blob: 159e2a0c3e80fa229fd2ff95f0e2015caf017ab7 [file] [log] [blame]
Linus Torvalds1da177e2005-04-16 15:20:36 -07001 CPUSETS
2 -------
3
4Copyright (C) 2004 BULL SA.
5Written by Simon.Derr@bull.net
6
Christoph Lameterb4fb3762006-03-14 19:50:20 -08007Portions Copyright (c) 2004-2006 Silicon Graphics, Inc.
Linus Torvalds1da177e2005-04-16 15:20:36 -07008Modified by Paul Jackson <pj@sgi.com>
Christoph Lameterb4fb3762006-03-14 19:50:20 -08009Modified by Christoph Lameter <clameter@sgi.com>
Linus Torvalds1da177e2005-04-16 15:20:36 -070010
11CONTENTS:
12=========
13
141. Cpusets
15 1.1 What are cpusets ?
16 1.2 Why are cpusets needed ?
17 1.3 How are cpusets implemented ?
Paul Jacksonbd5e09c2006-01-08 01:01:50 -080018 1.4 What are exclusive cpusets ?
19 1.5 What does notify_on_release do ?
Paul Jackson90c9cc42006-01-08 01:01:51 -080020 1.6 What is memory_pressure ?
Paul Jackson825a46a2006-03-24 03:16:03 -080021 1.7 What is memory spread ?
22 1.8 How do I use cpusets ?
Linus Torvalds1da177e2005-04-16 15:20:36 -0700232. Usage Examples and Syntax
24 2.1 Basic Usage
25 2.2 Adding/removing cpus
26 2.3 Setting flags
27 2.4 Attaching processes
283. Questions
294. Contact
30
311. Cpusets
32==========
33
341.1 What are cpusets ?
35----------------------
36
37Cpusets provide a mechanism for assigning a set of CPUs and Memory
38Nodes to a set of tasks.
39
40Cpusets constrain the CPU and Memory placement of tasks to only
41the resources within a tasks current cpuset. They form a nested
42hierarchy visible in a virtual file system. These are the essential
43hooks, beyond what is already present, required to manage dynamic
44job placement on large systems.
45
46Each task has a pointer to a cpuset. Multiple tasks may reference
47the same cpuset. Requests by a task, using the sched_setaffinity(2)
48system call to include CPUs in its CPU affinity mask, and using the
49mbind(2) and set_mempolicy(2) system calls to include Memory Nodes
50in its memory policy, are both filtered through that tasks cpuset,
51filtering out any CPUs or Memory Nodes not in that cpuset. The
52scheduler will not schedule a task on a CPU that is not allowed in
53its cpus_allowed vector, and the kernel page allocator will not
54allocate a page on a node that is not allowed in the requesting tasks
55mems_allowed vector.
56
Linus Torvalds1da177e2005-04-16 15:20:36 -070057User level code may create and destroy cpusets by name in the cpuset
58virtual file system, manage the attributes and permissions of these
59cpusets and which CPUs and Memory Nodes are assigned to each cpuset,
60specify and query to which cpuset a task is assigned, and list the
61task pids assigned to a cpuset.
62
63
641.2 Why are cpusets needed ?
65----------------------------
66
67The management of large computer systems, with many processors (CPUs),
68complex memory cache hierarchies and multiple Memory Nodes having
69non-uniform access times (NUMA) presents additional challenges for
70the efficient scheduling and memory placement of processes.
71
72Frequently more modest sized systems can be operated with adequate
73efficiency just by letting the operating system automatically share
74the available CPU and Memory resources amongst the requesting tasks.
75
76But larger systems, which benefit more from careful processor and
77memory placement to reduce memory access times and contention,
78and which typically represent a larger investment for the customer,
Jean Delvare33430dc2005-10-30 15:02:20 -080079can benefit from explicitly placing jobs on properly sized subsets of
Linus Torvalds1da177e2005-04-16 15:20:36 -070080the system.
81
82This can be especially valuable on:
83
84 * Web Servers running multiple instances of the same web application,
85 * Servers running different applications (for instance, a web server
86 and a database), or
87 * NUMA systems running large HPC applications with demanding
88 performance characteristics.
Dinakar Guniguntala85d7b942005-06-25 14:57:34 -070089 * Also cpu_exclusive cpusets are useful for servers running orthogonal
90 workloads such as RT applications requiring low latency and HPC
91 applications that are throughput sensitive
Linus Torvalds1da177e2005-04-16 15:20:36 -070092
93These subsets, or "soft partitions" must be able to be dynamically
94adjusted, as the job mix changes, without impacting other concurrently
Christoph Lameterb4fb3762006-03-14 19:50:20 -080095executing jobs. The location of the running jobs pages may also be moved
96when the memory locations are changed.
Linus Torvalds1da177e2005-04-16 15:20:36 -070097
98The kernel cpuset patch provides the minimum essential kernel
99mechanisms required to efficiently implement such subsets. It
100leverages existing CPU and Memory Placement facilities in the Linux
101kernel to avoid any additional impact on the critical scheduler or
102memory allocator code.
103
104
1051.3 How are cpusets implemented ?
106---------------------------------
107
Christoph Lameterb4fb3762006-03-14 19:50:20 -0800108Cpusets provide a Linux kernel mechanism to constrain which CPUs and
109Memory Nodes are used by a process or set of processes.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700110
111The Linux kernel already has a pair of mechanisms to specify on which
112CPUs a task may be scheduled (sched_setaffinity) and on which Memory
113Nodes it may obtain memory (mbind, set_mempolicy).
114
115Cpusets extends these two mechanisms as follows:
116
117 - Cpusets are sets of allowed CPUs and Memory Nodes, known to the
118 kernel.
119 - Each task in the system is attached to a cpuset, via a pointer
120 in the task structure to a reference counted cpuset structure.
121 - Calls to sched_setaffinity are filtered to just those CPUs
122 allowed in that tasks cpuset.
123 - Calls to mbind and set_mempolicy are filtered to just
124 those Memory Nodes allowed in that tasks cpuset.
125 - The root cpuset contains all the systems CPUs and Memory
126 Nodes.
127 - For any cpuset, one can define child cpusets containing a subset
128 of the parents CPU and Memory Node resources.
129 - The hierarchy of cpusets can be mounted at /dev/cpuset, for
130 browsing and manipulation from user space.
131 - A cpuset may be marked exclusive, which ensures that no other
132 cpuset (except direct ancestors and descendents) may contain
133 any overlapping CPUs or Memory Nodes.
Dinakar Guniguntala85d7b942005-06-25 14:57:34 -0700134 Also a cpu_exclusive cpuset would be associated with a sched
135 domain.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700136 - You can list all the tasks (by pid) attached to any cpuset.
137
138The implementation of cpusets requires a few, simple hooks
139into the rest of the kernel, none in performance critical paths:
140
Paul Jackson864913f2006-01-11 02:01:38 +0100141 - in init/main.c, to initialize the root cpuset at system boot.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700142 - in fork and exit, to attach and detach a task from its cpuset.
143 - in sched_setaffinity, to mask the requested CPUs by what's
144 allowed in that tasks cpuset.
145 - in sched.c migrate_all_tasks(), to keep migrating tasks within
146 the CPUs allowed by their cpuset, if possible.
Dinakar Guniguntala85d7b942005-06-25 14:57:34 -0700147 - in sched.c, a new API partition_sched_domains for handling
148 sched domain changes associated with cpu_exclusive cpusets
149 and related changes in both sched.c and arch/ia64/kernel/domain.c
Linus Torvalds1da177e2005-04-16 15:20:36 -0700150 - in the mbind and set_mempolicy system calls, to mask the requested
151 Memory Nodes by what's allowed in that tasks cpuset.
Paul Jackson864913f2006-01-11 02:01:38 +0100152 - in page_alloc.c, to restrict memory to allowed nodes.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700153 - in vmscan.c, to restrict page recovery to the current cpuset.
154
155In addition a new file system, of type "cpuset" may be mounted,
156typically at /dev/cpuset, to enable browsing and modifying the cpusets
157presently known to the kernel. No new system calls are added for
158cpusets - all support for querying and modifying cpusets is via
159this cpuset file system.
160
161Each task under /proc has an added file named 'cpuset', displaying
162the cpuset name, as the path relative to the root of the cpuset file
163system.
164
165The /proc/<pid>/status file for each task has two added lines,
166displaying the tasks cpus_allowed (on which CPUs it may be scheduled)
167and mems_allowed (on which Memory Nodes it may obtain memory),
168in the format seen in the following example:
169
170 Cpus_allowed: ffffffff,ffffffff,ffffffff,ffffffff
171 Mems_allowed: ffffffff,ffffffff
172
173Each cpuset is represented by a directory in the cpuset file system
174containing the following files describing that cpuset:
175
176 - cpus: list of CPUs in that cpuset
177 - mems: list of Memory Nodes in that cpuset
Paul Jackson45b07ef2006-01-08 01:00:56 -0800178 - memory_migrate flag: if set, move pages to cpusets nodes
Linus Torvalds1da177e2005-04-16 15:20:36 -0700179 - cpu_exclusive flag: is cpu placement exclusive?
180 - mem_exclusive flag: is memory placement exclusive?
181 - tasks: list of tasks (by pid) attached to that cpuset
Paul Jacksonbd5e09c2006-01-08 01:01:50 -0800182 - notify_on_release flag: run /sbin/cpuset_release_agent on exit?
Paul Jacksonbd5e09c2006-01-08 01:01:50 -0800183 - memory_pressure: measure of how much paging pressure in cpuset
184
185In addition, the root cpuset only has the following file:
186 - memory_pressure_enabled flag: compute memory_pressure?
Linus Torvalds1da177e2005-04-16 15:20:36 -0700187
188New cpusets are created using the mkdir system call or shell
189command. The properties of a cpuset, such as its flags, allowed
190CPUs and Memory Nodes, and attached tasks, are modified by writing
191to the appropriate file in that cpusets directory, as listed above.
192
193The named hierarchical structure of nested cpusets allows partitioning
194a large system into nested, dynamically changeable, "soft-partitions".
195
196The attachment of each task, automatically inherited at fork by any
197children of that task, to a cpuset allows organizing the work load
198on a system into related sets of tasks such that each set is constrained
199to using the CPUs and Memory Nodes of a particular cpuset. A task
200may be re-attached to any other cpuset, if allowed by the permissions
201on the necessary cpuset file system directories.
202
203Such management of a system "in the large" integrates smoothly with
204the detailed placement done on individual tasks and memory regions
205using the sched_setaffinity, mbind and set_mempolicy system calls.
206
207The following rules apply to each cpuset:
208
209 - Its CPUs and Memory Nodes must be a subset of its parents.
210 - It can only be marked exclusive if its parent is.
211 - If its cpu or memory is exclusive, they may not overlap any sibling.
212
213These rules, and the natural hierarchy of cpusets, enable efficient
214enforcement of the exclusive guarantee, without having to scan all
215cpusets every time any of them change to ensure nothing overlaps a
216exclusive cpuset. Also, the use of a Linux virtual file system (vfs)
217to represent the cpuset hierarchy provides for a familiar permission
218and name space for cpusets, with a minimum of additional kernel code.
219
Paul Jacksonbd5e09c2006-01-08 01:01:50 -0800220
2211.4 What are exclusive cpusets ?
222--------------------------------
223
224If a cpuset is cpu or mem exclusive, no other cpuset, other than
225a direct ancestor or descendent, may share any of the same CPUs or
226Memory Nodes.
227
228A cpuset that is cpu_exclusive has a scheduler (sched) domain
229associated with it. The sched domain consists of all CPUs in the
230current cpuset that are not part of any exclusive child cpusets.
231This ensures that the scheduler load balancing code only balances
232against the CPUs that are in the sched domain as defined above and
233not all of the CPUs in the system. This removes any overhead due to
234load balancing code trying to pull tasks outside of the cpu_exclusive
235cpuset only to be prevented by the tasks' cpus_allowed mask.
236
237A cpuset that is mem_exclusive restricts kernel allocations for
238page, buffer and other data commonly shared by the kernel across
239multiple users. All cpusets, whether mem_exclusive or not, restrict
240allocations of memory for user space. This enables configuring a
241system so that several independent jobs can share common kernel data,
242such as file system pages, while isolating each jobs user allocation in
243its own cpuset. To do this, construct a large mem_exclusive cpuset to
244hold all the jobs, and construct child, non-mem_exclusive cpusets for
245each individual job. Only a small amount of typical kernel memory,
246such as requests from interrupt handlers, is allowed to be taken
247outside even a mem_exclusive cpuset.
248
249
2501.5 What does notify_on_release do ?
251------------------------------------
252
253If the notify_on_release flag is enabled (1) in a cpuset, then whenever
254the last task in the cpuset leaves (exits or attaches to some other
255cpuset) and the last child cpuset of that cpuset is removed, then
256the kernel runs the command /sbin/cpuset_release_agent, supplying the
257pathname (relative to the mount point of the cpuset file system) of the
258abandoned cpuset. This enables automatic removal of abandoned cpusets.
259The default value of notify_on_release in the root cpuset at system
260boot is disabled (0). The default value of other cpusets at creation
261is the current value of their parents notify_on_release setting.
262
263
Paul Jackson90c9cc42006-01-08 01:01:51 -08002641.6 What is memory_pressure ?
Paul Jacksonbd5e09c2006-01-08 01:01:50 -0800265-----------------------------
266The memory_pressure of a cpuset provides a simple per-cpuset metric
267of the rate that the tasks in a cpuset are attempting to free up in
268use memory on the nodes of the cpuset to satisfy additional memory
269requests.
270
271This enables batch managers monitoring jobs running in dedicated
272cpusets to efficiently detect what level of memory pressure that job
273is causing.
274
275This is useful both on tightly managed systems running a wide mix of
276submitted jobs, which may choose to terminate or re-prioritize jobs that
277are trying to use more memory than allowed on the nodes assigned them,
278and with tightly coupled, long running, massively parallel scientific
279computing jobs that will dramatically fail to meet required performance
280goals if they start to use more memory than allowed to them.
281
282This mechanism provides a very economical way for the batch manager
283to monitor a cpuset for signs of memory pressure. It's up to the
284batch manager or other user code to decide what to do about it and
285take action.
286
287==> Unless this feature is enabled by writing "1" to the special file
288 /dev/cpuset/memory_pressure_enabled, the hook in the rebalance
289 code of __alloc_pages() for this metric reduces to simply noticing
290 that the cpuset_memory_pressure_enabled flag is zero. So only
291 systems that enable this feature will compute the metric.
292
293Why a per-cpuset, running average:
294
295 Because this meter is per-cpuset, rather than per-task or mm,
296 the system load imposed by a batch scheduler monitoring this
297 metric is sharply reduced on large systems, because a scan of
298 the tasklist can be avoided on each set of queries.
299
300 Because this meter is a running average, instead of an accumulating
301 counter, a batch scheduler can detect memory pressure with a
302 single read, instead of having to read and accumulate results
303 for a period of time.
304
305 Because this meter is per-cpuset rather than per-task or mm,
306 the batch scheduler can obtain the key information, memory
307 pressure in a cpuset, with a single read, rather than having to
308 query and accumulate results over all the (dynamically changing)
309 set of tasks in the cpuset.
310
311A per-cpuset simple digital filter (requires a spinlock and 3 words
312of data per-cpuset) is kept, and updated by any task attached to that
313cpuset, if it enters the synchronous (direct) page reclaim code.
314
315A per-cpuset file provides an integer number representing the recent
316(half-life of 10 seconds) rate of direct page reclaims caused by
317the tasks in the cpuset, in units of reclaims attempted per second,
318times 1000.
319
320
Paul Jackson825a46a2006-03-24 03:16:03 -08003211.7 What is memory spread ?
322---------------------------
323There are two boolean flag files per cpuset that control where the
324kernel allocates pages for the file system buffers and related in
325kernel data structures. They are called 'memory_spread_page' and
326'memory_spread_slab'.
327
328If the per-cpuset boolean flag file 'memory_spread_page' is set, then
329the kernel will spread the file system buffers (page cache) evenly
330over all the nodes that the faulting task is allowed to use, instead
331of preferring to put those pages on the node where the task is running.
332
333If the per-cpuset boolean flag file 'memory_spread_slab' is set,
334then the kernel will spread some file system related slab caches,
335such as for inodes and dentries evenly over all the nodes that the
336faulting task is allowed to use, instead of preferring to put those
337pages on the node where the task is running.
338
339The setting of these flags does not affect anonymous data segment or
340stack segment pages of a task.
341
342By default, both kinds of memory spreading are off, and memory
343pages are allocated on the node local to where the task is running,
344except perhaps as modified by the tasks NUMA mempolicy or cpuset
345configuration, so long as sufficient free memory pages are available.
346
347When new cpusets are created, they inherit the memory spread settings
348of their parent.
349
350Setting memory spreading causes allocations for the affected page
351or slab caches to ignore the tasks NUMA mempolicy and be spread
352instead. Tasks using mbind() or set_mempolicy() calls to set NUMA
353mempolicies will not notice any change in these calls as a result of
354their containing tasks memory spread settings. If memory spreading
355is turned off, then the currently specified NUMA mempolicy once again
356applies to memory page allocations.
357
358Both 'memory_spread_page' and 'memory_spread_slab' are boolean flag
359files. By default they contain "0", meaning that the feature is off
360for that cpuset. If a "1" is written to that file, then that turns
361the named feature on.
362
363The implementation is simple.
364
365Setting the flag 'memory_spread_page' turns on a per-process flag
366PF_SPREAD_PAGE for each task that is in that cpuset or subsequently
367joins that cpuset. The page allocation calls for the page cache
368is modified to perform an inline check for this PF_SPREAD_PAGE task
369flag, and if set, a call to a new routine cpuset_mem_spread_node()
370returns the node to prefer for the allocation.
371
372Similarly, setting 'memory_spread_cache' turns on the flag
373PF_SPREAD_SLAB, and appropriately marked slab caches will allocate
374pages from the node returned by cpuset_mem_spread_node().
375
376The cpuset_mem_spread_node() routine is also simple. It uses the
377value of a per-task rotor cpuset_mem_spread_rotor to select the next
378node in the current tasks mems_allowed to prefer for the allocation.
379
380This memory placement policy is also known (in other contexts) as
381round-robin or interleave.
382
383This policy can provide substantial improvements for jobs that need
384to place thread local data on the corresponding node, but that need
385to access large file system data sets that need to be spread across
386the several nodes in the jobs cpuset in order to fit. Without this
387policy, especially for jobs that might have one thread reading in the
388data set, the memory allocation across the nodes in the jobs cpuset
389can become very uneven.
390
391
3921.8 How do I use cpusets ?
Linus Torvalds1da177e2005-04-16 15:20:36 -0700393--------------------------
394
395In order to minimize the impact of cpusets on critical kernel
396code, such as the scheduler, and due to the fact that the kernel
397does not support one task updating the memory placement of another
398task directly, the impact on a task of changing its cpuset CPU
399or Memory Node placement, or of changing to which cpuset a task
400is attached, is subtle.
401
402If a cpuset has its Memory Nodes modified, then for each task attached
403to that cpuset, the next time that the kernel attempts to allocate
404a page of memory for that task, the kernel will notice the change
405in the tasks cpuset, and update its per-task memory placement to
406remain within the new cpusets memory placement. If the task was using
407mempolicy MPOL_BIND, and the nodes to which it was bound overlap with
408its new cpuset, then the task will continue to use whatever subset
409of MPOL_BIND nodes are still allowed in the new cpuset. If the task
410was using MPOL_BIND and now none of its MPOL_BIND nodes are allowed
411in the new cpuset, then the task will be essentially treated as if it
412was MPOL_BIND bound to the new cpuset (even though its numa placement,
413as queried by get_mempolicy(), doesn't change). If a task is moved
414from one cpuset to another, then the kernel will adjust the tasks
415memory placement, as above, the next time that the kernel attempts
416to allocate a page of memory for that task.
417
418If a cpuset has its CPUs modified, then each task using that
419cpuset does _not_ change its behavior automatically. In order to
420minimize the impact on the critical scheduling code in the kernel,
421tasks will continue to use their prior CPU placement until they
422are rebound to their cpuset, by rewriting their pid to the 'tasks'
423file of their cpuset. If a task had been bound to some subset of its
424cpuset using the sched_setaffinity() call, and if any of that subset
425is still allowed in its new cpuset settings, then the task will be
426restricted to the intersection of the CPUs it was allowed on before,
427and its new cpuset CPU placement. If, on the other hand, there is
428no overlap between a tasks prior placement and its new cpuset CPU
429placement, then the task will be allowed to run on any CPU allowed
430in its new cpuset. If a task is moved from one cpuset to another,
431its CPU placement is updated in the same way as if the tasks pid is
432rewritten to the 'tasks' file of its current cpuset.
433
434In summary, the memory placement of a task whose cpuset is changed is
435updated by the kernel, on the next allocation of a page for that task,
436but the processor placement is not updated, until that tasks pid is
437rewritten to the 'tasks' file of its cpuset. This is done to avoid
438impacting the scheduler code in the kernel with a check for changes
439in a tasks processor placement.
440
Paul Jackson45b07ef2006-01-08 01:00:56 -0800441Normally, once a page is allocated (given a physical page
442of main memory) then that page stays on whatever node it
443was allocated, so long as it remains allocated, even if the
444cpusets memory placement policy 'mems' subsequently changes.
445If the cpuset flag file 'memory_migrate' is set true, then when
446tasks are attached to that cpuset, any pages that task had
447allocated to it on nodes in its previous cpuset are migrated
Christoph Lameterb4fb3762006-03-14 19:50:20 -0800448to the tasks new cpuset. The relative placement of the page within
449the cpuset is preserved during these migration operations if possible.
450For example if the page was on the second valid node of the prior cpuset
451then the page will be placed on the second valid node of the new cpuset.
452
Paul Jackson45b07ef2006-01-08 01:00:56 -0800453Also if 'memory_migrate' is set true, then if that cpusets
454'mems' file is modified, pages allocated to tasks in that
455cpuset, that were on nodes in the previous setting of 'mems',
Christoph Lameterb4fb3762006-03-14 19:50:20 -0800456will be moved to nodes in the new setting of 'mems.'
457Pages that were not in the tasks prior cpuset, or in the cpusets
458prior 'mems' setting, will not be moved.
Paul Jackson45b07ef2006-01-08 01:00:56 -0800459
Tobias Klauserd533f672005-09-10 00:26:46 -0700460There is an exception to the above. If hotplug functionality is used
Linus Torvalds1da177e2005-04-16 15:20:36 -0700461to remove all the CPUs that are currently assigned to a cpuset,
462then the kernel will automatically update the cpus_allowed of all
Paul Jacksonb39c4fa2005-05-20 13:59:15 -0700463tasks attached to CPUs in that cpuset to allow all CPUs. When memory
Linus Torvalds1da177e2005-04-16 15:20:36 -0700464hotplug functionality for removing Memory Nodes is available, a
465similar exception is expected to apply there as well. In general,
466the kernel prefers to violate cpuset placement, over starving a task
467that has had all its allowed CPUs or Memory Nodes taken offline. User
468code should reconfigure cpusets to only refer to online CPUs and Memory
469Nodes when using hotplug to add or remove such resources.
470
471There is a second exception to the above. GFP_ATOMIC requests are
472kernel internal allocations that must be satisfied, immediately.
473The kernel may drop some request, in rare cases even panic, if a
474GFP_ATOMIC alloc fails. If the request cannot be satisfied within
475the current tasks cpuset, then we relax the cpuset, and look for
476memory anywhere we can find it. It's better to violate the cpuset
477than stress the kernel.
478
479To start a new job that is to be contained within a cpuset, the steps are:
480
481 1) mkdir /dev/cpuset
482 2) mount -t cpuset none /dev/cpuset
483 3) Create the new cpuset by doing mkdir's and write's (or echo's) in
484 the /dev/cpuset virtual file system.
485 4) Start a task that will be the "founding father" of the new job.
486 5) Attach that task to the new cpuset by writing its pid to the
487 /dev/cpuset tasks file for that cpuset.
488 6) fork, exec or clone the job tasks from this founding father task.
489
490For example, the following sequence of commands will setup a cpuset
491named "Charlie", containing just CPUs 2 and 3, and Memory Node 1,
492and then start a subshell 'sh' in that cpuset:
493
494 mount -t cpuset none /dev/cpuset
495 cd /dev/cpuset
496 mkdir Charlie
497 cd Charlie
498 /bin/echo 2-3 > cpus
499 /bin/echo 1 > mems
500 /bin/echo $$ > tasks
501 sh
502 # The subshell 'sh' is now running in cpuset Charlie
503 # The next line should display '/Charlie'
504 cat /proc/self/cpuset
505
Linus Torvalds1da177e2005-04-16 15:20:36 -0700506In the future, a C library interface to cpusets will likely be
507available. For now, the only way to query or modify cpusets is
508via the cpuset file system, using the various cd, mkdir, echo, cat,
509rmdir commands from the shell, or their equivalent from C.
510
511The sched_setaffinity calls can also be done at the shell prompt using
512SGI's runon or Robert Love's taskset. The mbind and set_mempolicy
513calls can be done at the shell prompt using the numactl command
514(part of Andi Kleen's numa package).
515
5162. Usage Examples and Syntax
517============================
518
5192.1 Basic Usage
520---------------
521
522Creating, modifying, using the cpusets can be done through the cpuset
523virtual filesystem.
524
525To mount it, type:
526# mount -t cpuset none /dev/cpuset
527
528Then under /dev/cpuset you can find a tree that corresponds to the
529tree of the cpusets in the system. For instance, /dev/cpuset
530is the cpuset that holds the whole system.
531
532If you want to create a new cpuset under /dev/cpuset:
533# cd /dev/cpuset
534# mkdir my_cpuset
535
536Now you want to do something with this cpuset.
537# cd my_cpuset
538
539In this directory you can find several files:
540# ls
541cpus cpu_exclusive mems mem_exclusive tasks
542
543Reading them will give you information about the state of this cpuset:
544the CPUs and Memory Nodes it can use, the processes that are using
545it, its properties. By writing to these files you can manipulate
546the cpuset.
547
548Set some flags:
549# /bin/echo 1 > cpu_exclusive
550
551Add some cpus:
552# /bin/echo 0-7 > cpus
553
554Now attach your shell to this cpuset:
555# /bin/echo $$ > tasks
556
557You can also create cpusets inside your cpuset by using mkdir in this
558directory.
559# mkdir my_sub_cs
560
561To remove a cpuset, just use rmdir:
562# rmdir my_sub_cs
563This will fail if the cpuset is in use (has cpusets inside, or has
564processes attached).
565
5662.2 Adding/removing cpus
567------------------------
568
569This is the syntax to use when writing in the cpus or mems files
570in cpuset directories:
571
572# /bin/echo 1-4 > cpus -> set cpus list to cpus 1,2,3,4
573# /bin/echo 1,2,3,4 > cpus -> set cpus list to cpus 1,2,3,4
574
5752.3 Setting flags
576-----------------
577
578The syntax is very simple:
579
580# /bin/echo 1 > cpu_exclusive -> set flag 'cpu_exclusive'
581# /bin/echo 0 > cpu_exclusive -> unset flag 'cpu_exclusive'
582
5832.4 Attaching processes
584-----------------------
585
586# /bin/echo PID > tasks
587
588Note that it is PID, not PIDs. You can only attach ONE task at a time.
589If you have several tasks to attach, you have to do it one after another:
590
591# /bin/echo PID1 > tasks
592# /bin/echo PID2 > tasks
593 ...
594# /bin/echo PIDn > tasks
595
596
5973. Questions
598============
599
600Q: what's up with this '/bin/echo' ?
601A: bash's builtin 'echo' command does not check calls to write() against
602 errors. If you use it in the cpuset file system, you won't be
603 able to tell whether a command succeeded or failed.
604
605Q: When I attach processes, only the first of the line gets really attached !
606A: We can only return one error code per call to write(). So you should also
607 put only ONE pid.
608
6094. Contact
610==========
611
612Web: http://www.bullopensource.org/cpuset