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Based on kernel version 2.6.27. Page generated on 2008-10-13 09:53 EST.

1					CPUSETS
2					-------
3	
4	Copyright (C) 2004 BULL SA.
5	Written by Simon.Derr[AT]bull[DOT]net
6	
7	Portions Copyright (c) 2004-2006 Silicon Graphics, Inc.
8	Modified by Paul Jackson <pj[AT]sgi[DOT]com>
9	Modified by Christoph Lameter <clameter[AT]sgi[DOT]com>
10	Modified by Paul Menage <menage[AT]google[DOT]com>
11	Modified by Hidetoshi Seto <seto.hidetoshi[AT]jp.fujitsu[DOT]com>
12	
13	CONTENTS:
14	=========
15	
16	1. Cpusets
17	  1.1 What are cpusets ?
18	  1.2 Why are cpusets needed ?
19	  1.3 How are cpusets implemented ?
20	  1.4 What are exclusive cpusets ?
21	  1.5 What is memory_pressure ?
22	  1.6 What is memory spread ?
23	  1.7 What is sched_load_balance ?
24	  1.8 What is sched_relax_domain_level ?
25	  1.9 How do I use cpusets ?
26	2. Usage Examples and Syntax
27	  2.1 Basic Usage
28	  2.2 Adding/removing cpus
29	  2.3 Setting flags
30	  2.4 Attaching processes
31	3. Questions
32	4. Contact
33	
34	1. Cpusets
35	==========
36	
37	1.1 What are cpusets ?
38	----------------------
39	
40	Cpusets provide a mechanism for assigning a set of CPUs and Memory
41	Nodes to a set of tasks.   In this document "Memory Node" refers to
42	an on-line node that contains memory.
43	
44	Cpusets constrain the CPU and Memory placement of tasks to only
45	the resources within a tasks current cpuset.  They form a nested
46	hierarchy visible in a virtual file system.  These are the essential
47	hooks, beyond what is already present, required to manage dynamic
48	job placement on large systems.
49	
50	Cpusets use the generic cgroup subsystem described in
51	Documentation/cgroup.txt.
52	
53	Requests by a task, using the sched_setaffinity(2) system call to
54	include CPUs in its CPU affinity mask, and using the mbind(2) and
55	set_mempolicy(2) system calls to include Memory Nodes in its memory
56	policy, are both filtered through that tasks cpuset, filtering out any
57	CPUs or Memory Nodes not in that cpuset.  The scheduler will not
58	schedule a task on a CPU that is not allowed in its cpus_allowed
59	vector, and the kernel page allocator will not allocate a page on a
60	node that is not allowed in the requesting tasks mems_allowed vector.
61	
62	User level code may create and destroy cpusets by name in the cgroup
63	virtual file system, manage the attributes and permissions of these
64	cpusets and which CPUs and Memory Nodes are assigned to each cpuset,
65	specify and query to which cpuset a task is assigned, and list the
66	task pids assigned to a cpuset.
67	
68	
69	1.2 Why are cpusets needed ?
70	----------------------------
71	
72	The management of large computer systems, with many processors (CPUs),
73	complex memory cache hierarchies and multiple Memory Nodes having
74	non-uniform access times (NUMA) presents additional challenges for
75	the efficient scheduling and memory placement of processes.
76	
77	Frequently more modest sized systems can be operated with adequate
78	efficiency just by letting the operating system automatically share
79	the available CPU and Memory resources amongst the requesting tasks.
80	
81	But larger systems, which benefit more from careful processor and
82	memory placement to reduce memory access times and contention,
83	and which typically represent a larger investment for the customer,
84	can benefit from explicitly placing jobs on properly sized subsets of
85	the system.
86	
87	This can be especially valuable on:
88	
89	    * Web Servers running multiple instances of the same web application,
90	    * Servers running different applications (for instance, a web server
91	      and a database), or
92	    * NUMA systems running large HPC applications with demanding
93	      performance characteristics.
94	
95	These subsets, or "soft partitions" must be able to be dynamically
96	adjusted, as the job mix changes, without impacting other concurrently
97	executing jobs. The location of the running jobs pages may also be moved
98	when the memory locations are changed.
99	
100	The kernel cpuset patch provides the minimum essential kernel
101	mechanisms required to efficiently implement such subsets.  It
102	leverages existing CPU and Memory Placement facilities in the Linux
103	kernel to avoid any additional impact on the critical scheduler or
104	memory allocator code.
105	
106	
107	1.3 How are cpusets implemented ?
108	---------------------------------
109	
110	Cpusets provide a Linux kernel mechanism to constrain which CPUs and
111	Memory Nodes are used by a process or set of processes.
112	
113	The Linux kernel already has a pair of mechanisms to specify on which
114	CPUs a task may be scheduled (sched_setaffinity) and on which Memory
115	Nodes it may obtain memory (mbind, set_mempolicy).
116	
117	Cpusets extends these two mechanisms as follows:
118	
119	 - Cpusets are sets of allowed CPUs and Memory Nodes, known to the
120	   kernel.
121	 - Each task in the system is attached to a cpuset, via a pointer
122	   in the task structure to a reference counted cgroup structure.
123	 - Calls to sched_setaffinity are filtered to just those CPUs
124	   allowed in that tasks cpuset.
125	 - Calls to mbind and set_mempolicy are filtered to just
126	   those Memory Nodes allowed in that tasks cpuset.
127	 - The root cpuset contains all the systems CPUs and Memory
128	   Nodes.
129	 - For any cpuset, one can define child cpusets containing a subset
130	   of the parents CPU and Memory Node resources.
131	 - The hierarchy of cpusets can be mounted at /dev/cpuset, for
132	   browsing and manipulation from user space.
133	 - A cpuset may be marked exclusive, which ensures that no other
134	   cpuset (except direct ancestors and descendents) may contain
135	   any overlapping CPUs or Memory Nodes.
136	 - You can list all the tasks (by pid) attached to any cpuset.
137	
138	The implementation of cpusets requires a few, simple hooks
139	into the rest of the kernel, none in performance critical paths:
140	
141	 - in init/main.c, to initialize the root cpuset at system boot.
142	 - 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.
147	 - in the mbind and set_mempolicy system calls, to mask the requested
148	   Memory Nodes by what's allowed in that tasks cpuset.
149	 - in page_alloc.c, to restrict memory to allowed nodes.
150	 - in vmscan.c, to restrict page recovery to the current cpuset.
151	
152	You should mount the "cgroup" filesystem type in order to enable
153	browsing and modifying the cpusets presently known to the kernel.  No
154	new system calls are added for cpusets - all support for querying and
155	modifying cpusets is via this cpuset file system.
156	
157	The /proc/<pid>/status file for each task has four added lines,
158	displaying the tasks cpus_allowed (on which CPUs it may be scheduled)
159	and mems_allowed (on which Memory Nodes it may obtain memory),
160	in the two formats seen in the following example:
161	
162	  Cpus_allowed:   ffffffff,ffffffff,ffffffff,ffffffff
163	  Cpus_allowed_list:      0-127
164	  Mems_allowed:   ffffffff,ffffffff
165	  Mems_allowed_list:      0-63
166	
167	Each cpuset is represented by a directory in the cgroup file system
168	containing (on top of the standard cgroup files) the following
169	files describing that cpuset:
170	
171	 - cpus: list of CPUs in that cpuset
172	 - mems: list of Memory Nodes in that cpuset
173	 - memory_migrate flag: if set, move pages to cpusets nodes
174	 - cpu_exclusive flag: is cpu placement exclusive?
175	 - mem_exclusive flag: is memory placement exclusive?
176	 - mem_hardwall flag:  is memory allocation hardwalled
177	 - memory_pressure: measure of how much paging pressure in cpuset
178	
179	In addition, the root cpuset only has the following file:
180	 - memory_pressure_enabled flag: compute memory_pressure?
181	
182	New cpusets are created using the mkdir system call or shell
183	command.  The properties of a cpuset, such as its flags, allowed
184	CPUs and Memory Nodes, and attached tasks, are modified by writing
185	to the appropriate file in that cpusets directory, as listed above.
186	
187	The named hierarchical structure of nested cpusets allows partitioning
188	a large system into nested, dynamically changeable, "soft-partitions".
189	
190	The attachment of each task, automatically inherited at fork by any
191	children of that task, to a cpuset allows organizing the work load
192	on a system into related sets of tasks such that each set is constrained
193	to using the CPUs and Memory Nodes of a particular cpuset.  A task
194	may be re-attached to any other cpuset, if allowed by the permissions
195	on the necessary cpuset file system directories.
196	
197	Such management of a system "in the large" integrates smoothly with
198	the detailed placement done on individual tasks and memory regions
199	using the sched_setaffinity, mbind and set_mempolicy system calls.
200	
201	The following rules apply to each cpuset:
202	
203	 - Its CPUs and Memory Nodes must be a subset of its parents.
204	 - It can't be marked exclusive unless its parent is.
205	 - If its cpu or memory is exclusive, they may not overlap any sibling.
206	
207	These rules, and the natural hierarchy of cpusets, enable efficient
208	enforcement of the exclusive guarantee, without having to scan all
209	cpusets every time any of them change to ensure nothing overlaps a
210	exclusive cpuset.  Also, the use of a Linux virtual file system (vfs)
211	to represent the cpuset hierarchy provides for a familiar permission
212	and name space for cpusets, with a minimum of additional kernel code.
213	
214	The cpus and mems files in the root (top_cpuset) cpuset are
215	read-only.  The cpus file automatically tracks the value of
216	cpu_online_map using a CPU hotplug notifier, and the mems file
217	automatically tracks the value of node_states[N_HIGH_MEMORY]--i.e.,
218	nodes with memory--using the cpuset_track_online_nodes() hook.
219	
220	
221	1.4 What are exclusive cpusets ?
222	--------------------------------
223	
224	If a cpuset is cpu or mem exclusive, no other cpuset, other than
225	a direct ancestor or descendent, may share any of the same CPUs or
226	Memory Nodes.
227	
228	A cpuset that is mem_exclusive *or* mem_hardwall is "hardwalled",
229	i.e. it restricts kernel allocations for page, buffer and other data
230	commonly shared by the kernel across multiple users.  All cpusets,
231	whether hardwalled or not, restrict allocations of memory for user
232	space.  This enables configuring a system so that several independent
233	jobs can share common kernel data, such as file system pages, while
234	isolating each job's user allocation in its own cpuset.  To do this,
235	construct a large mem_exclusive cpuset to hold all the jobs, and
236	construct child, non-mem_exclusive cpusets for each individual job.
237	Only a small amount of typical kernel memory, such as requests from
238	interrupt handlers, is allowed to be taken outside even a
239	mem_exclusive cpuset.
240	
241	
242	1.5 What is memory_pressure ?
243	-----------------------------
244	The memory_pressure of a cpuset provides a simple per-cpuset metric
245	of the rate that the tasks in a cpuset are attempting to free up in
246	use memory on the nodes of the cpuset to satisfy additional memory
247	requests.
248	
249	This enables batch managers monitoring jobs running in dedicated
250	cpusets to efficiently detect what level of memory pressure that job
251	is causing.
252	
253	This is useful both on tightly managed systems running a wide mix of
254	submitted jobs, which may choose to terminate or re-prioritize jobs that
255	are trying to use more memory than allowed on the nodes assigned them,
256	and with tightly coupled, long running, massively parallel scientific
257	computing jobs that will dramatically fail to meet required performance
258	goals if they start to use more memory than allowed to them.
259	
260	This mechanism provides a very economical way for the batch manager
261	to monitor a cpuset for signs of memory pressure.  It's up to the
262	batch manager or other user code to decide what to do about it and
263	take action.
264	
265	==> Unless this feature is enabled by writing "1" to the special file
266	    /dev/cpuset/memory_pressure_enabled, the hook in the rebalance
267	    code of __alloc_pages() for this metric reduces to simply noticing
268	    that the cpuset_memory_pressure_enabled flag is zero.  So only
269	    systems that enable this feature will compute the metric.
270	
271	Why a per-cpuset, running average:
272	
273	    Because this meter is per-cpuset, rather than per-task or mm,
274	    the system load imposed by a batch scheduler monitoring this
275	    metric is sharply reduced on large systems, because a scan of
276	    the tasklist can be avoided on each set of queries.
277	
278	    Because this meter is a running average, instead of an accumulating
279	    counter, a batch scheduler can detect memory pressure with a
280	    single read, instead of having to read and accumulate results
281	    for a period of time.
282	
283	    Because this meter is per-cpuset rather than per-task or mm,
284	    the batch scheduler can obtain the key information, memory
285	    pressure in a cpuset, with a single read, rather than having to
286	    query and accumulate results over all the (dynamically changing)
287	    set of tasks in the cpuset.
288	
289	A per-cpuset simple digital filter (requires a spinlock and 3 words
290	of data per-cpuset) is kept, and updated by any task attached to that
291	cpuset, if it enters the synchronous (direct) page reclaim code.
292	
293	A per-cpuset file provides an integer number representing the recent
294	(half-life of 10 seconds) rate of direct page reclaims caused by
295	the tasks in the cpuset, in units of reclaims attempted per second,
296	times 1000.
297	
298	
299	1.6 What is memory spread ?
300	---------------------------
301	There are two boolean flag files per cpuset that control where the
302	kernel allocates pages for the file system buffers and related in
303	kernel data structures.  They are called 'memory_spread_page' and
304	'memory_spread_slab'.
305	
306	If the per-cpuset boolean flag file 'memory_spread_page' is set, then
307	the kernel will spread the file system buffers (page cache) evenly
308	over all the nodes that the faulting task is allowed to use, instead
309	of preferring to put those pages on the node where the task is running.
310	
311	If the per-cpuset boolean flag file 'memory_spread_slab' is set,
312	then the kernel will spread some file system related slab caches,
313	such as for inodes and dentries evenly over all the nodes that the
314	faulting task is allowed to use, instead of preferring to put those
315	pages on the node where the task is running.
316	
317	The setting of these flags does not affect anonymous data segment or
318	stack segment pages of a task.
319	
320	By default, both kinds of memory spreading are off, and memory
321	pages are allocated on the node local to where the task is running,
322	except perhaps as modified by the tasks NUMA mempolicy or cpuset
323	configuration, so long as sufficient free memory pages are available.
324	
325	When new cpusets are created, they inherit the memory spread settings
326	of their parent.
327	
328	Setting memory spreading causes allocations for the affected page
329	or slab caches to ignore the tasks NUMA mempolicy and be spread
330	instead.    Tasks using mbind() or set_mempolicy() calls to set NUMA
331	mempolicies will not notice any change in these calls as a result of
332	their containing tasks memory spread settings.  If memory spreading
333	is turned off, then the currently specified NUMA mempolicy once again
334	applies to memory page allocations.
335	
336	Both 'memory_spread_page' and 'memory_spread_slab' are boolean flag
337	files.  By default they contain "0", meaning that the feature is off
338	for that cpuset.  If a "1" is written to that file, then that turns
339	the named feature on.
340	
341	The implementation is simple.
342	
343	Setting the flag 'memory_spread_page' turns on a per-process flag
344	PF_SPREAD_PAGE for each task that is in that cpuset or subsequently
345	joins that cpuset.  The page allocation calls for the page cache
346	is modified to perform an inline check for this PF_SPREAD_PAGE task
347	flag, and if set, a call to a new routine cpuset_mem_spread_node()
348	returns the node to prefer for the allocation.
349	
350	Similarly, setting 'memory_spread_slab' turns on the flag
351	PF_SPREAD_SLAB, and appropriately marked slab caches will allocate
352	pages from the node returned by cpuset_mem_spread_node().
353	
354	The cpuset_mem_spread_node() routine is also simple.  It uses the
355	value of a per-task rotor cpuset_mem_spread_rotor to select the next
356	node in the current tasks mems_allowed to prefer for the allocation.
357	
358	This memory placement policy is also known (in other contexts) as
359	round-robin or interleave.
360	
361	This policy can provide substantial improvements for jobs that need
362	to place thread local data on the corresponding node, but that need
363	to access large file system data sets that need to be spread across
364	the several nodes in the jobs cpuset in order to fit.  Without this
365	policy, especially for jobs that might have one thread reading in the
366	data set, the memory allocation across the nodes in the jobs cpuset
367	can become very uneven.
368	
369	1.7 What is sched_load_balance ?
370	--------------------------------
371	
372	The kernel scheduler (kernel/sched.c) automatically load balances
373	tasks.  If one CPU is underutilized, kernel code running on that
374	CPU will look for tasks on other more overloaded CPUs and move those
375	tasks to itself, within the constraints of such placement mechanisms
376	as cpusets and sched_setaffinity.
377	
378	The algorithmic cost of load balancing and its impact on key shared
379	kernel data structures such as the task list increases more than
380	linearly with the number of CPUs being balanced.  So the scheduler
381	has support to  partition the systems CPUs into a number of sched
382	domains such that it only load balances within each sched domain.
383	Each sched domain covers some subset of the CPUs in the system;
384	no two sched domains overlap; some CPUs might not be in any sched
385	domain and hence won't be load balanced.
386	
387	Put simply, it costs less to balance between two smaller sched domains
388	than one big one, but doing so means that overloads in one of the
389	two domains won't be load balanced to the other one.
390	
391	By default, there is one sched domain covering all CPUs, except those
392	marked isolated using the kernel boot time "isolcpus=" argument.
393	
394	This default load balancing across all CPUs is not well suited for
395	the following two situations:
396	 1) On large systems, load balancing across many CPUs is expensive.
397	    If the system is managed using cpusets to place independent jobs
398	    on separate sets of CPUs, full load balancing is unnecessary.
399	 2) Systems supporting realtime on some CPUs need to minimize
400	    system overhead on those CPUs, including avoiding task load
401	    balancing if that is not needed.
402	
403	When the per-cpuset flag "sched_load_balance" is enabled (the default
404	setting), it requests that all the CPUs in that cpusets allowed 'cpus'
405	be contained in a single sched domain, ensuring that load balancing
406	can move a task (not otherwised pinned, as by sched_setaffinity)
407	from any CPU in that cpuset to any other.
408	
409	When the per-cpuset flag "sched_load_balance" is disabled, then the
410	scheduler will avoid load balancing across the CPUs in that cpuset,
411	--except-- in so far as is necessary because some overlapping cpuset
412	has "sched_load_balance" enabled.
413	
414	So, for example, if the top cpuset has the flag "sched_load_balance"
415	enabled, then the scheduler will have one sched domain covering all
416	CPUs, and the setting of the "sched_load_balance" flag in any other
417	cpusets won't matter, as we're already fully load balancing.
418	
419	Therefore in the above two situations, the top cpuset flag
420	"sched_load_balance" should be disabled, and only some of the smaller,
421	child cpusets have this flag enabled.
422	
423	When doing this, you don't usually want to leave any unpinned tasks in
424	the top cpuset that might use non-trivial amounts of CPU, as such tasks
425	may be artificially constrained to some subset of CPUs, depending on
426	the particulars of this flag setting in descendent cpusets.  Even if
427	such a task could use spare CPU cycles in some other CPUs, the kernel
428	scheduler might not consider the possibility of load balancing that
429	task to that underused CPU.
430	
431	Of course, tasks pinned to a particular CPU can be left in a cpuset
432	that disables "sched_load_balance" as those tasks aren't going anywhere
433	else anyway.
434	
435	There is an impedance mismatch here, between cpusets and sched domains.
436	Cpusets are hierarchical and nest.  Sched domains are flat; they don't
437	overlap and each CPU is in at most one sched domain.
438	
439	It is necessary for sched domains to be flat because load balancing
440	across partially overlapping sets of CPUs would risk unstable dynamics
441	that would be beyond our understanding.  So if each of two partially
442	overlapping cpusets enables the flag 'sched_load_balance', then we
443	form a single sched domain that is a superset of both.  We won't move
444	a task to a CPU outside it cpuset, but the scheduler load balancing
445	code might waste some compute cycles considering that possibility.
446	
447	This mismatch is why there is not a simple one-to-one relation
448	between which cpusets have the flag "sched_load_balance" enabled,
449	and the sched domain configuration.  If a cpuset enables the flag, it
450	will get balancing across all its CPUs, but if it disables the flag,
451	it will only be assured of no load balancing if no other overlapping
452	cpuset enables the flag.
453	
454	If two cpusets have partially overlapping 'cpus' allowed, and only
455	one of them has this flag enabled, then the other may find its
456	tasks only partially load balanced, just on the overlapping CPUs.
457	This is just the general case of the top_cpuset example given a few
458	paragraphs above.  In the general case, as in the top cpuset case,
459	don't leave tasks that might use non-trivial amounts of CPU in
460	such partially load balanced cpusets, as they may be artificially
461	constrained to some subset of the CPUs allowed to them, for lack of
462	load balancing to the other CPUs.
463	
464	1.7.1 sched_load_balance implementation details.
465	------------------------------------------------
466	
467	The per-cpuset flag 'sched_load_balance' defaults to enabled (contrary
468	to most cpuset flags.)  When enabled for a cpuset, the kernel will
469	ensure that it can load balance across all the CPUs in that cpuset
470	(makes sure that all the CPUs in the cpus_allowed of that cpuset are
471	in the same sched domain.)
472	
473	If two overlapping cpusets both have 'sched_load_balance' enabled,
474	then they will be (must be) both in the same sched domain.
475	
476	If, as is the default, the top cpuset has 'sched_load_balance' enabled,
477	then by the above that means there is a single sched domain covering
478	the whole system, regardless of any other cpuset settings.
479	
480	The kernel commits to user space that it will avoid load balancing
481	where it can.  It will pick as fine a granularity partition of sched
482	domains as it can while still providing load balancing for any set
483	of CPUs allowed to a cpuset having 'sched_load_balance' enabled.
484	
485	The internal kernel cpuset to scheduler interface passes from the
486	cpuset code to the scheduler code a partition of the load balanced
487	CPUs in the system. This partition is a set of subsets (represented
488	as an array of cpumask_t) of CPUs, pairwise disjoint, that cover all
489	the CPUs that must be load balanced.
490	
491	Whenever the 'sched_load_balance' flag changes, or CPUs come or go
492	from a cpuset with this flag enabled, or a cpuset with this flag
493	enabled is removed, the cpuset code builds a new such partition and
494	passes it to the scheduler sched domain setup code, to have the sched
495	domains rebuilt as necessary.
496	
497	This partition exactly defines what sched domains the scheduler should
498	setup - one sched domain for each element (cpumask_t) in the partition.
499	
500	The scheduler remembers the currently active sched domain partitions.
501	When the scheduler routine partition_sched_domains() is invoked from
502	the cpuset code to update these sched domains, it compares the new
503	partition requested with the current, and updates its sched domains,
504	removing the old and adding the new, for each change.
505	
506	
507	1.8 What is sched_relax_domain_level ?
508	--------------------------------------
509	
510	In sched domain, the scheduler migrates tasks in 2 ways; periodic load
511	balance on tick, and at time of some schedule events.
512	
513	When a task is woken up, scheduler try to move the task on idle CPU.
514	For example, if a task A running on CPU X activates another task B
515	on the same CPU X, and if CPU Y is X's sibling and performing idle,
516	then scheduler migrate task B to CPU Y so that task B can start on
517	CPU Y without waiting task A on CPU X.
518	
519	And if a CPU run out of tasks in its runqueue, the CPU try to pull
520	extra tasks from other busy CPUs to help them before it is going to
521	be idle.
522	
523	Of course it takes some searching cost to find movable tasks and/or
524	idle CPUs, the scheduler might not search all CPUs in the domain
525	everytime.  In fact, in some architectures, the searching ranges on
526	events are limited in the same socket or node where the CPU locates,
527	while the load balance on tick searchs all.
528	
529	For example, assume CPU Z is relatively far from CPU X.  Even if CPU Z
530	is idle while CPU X and the siblings are busy, scheduler can't migrate
531	woken task B from X to Z since it is out of its searching range.
532	As the result, task B on CPU X need to wait task A or wait load balance
533	on the next tick.  For some applications in special situation, waiting
534	1 tick may be too long.
535	
536	The 'sched_relax_domain_level' file allows you to request changing
537	this searching range as you like.  This file takes int value which
538	indicates size of searching range in levels ideally as follows,
539	otherwise initial value -1 that indicates the cpuset has no request.
540	
541	  -1  : no request. use system default or follow request of others.
542	   0  : no search.
543	   1  : search siblings (hyperthreads in a core).
544	   2  : search cores in a package.
545	   3  : search cpus in a node [= system wide on non-NUMA system]
546	 ( 4  : search nodes in a chunk of node [on NUMA system] )
547	 ( 5  : search system wide [on NUMA system] )
548	
549	The system default is architecture dependent.  The system default
550	can be changed using the relax_domain_level= boot parameter.
551	
552	This file is per-cpuset and affect the sched domain where the cpuset
553	belongs to.  Therefore if the flag 'sched_load_balance' of a cpuset
554	is disabled, then 'sched_relax_domain_level' have no effect since
555	there is no sched domain belonging the cpuset.
556	
557	If multiple cpusets are overlapping and hence they form a single sched
558	domain, the largest value among those is used.  Be careful, if one
559	requests 0 and others are -1 then 0 is used.
560	
561	Note that modifying this file will have both good and bad effects,
562	and whether it is acceptable or not will be depend on your situation.
563	Don't modify this file if you are not sure.
564	
565	If your situation is:
566	 - The migration costs between each cpu can be assumed considerably
567	   small(for you) due to your special application's behavior or
568	   special hardware support for CPU cache etc.
569	 - The searching cost doesn't have impact(for you) or you can make
570	   the searching cost enough small by managing cpuset to compact etc.
571	 - The latency is required even it sacrifices cache hit rate etc.
572	then increasing 'sched_relax_domain_level' would benefit you.
573	
574	
575	1.9 How do I use cpusets ?
576	--------------------------
577	
578	In order to minimize the impact of cpusets on critical kernel
579	code, such as the scheduler, and due to the fact that the kernel
580	does not support one task updating the memory placement of another
581	task directly, the impact on a task of changing its cpuset CPU
582	or Memory Node placement, or of changing to which cpuset a task
583	is attached, is subtle.
584	
585	If a cpuset has its Memory Nodes modified, then for each task attached
586	to that cpuset, the next time that the kernel attempts to allocate
587	a page of memory for that task, the kernel will notice the change
588	in the tasks cpuset, and update its per-task memory placement to
589	remain within the new cpusets memory placement.  If the task was using
590	mempolicy MPOL_BIND, and the nodes to which it was bound overlap with
591	its new cpuset, then the task will continue to use whatever subset
592	of MPOL_BIND nodes are still allowed in the new cpuset.  If the task
593	was using MPOL_BIND and now none of its MPOL_BIND nodes are allowed
594	in the new cpuset, then the task will be essentially treated as if it
595	was MPOL_BIND bound to the new cpuset (even though its numa placement,
596	as queried by get_mempolicy(), doesn't change).  If a task is moved
597	from one cpuset to another, then the kernel will adjust the tasks
598	memory placement, as above, the next time that the kernel attempts
599	to allocate a page of memory for that task.
600	
601	If a cpuset has its 'cpus' modified, then each task in that cpuset
602	will have its allowed CPU placement changed immediately.  Similarly,
603	if a tasks pid is written to a cpusets 'tasks' file, in either its
604	current cpuset or another cpuset, then its allowed CPU placement is
605	changed immediately.  If such a task had been bound to some subset
606	of its cpuset using the sched_setaffinity() call, the task will be
607	allowed to run on any CPU allowed in its new cpuset, negating the
608	affect of the prior sched_setaffinity() call.
609	
610	In summary, the memory placement of a task whose cpuset is changed is
611	updated by the kernel, on the next allocation of a page for that task,
612	but the processor placement is not updated, until that tasks pid is
613	rewritten to the 'tasks' file of its cpuset.  This is done to avoid
614	impacting the scheduler code in the kernel with a check for changes
615	in a tasks processor placement.
616	
617	Normally, once a page is allocated (given a physical page
618	of main memory) then that page stays on whatever node it
619	was allocated, so long as it remains allocated, even if the
620	cpusets memory placement policy 'mems' subsequently changes.
621	If the cpuset flag file 'memory_migrate' is set true, then when
622	tasks are attached to that cpuset, any pages that task had
623	allocated to it on nodes in its previous cpuset are migrated
624	to the tasks new cpuset. The relative placement of the page within
625	the cpuset is preserved during these migration operations if possible.
626	For example if the page was on the second valid node of the prior cpuset
627	then the page will be placed on the second valid node of the new cpuset.
628	
629	Also if 'memory_migrate' is set true, then if that cpusets
630	'mems' file is modified, pages allocated to tasks in that
631	cpuset, that were on nodes in the previous setting of 'mems',
632	will be moved to nodes in the new setting of 'mems.'
633	Pages that were not in the tasks prior cpuset, or in the cpusets
634	prior 'mems' setting, will not be moved.
635	
636	There is an exception to the above.  If hotplug functionality is used
637	to remove all the CPUs that are currently assigned to a cpuset,
638	then all the tasks in that cpuset will be moved to the nearest ancestor
639	with non-empty cpus.  But the moving of some (or all) tasks might fail if
640	cpuset is bound with another cgroup subsystem which has some restrictions
641	on task attaching.  In this failing case, those tasks will stay
642	in the original cpuset, and the kernel will automatically update
643	their cpus_allowed to allow all online CPUs.  When memory hotplug
644	functionality for removing Memory Nodes is available, a similar exception
645	is expected to apply there as well.  In general, the kernel prefers to
646	violate cpuset placement, over starving a task that has had all
647	its allowed CPUs or Memory Nodes taken offline.
648	
649	There is a second exception to the above.  GFP_ATOMIC requests are
650	kernel internal allocations that must be satisfied, immediately.
651	The kernel may drop some request, in rare cases even panic, if a
652	GFP_ATOMIC alloc fails.  If the request cannot be satisfied within
653	the current tasks cpuset, then we relax the cpuset, and look for
654	memory anywhere we can find it.  It's better to violate the cpuset
655	than stress the kernel.
656	
657	To start a new job that is to be contained within a cpuset, the steps are:
658	
659	 1) mkdir /dev/cpuset
660	 2) mount -t cgroup -ocpuset cpuset /dev/cpuset
661	 3) Create the new cpuset by doing mkdir's and write's (or echo's) in
662	    the /dev/cpuset virtual file system.
663	 4) Start a task that will be the "founding father" of the new job.
664	 5) Attach that task to the new cpuset by writing its pid to the
665	    /dev/cpuset tasks file for that cpuset.
666	 6) fork, exec or clone the job tasks from this founding father task.
667	
668	For example, the following sequence of commands will setup a cpuset
669	named "Charlie", containing just CPUs 2 and 3, and Memory Node 1,
670	and then start a subshell 'sh' in that cpuset:
671	
672	  mount -t cgroup -ocpuset cpuset /dev/cpuset
673	  cd /dev/cpuset
674	  mkdir Charlie
675	  cd Charlie
676	  /bin/echo 2-3 > cpus
677	  /bin/echo 1 > mems
678	  /bin/echo $$ > tasks
679	  sh
680	  # The subshell 'sh' is now running in cpuset Charlie
681	  # The next line should display '/Charlie'
682	  cat /proc/self/cpuset
683	
684	In the future, a C library interface to cpusets will likely be
685	available.  For now, the only way to query or modify cpusets is
686	via the cpuset file system, using the various cd, mkdir, echo, cat,
687	rmdir commands from the shell, or their equivalent from C.
688	
689	The sched_setaffinity calls can also be done at the shell prompt using
690	SGI's runon or Robert Love's taskset.  The mbind and set_mempolicy
691	calls can be done at the shell prompt using the numactl command
692	(part of Andi Kleen's numa package).
693	
694	2. Usage Examples and Syntax
695	============================
696	
697	2.1 Basic Usage
698	---------------
699	
700	Creating, modifying, using the cpusets can be done through the cpuset
701	virtual filesystem.
702	
703	To mount it, type:
704	# mount -t cgroup -o cpuset cpuset /dev/cpuset
705	
706	Then under /dev/cpuset you can find a tree that corresponds to the
707	tree of the cpusets in the system. For instance, /dev/cpuset
708	is the cpuset that holds the whole system.
709	
710	If you want to create a new cpuset under /dev/cpuset:
711	# cd /dev/cpuset
712	# mkdir my_cpuset
713	
714	Now you want to do something with this cpuset.
715	# cd my_cpuset
716	
717	In this directory you can find several files:
718	# ls
719	cpu_exclusive  memory_migrate      mems                      tasks
720	cpus           memory_pressure     notify_on_release
721	mem_exclusive  memory_spread_page  sched_load_balance
722	mem_hardwall   memory_spread_slab  sched_relax_domain_level
723	
724	Reading them will give you information about the state of this cpuset:
725	the CPUs and Memory Nodes it can use, the processes that are using
726	it, its properties.  By writing to these files you can manipulate
727	the cpuset.
728	
729	Set some flags:
730	# /bin/echo 1 > cpu_exclusive
731	
732	Add some cpus:
733	# /bin/echo 0-7 > cpus
734	
735	Add some mems:
736	# /bin/echo 0-7 > mems
737	
738	Now attach your shell to this cpuset:
739	# /bin/echo $$ > tasks
740	
741	You can also create cpusets inside your cpuset by using mkdir in this
742	directory.
743	# mkdir my_sub_cs
744	
745	To remove a cpuset, just use rmdir:
746	# rmdir my_sub_cs
747	This will fail if the cpuset is in use (has cpusets inside, or has
748	processes attached).
749	
750	Note that for legacy reasons, the "cpuset" filesystem exists as a
751	wrapper around the cgroup filesystem.
752	
753	The command
754	
755	mount -t cpuset X /dev/cpuset
756	
757	is equivalent to
758	
759	mount -t cgroup -ocpuset X /dev/cpuset
760	echo "/sbin/cpuset_release_agent" > /dev/cpuset/release_agent
761	
762	2.2 Adding/removing cpus
763	------------------------
764	
765	This is the syntax to use when writing in the cpus or mems files
766	in cpuset directories:
767	
768	# /bin/echo 1-4 > cpus		-> set cpus list to cpus 1,2,3,4
769	# /bin/echo 1,2,3,4 > cpus	-> set cpus list to cpus 1,2,3,4
770	
771	2.3 Setting flags
772	-----------------
773	
774	The syntax is very simple:
775	
776	# /bin/echo 1 > cpu_exclusive 	-> set flag 'cpu_exclusive'
777	# /bin/echo 0 > cpu_exclusive 	-> unset flag 'cpu_exclusive'
778	
779	2.4 Attaching processes
780	-----------------------
781	
782	# /bin/echo PID > tasks
783	
784	Note that it is PID, not PIDs. You can only attach ONE task at a time.
785	If you have several tasks to attach, you have to do it one after another:
786	
787	# /bin/echo PID1 > tasks
788	# /bin/echo PID2 > tasks
789		...
790	# /bin/echo PIDn > tasks
791	
792	
793	3. Questions
794	============
795	
796	Q: what's up with this '/bin/echo' ?
797	A: bash's builtin 'echo' command does not check calls to write() against
798	   errors. If you use it in the cpuset file system, you won't be
799	   able to tell whether a command succeeded or failed.
800	
801	Q: When I attach processes, only the first of the line gets really attached !
802	A: We can only return one error code per call to write(). So you should also
803	   put only ONE pid.
804	
805	4. Contact
806	==========
807	
808	Web: http://www.bullopensource.org/cpuset
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