the tasklist can be avoided on each set of queries.

    Because this meter is a running average, instead of an accumulating

    counter, a batch scheduler can detect memory pressure with a

    single read, instead of having to read and accumulate results

    for a period of time.

    Because this meter is per-cpuset rather than per-task or mm,

    the batch scheduler can obtain the key information, memory

    pressure in a cpuset, with a single read, rather than having to

    query and accumulate results over all the (dynamically changing)

    set of tasks in the cpuset.

A per-cpuset simple digital filter (requires a spinlock and 3 words

of data per-cpuset) is kept, and updated by any task attached to that

cpuset, if it enters the synchronous (direct) page reclaim code.

A per-cpuset file provides an integer number representing the recent

(half-life of 10 seconds) rate of direct page reclaims caused by

the tasks in the cpuset, in units of reclaims attempted per second,

times 1000.

1.6 What is memory spread ?

---------------------------

There are two boolean flag files per cpuset that control where the

kernel allocates pages for the file system buffers and related in

kernel data structures.  They are called 'memory_spread_page' and

'memory_spread_slab'.

If the per-cpuset boolean flag file 'memory_spread_page' is set, then

the kernel will spread the file system buffers (page cache) evenly

over all the nodes that the faulting task is allowed to use, instead

of preferring to put those pages on the node where the task is running.

If the per-cpuset boolean flag file 'memory_spread_slab' is set,

then the kernel will spread some file system related slab caches,

such as for inodes and dentries evenly over all the nodes that the

faulting task is allowed to use, instead of preferring to put those

pages on the node where the task is running.

The setting of these flags does not affect anonymous data segment or

stack segment pages of a task.

By default, both kinds of memory spreading are off, and memory

pages are allocated on the node local to where the task is running,

except perhaps as modified by the tasks NUMA mempolicy or cpuset

configuration, so long as sufficient free memory pages are available.

When new cpusets are created, they inherit the memory spread settings

of their parent.

Setting memory spreading causes allocations for the affected page

or slab caches to ignore the tasks NUMA mempolicy and be spread

instead.    Tasks using mbind() or set_mempolicy() calls to set NUMA

mempolicies will not notice any change in these calls as a result of

their containing tasks memory spread settings.  If memory spreading

is turned off, then the currently specified NUMA mempolicy once again

applies to memory page allocations.

Both 'memory_spread_page' and 'memory_spread_slab' are boolean flag

files.  By default they contain "0", meaning that the feature is off

for that cpuset.  If a "1" is written to that file, then that turns

the named feature on.

The implementation is simple.

Setting the flag 'memory_spread_page' turns on a per-process flag

PF_SPREAD_PAGE for each task that is in that cpuset or subsequently

joins that cpuset.  The page allocation calls for the page cache

is modified to perform an inline check for this PF_SPREAD_PAGE task

flag, and if set, a call to a new routine cpuset_mem_spread_node()

returns the node to prefer for the allocation.

Similarly, setting 'memory_spread_cache' turns on the flag

PF_SPREAD_SLAB, and appropriately marked slab caches will allocate

pages from the node returned by cpuset_mem_spread_node().

The cpuset_mem_spread_node() routine is also simple.  It uses the

value of a per-task rotor cpuset_mem_spread_rotor to select the next

node in the current tasks mems_allowed to prefer for the allocation.

This memory placement policy is also known (in other contexts) as

round-robin or interleave.

This policy can provide substantial improvements for jobs that need

to place thread local data on the corresponding node, but that need

to access large file system data sets that need to be spread across

the several nodes in the jobs cpuset in order to fit.  Without this

policy, especially for jobs that might have one thread reading in the

data set, the memory allocation across the nodes in the jobs cpuset

can become very uneven.

1.7 What is sched_load_balance ?

--------------------------------

The kernel scheduler (kernel/sched.c) automatically load balances

tasks.  If one CPU is underutilized, kernel code running on that

CPU will look for tasks on other more overloaded CPUs and move those

tasks to itself, within the constraints of such placement mechanisms

as cpusets and sched_setaffinity.

The algorithmic cost of load balancing and its impact on key shared

kernel data structures such as the task list increases more than

linearly with the number of CPUs being balanced.  So the scheduler

has support to  partition the systems CPUs into a number of sched

domains such that it only load balances within each sched domain.

Each sched domain covers some subset of the CPUs in the system;

no two sched domains overlap; some CPUs might not be in any sched

domain and hence won't be load balanced.

Put simply, it costs less to balance between two smaller sched domains

than one big one, but doing so means that overloads in one of the

two domains won't be load balanced to the other one.

By default, there is one sched domain covering all CPUs, except those

marked isolated using the kernel boot time "isolcpus=" argument.

This default load balancing across all CPUs is not well suited for

the following two situations:

 1) On large systems, load balancing across many CPUs is expensive.

    If the system is managed using cpusets to place independent jobs

    on separate sets of CPUs, full load balancing is unnecessary.

 2) Systems supporting realtime on some CPUs need to minimize

    system overhead on those CPUs, including avoiding task load

    balancing if that is not needed.

When the per-cpuset flag "sched_load_balance" is enabled (the default

setting), it requests that all the CPUs in that cpusets allowed 'cpus'

be contained in a single sched domain, ensuring that load balancing

can move a task (not otherwised pinned, as by sched_setaffinity)

from any CPU in that cpuset to any other.

When the per-cpuset flag "sched_load_balance" is disabled, then the

scheduler will avoid load balancing across the CPUs in that cpuset,

--except-- in so far as is necessary because some overlapping cpuset

has "sched_load_balance" enabled.

So, for example, if the top cpuset has the flag "sched_load_balance"

enabled, then the scheduler will have one sched domain covering all

CPUs, and the setting of the "sched_load_balance" flag in any other

cpusets won't matter, as we're already fully load balancing.

Therefore in the above two situations, the top cpuset flag

"sched_load_balance" should be disabled, and only some of the smaller,

child cpusets have this flag enabled.

When doing this, you don't usually want to leave any unpinned tasks in

the top cpuset that might use non-trivial amounts of CPU, as such tasks

may be artificially constrained to some subset of CPUs, depending on

the particulars of this flag setting in descendent cpusets.  Even if

such a task could use spare CPU cycles in some other CPUs, the kernel

scheduler might not consider the possibility of load balancing that

task to that underused CPU.

Of course, tasks pinned to a particular CPU can be left in a cpuset

that disables "sched_load_balance" as those tasks aren't going anywhere

else anyway.

There is an impedance mismatch here, between cpusets and sched domains.

Cpusets are hierarchical and nest.  Sched domains are flat; they don't

overlap and each CPU is in at most one sched domain.

It is necessary for sched domains to be flat because load balancing

across partially overlapping sets of CPUs would risk unstable dynamics

that would be beyond our understanding.  So if each of two partially

overlapping cpusets enables the flag 'sched_load_balance', then we

form a single sched domain that is a superset of both.  We won't move

a task to a CPU outside it cpuset, but the scheduler load balancing

code might waste some compute cycles considering that possibility.

This mismatch is why there is not a simple one-to-one relation

between which cpusets have the flag "sched_load_balance" enabled,

and the sched domain configuration.  If a cpuset enables the flag, it

will get balancing across all its CPUs, but if it disables the flag,

it will only be assured of no load balancing if no other overlapping

cpuset enables the flag.

If two cpusets have partially overlapping 'cpus' allowed, and only

one of them has this flag enabled, then the other may find its

tasks only partially load balanced, just on the overlapping CPUs.

This is just the general case of the top_cpuset example given a few

paragraphs above.  In the general case, as in the top cpuset case,

don't leave tasks that might use non-trivial amounts of CPU in

such partially load balanced cpusets, as they may be artificially

constrained to some subset of the CPUs allowed to them, for lack of

load balancing to the other CPUs.

1.7.1 sched_load_balance implementation details.

------------------------------------------------

The per-cpuset flag 'sched_load_balance' defaults to enabled (contrary

to most cpuset flags.)  When enabled for a cpuset, the kernel will

ensure that it can load balance across all the CPUs in that cpuset

(makes sure that all the CPUs in the cpus_allowed of that cpuset are

in the same sched domain.)

If two overlapping cpusets both have 'sched_load_balance' enabled,

then they will be (must be) both in the same sched domain.

If, as is the default, the top cpuset has 'sched_load_balance' enabled,

then by the above that means there is a single sched domain covering

the whole system, regardless of any other cpuset settings.

The kernel commits to user space that it will avoid load balancing

where it can.  It will pick as fine a granularity partition of sched

domains as it can while still providing load balancing for any set

of CPUs allowed to a cpuset having 'sched_load_balance' enabled.

The internal kernel cpuset to scheduler interface passes from the

cpuset code to the scheduler code a partition of the load balanced

CPUs in the system. This partition is a set of subsets (represented

as an array of cpumask_t) of CPUs, pairwise disjoint, that cover all

the CPUs that must be load balanced.

Whenever the 'sched_load_balance' flag changes, or CPUs come or go

from a cpuset with this flag enabled, or a cpuset with this flag

enabled is removed, the cpuset code builds a new such partition and

passes it to the scheduler sched domain setup code, to have the sched

domains rebuilt as necessary.

This partition exactly defines what sched domains the scheduler should

setup - one sched domain for each element (cpumask_t) in the partition.

The scheduler remembers the currently active sched domain partitions.

When the scheduler routine partition_sched_domains() is invoked from

the cpuset code to update these sched domains, it compares the new

partition requested with the current, and updates its sched domains,

removing the old and adding the new, for each change.

1.8 How do I use cpusets ?

--------------------------

In order to minimize the impact of cpusets on critical kernel

code, such as the scheduler, and due to the fact that the kernel

does not support one task updating the memory placement of another

task directly, the impact on a task of changing its cpuset CPU

or Memory Node placement, or of changing to which cpuset a task

is attached, is subtle.

If a cpuset has its Memory Nodes modified, then for each task attached

to that cpuset, the next time that the kernel attempts to allocate

a page of memory for that task, the kernel will notice the change

in the tasks cpuset, and update its per-task memory placement to

remain within the new cpusets memory placement.  If the task was using

mempolicy MPOL_BIND, and the nodes to which it was bound overlap with

its new cpuset, then the task will continue to use whatever subset

of MPOL_BIND nodes are still allowed in the new cpuset.  If the task

was using MPOL_BIND and now none of its MPOL_BIND nodes are allowed

in the new cpuset, then the task will be essentially treated as if it

was MPOL_BIND bound to the new cpuset (even though its numa placement,

as queried by get_mempolicy(), doesn't change).  If a task is moved

from one cpuset to another, then the kernel will adjust the tasks

memory placement, as above, the next time that the kernel attempts

to allocate a page of memory for that task.

If a cpuset has its CPUs modified, then each task using that

cpuset does _not_ change its behavior automatically.  In order to

minimize the impact on the critical scheduling code in the kernel,

tasks will continue to use their prior CPU placement until they

are rebound to their cpuset, by rewriting their pid to the 'tasks'

file of their cpuset.  If a task had been bound to some subset of its

cpuset using the sched_setaffinity() call, and if any of that subset

is still allowed in its new cpuset settings, then the task will be

restricted to the intersection of the CPUs it was allowed on before,

and its new cpuset CPU placement.  If, on the other hand, there is

no overlap between a tasks prior placement and its new cpuset CPU

placement, then the task will be allowed to run on any CPU allowed

in its new cpuset.  If a task is moved from one cpuset to another,

its CPU placement is updated in the same way as if the tasks pid is

rewritten to the 'tasks' file of its current cpuset.

In summary, the memory placement of a task whose cpuset is changed is

updated by the kernel, on the next allocation of a page for that task,

but the processor placement is not updated, until that tasks pid is

rewritten to the 'tasks' file of its cpuset.  This is done to avoid

impacting the scheduler code in the kernel with a check for changes

in a tasks processor placement.

Normally, once a page is allocated (given a physical page

of main memory) then that page stays on whatever node it

was allocated, so long as it remains allocated, even if the

cpusets memory placement policy 'mems' subsequently changes.

If the cpuset flag file 'memory_migrate' is set true, then when

tasks are attached to that cpuset, any pages that task had

allocated to it on nodes in its previous cpuset are migrated

to the tasks new cpuset. The relative placement of the page within

the cpuset is preserved during these migration operations if possible.

For example if the page was on the second valid node of the prior cpuset

then the page will be placed on the second valid node of the new cpuset.

Also if 'memory_migrate' is set true, then if that cpusets

'mems' file is modified, pages allocated to tasks in that

cpuset, that were on nodes in the previous setting of 'mems',

will be moved to nodes in the new setting of 'mems.'

Pages that were not in the tasks prior cpuset, or in the cpusets

prior 'mems' setting, will not be moved.

There is an exception to the above.  If hotplug functionality is used

to remove all the CPUs that are currently assigned to a cpuset,

then the kernel will automatically update the cpus_allowed of all

tasks attached to CPUs in that cpuset to allow all CPUs.  When memory

hotplug functionality for removing Memory Nodes is available, a

similar exception is expected to apply there as well.  In general,

the kernel prefers to violate cpuset placement, over starving a task

that has had all its allowed CPUs or Memory Nodes taken offline.  User

code should reconfigure cpusets to only refer to online CPUs and Memory

Nodes when using hotplug to add or remove such resources.

There is a second exception to the above.  GFP_ATOMIC requests are

kernel internal allocations that must be satisfied, immediately.

The kernel may drop some request, in rare cases even panic, if a

GFP_ATOMIC alloc fails.  If the request cannot be satisfied within

the current tasks cpuset, then we relax the cpuset, and look for

memory anywhere we can find it.  It's better to violate the cpuset

than stress the kernel.

To start a new job that is to be contained within a cpuset, the steps are:

 1) mkdir /dev/cpuset

 2) mount -t cgroup -ocpuset cpuset /dev/cpuset

 3) Create the new cpuset by doing mkdir's and write's (or echo's) in

    the /dev/cpuset virtual file system.

 4) Start a task that will be the "founding father" of the new job.

 5) Attach that task to the new cpuset by writing its pid to the

    /dev/cpuset tasks file for that cpuset.

 6) fork, exec or clone the job tasks from this founding father task.

For example, the following sequence of commands will setup a cpuset

named "Charlie", containing just CPUs 2 and 3, and Memory Node 1,

and then start a subshell 'sh' in that cpuset:

  mount -t cgroup -ocpuset cpuset /dev/cpuset

  cd /dev/cpuset

  mkdir Charlie

  cd Charlie

  /bin/echo 2-3 > cpus

  /bin/echo 1 > mems

  /bin/echo $$ > tasks

  sh

  # The subshell 'sh' is now running in cpuset Charlie

  # The next line should display '/Charlie'

  cat /proc/self/cpuset

In the future, a C library interface to cpusets will likely be

available.  For now, the only way to query or modify cpusets is

via the cpuset file system, using the various cd, mkdir, echo, cat,

rmdir commands from the shell, or their equivalent from C.

The sched_setaffinity calls can also be done at the shell prompt using

SGI's runon or Robert Love's taskset.  The mbind and set_mempolicy

calls can be done at the shell prompt using the numactl command

(part of Andi Kleen's numa package).

2. Usage Examples and Syntax

============================

2.1 Basic Usage

---------------

Creating, modifying, using the cpusets can be done through the cpuset

virtual filesystem.

To mount it, type:

# mount -t cgroup -o cpuset cpuset /dev/cpuset

Then under /dev/cpuset you can find a tree that corresponds to the

tree of the cpusets in the system. For instance, /dev/cpuset

is the cpuset that holds the whole system.

If you want to create a new cpuset under /dev/cpuset:

# cd /dev/cpuset

# mkdir my_cpuset

Now you want to do something with this cpuset.

# cd my_cpuset

In this directory you can find several files:

# ls

cpus  cpu_exclusive  mems  mem_exclusive  tasks

Reading them will give you information about the state of this cpuset:

the CPUs and Memory Nodes it can use, the processes that are using

it, its properties.  By writing to these files you can manipulate

the cpuset.

Set some flags:

# /bin/echo 1 > cpu_exclusive

Add some cpus:

# /bin/echo 0-7 > cpus

Add some mems:

# /bin/echo 0-7 > mems

Now attach your shell to this cpuset:

# /bin/echo $$ > tasks

You can also create cpusets inside your cpuset by using mkdir in this

directory.

# mkdir my_sub_cs

To remove a cpuset, just use rmdir:

# rmdir my_sub_cs

This will fail if the cpuset is in use (has cpusets inside, or has

processes attached).

Note that for legacy reasons, the "cpuset" filesystem exists as a

wrapper around the cgroup filesystem.

The command

mount -t cpuset X /dev/cpuset

is equivalent to

mount -t cgroup -ocpuset X /dev/cpuset

echo "/sbin/cpuset_release_agent" > /dev/cpuset/release_agent

2.2 Adding/removing cpus

------------------------

This is the syntax to use when writing in the cpus or mems files

in cpuset directories:

# /bin/echo 1-4 > cpus          -> set cpus list to cpus 1,2,3,4

# /bin/echo 1,2,3,4 > cpus      -> set cpus list to cpus 1,2,3,4

2.3 Setting flags

-----------------

The syntax is very simple:

# /bin/echo 1 > cpu_exclusive   -> set flag 'cpu_exclusive'

# /bin/echo 0 > cpu_exclusive   -> unset flag 'cpu_exclusive'

2.4 Attaching processes

-----------------------

# /bin/echo PID > tasks

Note that it is PID, not PIDs. You can only attach ONE task at a time.

If you have several tasks to attach, you have to do it one after another:

# /bin/echo PID1 > tasks

# /bin/echo PID2 > tasks

        ...

# /bin/echo PIDn > tasks

3. Questions

============

Q: what's up with this '/bin/echo' ?

A: bash's builtin 'echo' command does not check calls to write() against

   errors. If you use it in the cpuset file system, you won't be

   able to tell whether a command succeeded or failed.

Q: When I attach processes, only the first of the line gets really attached !

A: We can only return one error code per call to write(). So you should also

   put only ONE pid.

4. Contact

==========

Web: http://www.bullopensource.org/cpuset