Skip to content

HTTPS clone URL

Subversion checkout URL

You can clone with
or
.
Download ZIP
Newer
Older
100644 403 lines (305 sloc) 15.396 kB
6867c93 @y-goto Memory hotplug document
y-goto authored
1 ==============
2 Memory Hotplug
3 ==============
4
10020ca @y-goto memory hotplug: document the memory hotplug notifier
y-goto authored
5 Created: Jul 28 2007
6 Add description of notifier of memory hotplug Oct 11 2007
6867c93 @y-goto Memory hotplug document
y-goto authored
7
8 This document is about memory hotplug including how-to-use and current status.
9 Because Memory Hotplug is still under development, contents of this text will
10 be changed often.
11
12 1. Introduction
13 1.1 purpose of memory hotplug
14 1.2. Phases of memory hotplug
15 1.3. Unit of Memory online/offline operation
16 2. Kernel Configuration
17 3. sysfs files for memory hotplug
18 4. Physical memory hot-add phase
19 4.1 Hardware(Firmware) Support
20 4.2 Notify memory hot-add event by hand
21 5. Logical Memory hot-add phase
22 5.1. State of memory
23 5.2. How to online memory
24 6. Logical memory remove
25 6.1 Memory offline and ZONE_MOVABLE
26 6.2. How to offline memory
27 7. Physical memory remove
10020ca @y-goto memory hotplug: document the memory hotplug notifier
y-goto authored
28 8. Memory hotplug event notifier
29 9. Future Work List
6867c93 @y-goto Memory hotplug document
y-goto authored
30
31 Note(1): x86_64's has special implementation for memory hotplug.
32 This text does not describe it.
33 Note(2): This text assumes that sysfs is mounted at /sys.
34
35
36 ---------------
37 1. Introduction
38 ---------------
39
40 1.1 purpose of memory hotplug
41 ------------
42 Memory Hotplug allows users to increase/decrease the amount of memory.
43 Generally, there are two purposes.
44
45 (A) For changing the amount of memory.
46 This is to allow a feature like capacity on demand.
47 (B) For installing/removing DIMMs or NUMA-nodes physically.
48 This is to exchange DIMMs/NUMA-nodes, reduce power consumption, etc.
49
50 (A) is required by highly virtualized environments and (B) is required by
51 hardware which supports memory power management.
52
53 Linux memory hotplug is designed for both purpose.
54
55
56 1.2. Phases of memory hotplug
57 ---------------
58 There are 2 phases in Memory Hotplug.
59 1) Physical Memory Hotplug phase
60 2) Logical Memory Hotplug phase.
61
62 The First phase is to communicate hardware/firmware and make/erase
63 environment for hotplugged memory. Basically, this phase is necessary
64 for the purpose (B), but this is good phase for communication between
65 highly virtualized environments too.
66
67 When memory is hotplugged, the kernel recognizes new memory, makes new memory
68 management tables, and makes sysfs files for new memory's operation.
69
70 If firmware supports notification of connection of new memory to OS,
71 this phase is triggered automatically. ACPI can notify this event. If not,
72 "probe" operation by system administration is used instead.
73 (see Section 4.).
74
75 Logical Memory Hotplug phase is to change memory state into
19f5946 trivial: Miscellaneous documentation typo fixes
Matt LaPlante authored
76 available/unavailable for users. Amount of memory from user's view is
6867c93 @y-goto Memory hotplug document
y-goto authored
77 changed by this phase. The kernel makes all memory in it as free pages
78 when a memory range is available.
79
80 In this document, this phase is described as online/offline.
81
19f5946 trivial: Miscellaneous documentation typo fixes
Matt LaPlante authored
82 Logical Memory Hotplug phase is triggered by write of sysfs file by system
6867c93 @y-goto Memory hotplug document
y-goto authored
83 administrator. For the hot-add case, it must be executed after Physical Hotplug
84 phase by hand.
85 (However, if you writes udev's hotplug scripts for memory hotplug, these
86 phases can be execute in seamless way.)
87
88
89 1.3. Unit of Memory online/offline operation
90 ------------
91 Memory hotplug uses SPARSEMEM memory model. SPARSEMEM divides the whole memory
92 into chunks of the same size. The chunk is called a "section". The size of
93 a section is architecture dependent. For example, power uses 16MiB, ia64 uses
94 1GiB. The unit of online/offline operation is "one section". (see Section 3.)
95
96 To determine the size of sections, please read this file:
97
98 /sys/devices/system/memory/block_size_bytes
99
100 This file shows the size of sections in byte.
101
102 -----------------------
103 2. Kernel Configuration
104 -----------------------
105 To use memory hotplug feature, kernel must be compiled with following
106 config options.
107
108 - For all memory hotplug
109 Memory model -> Sparse Memory (CONFIG_SPARSEMEM)
110 Allow for memory hot-add (CONFIG_MEMORY_HOTPLUG)
111
112 - To enable memory removal, the followings are also necessary
113 Allow for memory hot remove (CONFIG_MEMORY_HOTREMOVE)
114 Page Migration (CONFIG_MIGRATION)
115
116 - For ACPI memory hotplug, the followings are also necessary
117 Memory hotplug (under ACPI Support menu) (CONFIG_ACPI_HOTPLUG_MEMORY)
118 This option can be kernel module.
119
120 - As a related configuration, if your box has a feature of NUMA-node hotplug
121 via ACPI, then this option is necessary too.
122 ACPI0004,PNP0A05 and PNP0A06 Container Driver (under ACPI Support menu)
123 (CONFIG_ACPI_CONTAINER).
124 This option can be kernel module too.
125
126 --------------------------------
c04fc58 mm: show node to memory section relationship with symlinks in sysfs
Gary Hade authored
127 4 sysfs files for memory hotplug
6867c93 @y-goto Memory hotplug document
y-goto authored
128 --------------------------------
0c2c99b memory hotplug: Allow memory blocks to span multiple memory sections
Nathan Fontenot authored
129 All sections have their device information in sysfs. Each section is part of
130 a memory block under /sys/devices/system/memory as
6867c93 @y-goto Memory hotplug document
y-goto authored
131
132 /sys/devices/system/memory/memoryXXX
0c2c99b memory hotplug: Allow memory blocks to span multiple memory sections
Nathan Fontenot authored
133 (XXX is the section id.)
6867c93 @y-goto Memory hotplug document
y-goto authored
134
0c2c99b memory hotplug: Allow memory blocks to span multiple memory sections
Nathan Fontenot authored
135 Now, XXX is defined as (start_address_of_section / section_size) of the first
136 section contained in the memory block. The files 'phys_index' and
137 'end_phys_index' under each directory report the beginning and end section id's
138 for the memory block covered by the sysfs directory. It is expected that all
139 memory sections in this range are present and no memory holes exist in the
140 range. Currently there is no way to determine if there is a memory hole, but
141 the existence of one should not affect the hotplug capabilities of the memory
142 block.
6867c93 @y-goto Memory hotplug document
y-goto authored
143
144 For example, assume 1GiB section size. A device for a memory starting at
145 0x100000000 is /sys/device/system/memory/memory4
146 (0x100000000 / 1Gib = 4)
147 This device covers address range [0x100000000 ... 0x140000000)
148
0c2c99b memory hotplug: Allow memory blocks to span multiple memory sections
Nathan Fontenot authored
149 Under each section, you can see 4 or 5 files, the end_phys_index file being
150 a recent addition and not present on older kernels.
6867c93 @y-goto Memory hotplug document
y-goto authored
151
0c2c99b memory hotplug: Allow memory blocks to span multiple memory sections
Nathan Fontenot authored
152 /sys/devices/system/memory/memoryXXX/start_phys_index
153 /sys/devices/system/memory/memoryXXX/end_phys_index
6867c93 @y-goto Memory hotplug document
y-goto authored
154 /sys/devices/system/memory/memoryXXX/phys_device
155 /sys/devices/system/memory/memoryXXX/state
c04fc58 mm: show node to memory section relationship with symlinks in sysfs
Gary Hade authored
156 /sys/devices/system/memory/memoryXXX/removable
6867c93 @y-goto Memory hotplug document
y-goto authored
157
0c2c99b memory hotplug: Allow memory blocks to span multiple memory sections
Nathan Fontenot authored
158 'phys_index' : read-only and contains section id of the first section
159 in the memory block, same as XXX.
160 'end_phys_index' : read-only and contains section id of the last section
161 in the memory block.
162 'state' : read-write
163 at read: contains online/offline state of memory.
164 at write: user can specify "online", "offline" command
165 which will be performed on al sections in the block.
166 'phys_device' : read-only: designed to show the name of physical memory
167 device. This is not well implemented now.
168 'removable' : read-only: contains an integer value indicating
169 whether the memory block is removable or not
170 removable. A value of 1 indicates that the memory
171 block is removable and a value of 0 indicates that
172 it is not removable. A memory block is removable only if
173 every section in the block is removable.
6867c93 @y-goto Memory hotplug document
y-goto authored
174
175 NOTE:
176 These directories/files appear after physical memory hotplug phase.
177
dee5d0d mm: add numa node symlink for memory section in sysfs
Alex Chiang authored
178 If CONFIG_NUMA is enabled the memoryXXX/ directories can also be accessed
179 via symbolic links located in the /sys/devices/system/node/node* directories.
180
181 For example:
c04fc58 mm: show node to memory section relationship with symlinks in sysfs
Gary Hade authored
182 /sys/devices/system/node/node0/memory9 -> ../../memory/memory9
6867c93 @y-goto Memory hotplug document
y-goto authored
183
dee5d0d mm: add numa node symlink for memory section in sysfs
Alex Chiang authored
184 A backlink will also be created:
185 /sys/devices/system/memory/memory9/node0 -> ../../node/node0
186
6867c93 @y-goto Memory hotplug document
y-goto authored
187 --------------------------------
188 4. Physical memory hot-add phase
189 --------------------------------
190
191 4.1 Hardware(Firmware) Support
192 ------------
193 On x86_64/ia64 platform, memory hotplug by ACPI is supported.
194
195 In general, the firmware (ACPI) which supports memory hotplug defines
196 memory class object of _HID "PNP0C80". When a notify is asserted to PNP0C80,
197 Linux's ACPI handler does hot-add memory to the system and calls a hotplug udev
198 script. This will be done automatically.
199
200 But scripts for memory hotplug are not contained in generic udev package(now).
201 You may have to write it by yourself or online/offline memory by hand.
202 Please see "How to online memory", "How to offline memory" in this text.
203
204 If firmware supports NUMA-node hotplug, and defines an object _HID "ACPI0004",
205 "PNP0A05", or "PNP0A06", notification is asserted to it, and ACPI handler
206 calls hotplug code for all of objects which are defined in it.
207 If memory device is found, memory hotplug code will be called.
208
209
210 4.2 Notify memory hot-add event by hand
211 ------------
212 In some environments, especially virtualized environment, firmware will not
213 notify memory hotplug event to the kernel. For such environment, "probe"
214 interface is supported. This interface depends on CONFIG_ARCH_MEMORY_PROBE.
215
216 Now, CONFIG_ARCH_MEMORY_PROBE is supported only by powerpc but it does not
217 contain highly architecture codes. Please add config if you need "probe"
218 interface.
219
220 Probe interface is located at
221 /sys/devices/system/memory/probe
222
223 You can tell the physical address of new memory to the kernel by
224
225 % echo start_address_of_new_memory > /sys/devices/system/memory/probe
226
227 Then, [start_address_of_new_memory, start_address_of_new_memory + section_size)
228 memory range is hot-added. In this case, hotplug script is not called (in
229 current implementation). You'll have to online memory by yourself.
230 Please see "How to online memory" in this text.
231
232
233
234 ------------------------------
235 5. Logical Memory hot-add phase
236 ------------------------------
237
238 5.1. State of memory
239 ------------
240 To see (online/offline) state of memory section, read 'state' file.
241
242 % cat /sys/device/system/memory/memoryXXX/state
243
244
245 If the memory section is online, you'll read "online".
246 If the memory section is offline, you'll read "offline".
247
248
249 5.2. How to online memory
250 ------------
251 Even if the memory is hot-added, it is not at ready-to-use state.
252 For using newly added memory, you have to "online" the memory section.
253
254 For onlining, you have to write "online" to the section's state file as:
255
256 % echo online > /sys/devices/system/memory/memoryXXX/state
257
258 After this, section memoryXXX's state will be 'online' and the amount of
259 available memory will be increased.
260
261 Currently, newly added memory is added as ZONE_NORMAL (for powerpc, ZONE_DMA).
262 This may be changed in future.
263
264
265
266 ------------------------
267 6. Logical memory remove
268 ------------------------
269
270 6.1 Memory offline and ZONE_MOVABLE
271 ------------
272 Memory offlining is more complicated than memory online. Because memory offline
273 has to make the whole memory section be unused, memory offline can fail if
274 the section includes memory which cannot be freed.
275
276 In general, memory offline can use 2 techniques.
277
278 (1) reclaim and free all memory in the section.
279 (2) migrate all pages in the section.
280
281 In the current implementation, Linux's memory offline uses method (2), freeing
282 all pages in the section by page migration. But not all pages are
283 migratable. Under current Linux, migratable pages are anonymous pages and
284 page caches. For offlining a section by migration, the kernel has to guarantee
285 that the section contains only migratable pages.
286
287 Now, a boot option for making a section which consists of migratable pages is
288 supported. By specifying "kernelcore=" or "movablecore=" boot option, you can
289 create ZONE_MOVABLE...a zone which is just used for movable pages.
290 (See also Documentation/kernel-parameters.txt)
291
292 Assume the system has "TOTAL" amount of memory at boot time, this boot option
293 creates ZONE_MOVABLE as following.
294
295 1) When kernelcore=YYYY boot option is used,
296 Size of memory not for movable pages (not for offline) is YYYY.
297 Size of memory for movable pages (for offline) is TOTAL-YYYY.
298
299 2) When movablecore=ZZZZ boot option is used,
300 Size of memory not for movable pages (not for offline) is TOTAL - ZZZZ.
301 Size of memory for movable pages (for offline) is ZZZZ.
302
303
304 Note) Unfortunately, there is no information to show which section belongs
305 to ZONE_MOVABLE. This is TBD.
306
307
308 6.2. How to offline memory
309 ------------
310 You can offline a section by using the same sysfs interface that was used in
311 memory onlining.
312
313 % echo offline > /sys/devices/system/memory/memoryXXX/state
314
315 If offline succeeds, the state of the memory section is changed to be "offline".
316 If it fails, some error core (like -EBUSY) will be returned by the kernel.
317 Even if a section does not belong to ZONE_MOVABLE, you can try to offline it.
318 If it doesn't contain 'unmovable' memory, you'll get success.
319
320 A section under ZONE_MOVABLE is considered to be able to be offlined easily.
321 But under some busy state, it may return -EBUSY. Even if a memory section
322 cannot be offlined due to -EBUSY, you can retry offlining it and may be able to
323 offline it (or not).
324 (For example, a page is referred to by some kernel internal call and released
325 soon.)
326
327 Consideration:
328 Memory hotplug's design direction is to make the possibility of memory offlining
329 higher and to guarantee unplugging memory under any situation. But it needs
330 more work. Returning -EBUSY under some situation may be good because the user
331 can decide to retry more or not by himself. Currently, memory offlining code
332 does some amount of retry with 120 seconds timeout.
333
334 -------------------------
335 7. Physical memory remove
336 -------------------------
337 Need more implementation yet....
338 - Notification completion of remove works by OS to firmware.
339 - Guard from remove if not yet.
340
10020ca @y-goto memory hotplug: document the memory hotplug notifier
y-goto authored
341 --------------------------------
342 8. Memory hotplug event notifier
343 --------------------------------
c94bed8 @standby24x7 Documentation: Fix typo in multiple files in Documentation
standby24x7 authored
344 Memory hotplug has event notifier. There are 6 types of notification.
10020ca @y-goto memory hotplug: document the memory hotplug notifier
y-goto authored
345
346 MEMORY_GOING_ONLINE
347 Generated before new memory becomes available in order to be able to
348 prepare subsystems to handle memory. The page allocator is still unable
349 to allocate from the new memory.
350
351 MEMORY_CANCEL_ONLINE
352 Generated if MEMORY_GOING_ONLINE fails.
353
354 MEMORY_ONLINE
19f5946 trivial: Miscellaneous documentation typo fixes
Matt LaPlante authored
355 Generated when memory has successfully brought online. The callback may
10020ca @y-goto memory hotplug: document the memory hotplug notifier
y-goto authored
356 allocate pages from the new memory.
357
358 MEMORY_GOING_OFFLINE
359 Generated to begin the process of offlining memory. Allocations are no
360 longer possible from the memory but some of the memory to be offlined
361 is still in use. The callback can be used to free memory known to a
362 subsystem from the indicated memory section.
363
364 MEMORY_CANCEL_OFFLINE
365 Generated if MEMORY_GOING_OFFLINE fails. Memory is available again from
366 the section that we attempted to offline.
367
368 MEMORY_OFFLINE
369 Generated after offlining memory is complete.
370
371 A callback routine can be registered by
372 hotplug_memory_notifier(callback_func, priority)
373
374 The second argument of callback function (action) is event types of above.
375 The third argument is passed by pointer of struct memory_notify.
376
377 struct memory_notify {
378 unsigned long start_pfn;
379 unsigned long nr_pages;
19f5946 trivial: Miscellaneous documentation typo fixes
Matt LaPlante authored
380 int status_change_nid;
10020ca @y-goto memory hotplug: document the memory hotplug notifier
y-goto authored
381 }
382
383 start_pfn is start_pfn of online/offline memory.
384 nr_pages is # of pages of online/offline memory.
385 status_change_nid is set node id when N_HIGH_MEMORY of nodemask is (will be)
386 set/clear. It means a new(memoryless) node gets new memory by online and a
387 node loses all memory. If this is -1, then nodemask status is not changed.
388 If status_changed_nid >= 0, callback should create/discard structures for the
389 node if necessary.
390
6867c93 @y-goto Memory hotplug document
y-goto authored
391 --------------
10020ca @y-goto memory hotplug: document the memory hotplug notifier
y-goto authored
392 9. Future Work
6867c93 @y-goto Memory hotplug document
y-goto authored
393 --------------
394 - allowing memory hot-add to ZONE_MOVABLE. maybe we need some switch like
395 sysctl or new control file.
396 - showing memory section and physical device relationship.
397 - showing memory section is under ZONE_MOVABLE or not
398 - test and make it better memory offlining.
399 - support HugeTLB page migration and offlining.
400 - memmap removing at memory offline.
401 - physical remove memory.
402
Something went wrong with that request. Please try again.