docs/specs/ppc-spapr-numa.rst

5 NUMA in sPAPR works different than the System Locality Distance
6 Information Table (SLIT) in ACPI. The logic is explained in the LOPAPR
9 of the elements that impacts how QEMU views NUMA in pseries.
16 everyone else outside of the group.
18 The format of the ibm,associativity property varies with the value of
19 bit 0 of byte 5 of the ibm,architecture-vec-5 property. The format with
20 bit 0 equal to zero is deprecated. The current format, with the bit 0
21 with the value of one, makes ibm,associativity property represent the
22 physical hierarchy of the platform, as one or more lists that starts
23 with the highest level grouping up to the smallest. Considering the
36 The ibm,associativity property for the processors would be:
41 Each allocable resource has an ibm,associativity property. The LOPAPR
43 considering that the same resource can have multiple connections to the
49 The ibm,associativity-reference-points property is an array that is used
50 to define the relevant performance/distance  related boundaries, defining
51 the NUMA levels for the platform.
53 The definition of its elements also varies with the value of bit 0 of byte 5
54 of the ibm,architecture-vec-5 property. The format with bit 0 equal to zero
55 is also deprecated. With the current format, each integer of the
57 the first element is 1) of the ibm,associativity array. The first
58 boundary is the most significant to application performance, followed by
59 less significant boundaries. Allocated resources that belongs to the
61 that matches the relevancy of the boundary itself. Resources that belongs
62 to the same first boundary will have the shortest distance from each
66 Using the previous example, the following setting reference points defines
71 The first NUMA level (0x3) is interpreted as the third element of each
72 ibm,associativity array, the second level is the second element and
73 the third level is the first element. Let's also consider that elements
74 belonging to the first NUMA level have distance equal to 10 from each
75 other, and each NUMA level doubles the distance from the previous. This
76 means that the second would be 20 and the third level 40. For the P1 and
77 P2 processors, we would have the following NUMA levels:
95   P1 and P2 have the same third NUMA level, MOD1: Distance between them = 40
97 Changing the ibm,associativity-reference-points array changes the performance
98 distance attributes for the same associativity arrays, as the following
114   NUMA configuration, distance between them is one boundary above the first
118 In a hypothetical platform where all resources inside the same hardware module
119 is considered to be on the same performance boundary:
133   P1 and P2 belongs to the same first order boundary. The distance between then
137 How the pseries Linux guest calculates NUMA distances
140 Another key difference between ACPI SLIT and the LOPAPR regarding NUMA is
141 how the distances are expressed. The SLIT table provides the NUMA distance
142 value between the relevant resources. LOPAPR does not provide a standard
143 way to calculate it. We have the ibm,associativity for each resource, which
144 provides a common-performance hierarchy,  and the ibm,associativity-reference-points
148 The result is that each OS is free to implement and to interpret the distance
149 as it sees fit. For the pseries Linux guest, each level of NUMA duplicates
150 the distance of the previous level, and the maximum amount of levels is
151 limited to MAX_DISTANCE_REF_POINTS = 4 (from arch/powerpc/mm/numa.c in the
152 kernel tree). This results in the following distances:
154 * both resources in the first NUMA level: 10
164 Starting in QEMU 5.2, the pseries machine considers user input when setting NUMA
165 topology of the guest. The overall design is:
168   for 4 distinct NUMA distance values based on the NUMA levels
177 the machine itself, e.g. NVLink 2 GPUs). The now legacy support has been
179 (and 80 if using NVLink2 GPUs), and there is no need to disrupt the
182 To bring the user experience x86 users have when tuning up NUMA, we had
183 to operate under the current pseries Linux kernel logic described in
184 `How the pseries Linux guest calculates NUMA distances`_. The result
192 that to the values that the Linux kernel operates on (10, 20, 40, 80, 160).
201 The reasoning behind this approximation is to avoid any round up to the local
202 distance (10), keeping it exclusive to the 4th NUMA level (which is still
203 exclusive to the node_id). All other ranges were chosen under the developer
204 discretion of what would be (somewhat) sensible considering the user input.
205 Any other strategy can be used here, but in the end the reality is that we'll
206 have to accept that a large array of values will be translated to the same
207 NUMA topology in the guest, e.g. this user input:
225 Will both be translated to the same values internally:
234 Users are encouraged to use only the kernel values in the NUMA definition to
235 avoid being taken by surprise with that the guest is actually seeing in the
236 topology. There are enough potential surprises that are inherent to the
244 resource. This would be used to represent that the resource has multiple
245 connections with the board, and then the operational system, when deciding
246 NUMA distancing, should consider the associativity information that provides
247 the shortest distance.
249 The spapr implementation does not support multiple associativity arrays per
250 resource, neither does the pseries Linux kernel. We'll have to represent the
254 Consider the following NUMA topology entered by user input:
264 All the associativity arrays are initialized with NUMA id in all associativity
273 Honoring just the relative distances of node 0 to every other node, we find the
274 NUMA level matches (considering the reference points {0x4, 0x3, 0x2, 0x1}) for
283 We'll copy the associativity domains of node 0 to all other nodes, based on
284 the NUMA level matches. Between 0 and 1, a match in 0x2, we'll also copy
285 the domains 0x2 and 0x1 from 0 to 1 as well. This will give us:
290 Doing the same to node 2 and node 3, these are the associativity arrays
298 The distances related to node 0 are accounted for. For node 1, and keeping
299 in mind that we don't need to revisit node 0 again, the distance from
301 match in 0x2. Repeating the same logic of copying all domains up to
302 the NUMA level match:
309 In the last step we will analyze just nodes 2 and 3. The desired distance
318 The kernel will read these arrays and will calculate the following NUMA topology for
319 the guest:
329 Note that this is not what the user wanted - the desired distance between
330 0 and 3 is 40, we calculated it as 20. This is what the current logic and
331 implementation constraints of the kernel and QEMU will provide inside the
334 Users are welcome to use this knowledge and experiment with the input to get
335 the NUMA topology they want, or as closer as they want. The important thing
339 Limitations of the implementation
342 As mentioned above, the pSeries NUMA distance logic is, in fact, a way to approximate
343 user choice. The Linux kernel, and PAPR itself, does not provide QEMU with the ways
344 to fully map user input to actual NUMA distance the guest will use. These limitations
348   the distance from node A to B is always the same as B to A. We do not support
349   any A-B pair where the distance back and forth is asymmetric. For example, the
350   following topology isn't supported and the pSeries guest will not boot with this
360 * 'non-transitive' topologies will be poorly translated to the guest. This is the
361   kind of topology where the distance from a node A to B is X, B to C is X, but
362   the distance A to C is not X. E.g.:
372   In the example above, distance 0 to 2 is 20, 2 to 3 is 20, but 0 to 3 is 40.
373   The kernel will always match with the shortest associativity domain possible,
374   and we're attempting to retain the previous established relations between the
375   nodes. This means that a distance equal to 20 between nodes 0 and 2 and the
376   same distance 20 between nodes 2 and 3 will cause the distance between 0 and 3
383 In short, we can summarize the NUMA distances seem in pseries Linux guests, using
386 * local distance, i.e. the distance of the resource to its own NUMA node: 10
390 The way the pseries Linux guest calculates NUMA distances has a direct effect
392 the default ibm,associativity-reference-points being used in the pseries
397 The first and second level are equal, 0x4, and a third one was added in
399 regardless of how the ibm,associativity properties are being created in
400 the device tree, the pseries Linux guest will only recognize three scenarios
403 * if the resources belongs to the same first NUMA level = 10
404 * second level is skipped since it's equal to the first
406   to the same third NUMA level, having distance = 40
409 This also means that user input in QEMU command line does not change the
410 NUMA distancing inside the guest for the pseries machine.