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Diffstat (limited to 'docs/specs/ppc-spapr-numa.rst')
| -rw-r--r-- | docs/specs/ppc-spapr-numa.rst | 235 |
1 files changed, 227 insertions, 8 deletions
diff --git a/docs/specs/ppc-spapr-numa.rst b/docs/specs/ppc-spapr-numa.rst index e762038022..5fca2bdd8e 100644 --- a/docs/specs/ppc-spapr-numa.rst +++ b/docs/specs/ppc-spapr-numa.rst @@ -158,9 +158,235 @@ kernel tree). This results in the following distances: * resources four NUMA levels apart: 160 -Consequences for QEMU NUMA tuning +pseries NUMA mechanics +====================== + +Starting in QEMU 5.2, the pseries machine considers user input when setting NUMA +topology of the guest. The overall design is: + +* ibm,associativity-reference-points is set to {0x4, 0x3, 0x2, 0x1}, allowing + for 4 distinct NUMA distance values based on the NUMA levels + +* ibm,max-associativity-domains supports multiple associativity domains in all + NUMA levels, granting user flexibility + +* ibm,associativity for all resources varies with user input + +These changes are only effective for pseries-5.2 and newer machines that are +created with more than one NUMA node (disconsidering NUMA nodes created by +the machine itself, e.g. NVLink 2 GPUs). The now legacy support has been +around for such a long time, with users seeing NUMA distances 10 and 40 +(and 80 if using NVLink2 GPUs), and there is no need to disrupt the +existing experience of those guests. + +To bring the user experience x86 users have when tuning up NUMA, we had +to operate under the current pseries Linux kernel logic described in +`How the pseries Linux guest calculates NUMA distances`_. The result +is that we needed to translate NUMA distance user input to pseries +Linux kernel input. + +Translating user distance to kernel distance +-------------------------------------------- + +User input for NUMA distance can vary from 10 to 254. We need to translate +that to the values that the Linux kernel operates on (10, 20, 40, 80, 160). +This is how it is being done: + +* user distance 11 to 30 will be interpreted as 20 +* user distance 31 to 60 will be interpreted as 40 +* user distance 61 to 120 will be interpreted as 80 +* user distance 121 and beyond will be interpreted as 160 +* user distance 10 stays 10 + +The reasoning behind this aproximation is to avoid any round up to the local +distance (10), keeping it exclusive to the 4th NUMA level (which is still +exclusive to the node_id). All other ranges were chosen under the developer +discretion of what would be (somewhat) sensible considering the user input. +Any other strategy can be used here, but in the end the reality is that we'll +have to accept that a large array of values will be translated to the same +NUMA topology in the guest, e.g. this user input: + +:: + + 0 1 2 + 0 10 31 120 + 1 31 10 30 + 2 120 30 10 + +And this other user input: + +:: + + 0 1 2 + 0 10 60 61 + 1 60 10 11 + 2 61 11 10 + +Will both be translated to the same values internally: + +:: + + 0 1 2 + 0 10 40 80 + 1 40 10 20 + 2 80 20 10 + +Users are encouraged to use only the kernel values in the NUMA definition to +avoid being taken by surprise with that the guest is actually seeing in the +topology. There are enough potential surprises that are inherent to the +associativity domain assignment process, discussed below. + + +How associativity domains are assigned +-------------------------------------- + +LOPAPR allows more than one associativity array (or 'string') per allocated +resource. This would be used to represent that the resource has multiple +connections with the board, and then the operational system, when deciding +NUMA distancing, should consider the associativity information that provides +the shortest distance. + +The spapr implementation does not support multiple associativity arrays per +resource, neither does the pseries Linux kernel. We'll have to represent the +NUMA topology using one associativity per resource, which means that choices +and compromises are going to be made. + +Consider the following NUMA topology entered by user input: + +:: + + 0 1 2 3 + 0 10 40 20 40 + 1 40 10 80 40 + 2 20 80 10 20 + 3 40 40 20 10 + +All the associativity arrays are initialized with NUMA id in all associativity +domains: + +* node 0: 0 0 0 0 +* node 1: 1 1 1 1 +* node 2: 2 2 2 2 +* node 3: 3 3 3 3 + + +Honoring just the relative distances of node 0 to every other node, we find the +NUMA level matches (considering the reference points {0x4, 0x3, 0x2, 0x1}) for +each distance: + +* distance from 0 to 1 is 40 (no match at 0x4 and 0x3, will match + at 0x2) +* distance from 0 to 2 is 20 (no match at 0x4, will match at 0x3) +* distance from 0 to 3 is 40 (no match at 0x4 and 0x3, will match + at 0x2) + +We'll copy the associativity domains of node 0 to all other nodes, based on +the NUMA level matches. Between 0 and 1, a match in 0x2, we'll also copy +the domains 0x2 and 0x1 from 0 to 1 as well. This will give us: + +* node 0: 0 0 0 0 +* node 1: 0 0 1 1 + +Doing the same to node 2 and node 3, these are the associativity arrays +after considering all matches with node 0: + +* node 0: 0 0 0 0 +* node 1: 0 0 1 1 +* node 2: 0 0 0 2 +* node 3: 0 0 3 3 + +The distances related to node 0 are accounted for. For node 1, and keeping +in mind that we don't need to revisit node 0 again, the distance from +node 1 to 2 is 80, matching at 0x1, and distance from 1 to 3 is 40, +match in 0x2. Repeating the same logic of copying all domains up to +the NUMA level match: + +* node 0: 0 0 0 0 +* node 1: 1 0 1 1 +* node 2: 1 0 0 2 +* node 3: 1 0 3 3 + +In the last step we will analyze just nodes 2 and 3. The desired distance +between 2 and 3 is 20, i.e. a match in 0x3: + +* node 0: 0 0 0 0 +* node 1: 1 0 1 1 +* node 2: 1 0 0 2 +* node 3: 1 0 0 3 + + +The kernel will read these arrays and will calculate the following NUMA topology for +the guest: + +:: + + 0 1 2 3 + 0 10 40 20 20 + 1 40 10 40 40 + 2 20 40 10 20 + 3 20 40 20 10 + +Note that this is not what the user wanted - the desired distance between +0 and 3 is 40, we calculated it as 20. This is what the current logic and +implementation constraints of the kernel and QEMU will provide inside the +LOPAPR specification. + +Users are welcome to use this knowledge and experiment with the input to get +the NUMA topology they want, or as closer as they want. The important thing +is to keep expectations up to par with what we are capable of provide at this +moment: an approximation. + +Limitations of the implementation --------------------------------- +As mentioned above, the pSeries NUMA distance logic is, in fact, a way to approximate +user choice. The Linux kernel, and PAPR itself, does not provide QEMU with the ways +to fully map user input to actual NUMA distance the guest will use. These limitations +creates two notable limitations in our support: + +* Asymmetrical topologies aren't supported. We only support NUMA topologies where + the distance from node A to B is always the same as B to A. We do not support + any A-B pair where the distance back and forth is asymmetric. For example, the + following topology isn't supported and the pSeries guest will not boot with this + user input: + +:: + + 0 1 + 0 10 40 + 1 20 10 + + +* 'non-transitive' topologies will be poorly translated to the guest. This is the + kind of topology where the distance from a node A to B is X, B to C is X, but + the distance A to C is not X. E.g.: + +:: + + 0 1 2 3 + 0 10 20 20 40 + 1 20 10 80 40 + 2 20 80 10 20 + 3 40 40 20 10 + + In the example above, distance 0 to 2 is 20, 2 to 3 is 20, but 0 to 3 is 40. + The kernel will always match with the shortest associativity domain possible, + and we're attempting to retain the previous established relations between the + nodes. This means that a distance equal to 20 between nodes 0 and 2 and the + same distance 20 between nodes 2 and 3 will cause the distance between 0 and 3 + to also be 20. + + +Legacy (5.1 and older) pseries NUMA mechanics +============================================= + +In short, we can summarize the NUMA distances seem in pseries Linux guests, using +QEMU up to 5.1, as follows: + +* local distance, i.e. the distance of the resource to its own NUMA node: 10 +* if it's a NVLink GPU device, distance: 80 +* every other resource, distance: 40 + The way the pseries Linux guest calculates NUMA distances has a direct effect on what QEMU users can expect when doing NUMA tuning. As of QEMU 5.1, this is the default ibm,associativity-reference-points being used in the pseries @@ -180,12 +406,5 @@ as far as NUMA distance goes: to the same third NUMA level, having distance = 40 * for NVLink GPUs, distance = 80 from everything else -In short, we can summarize the NUMA distances seem in pseries Linux guests, using -QEMU up to 5.1, as follows: - -* local distance, i.e. the distance of the resource to its own NUMA node: 10 -* if it's a NVLink GPU device, distance: 80 -* every other resource, distance: 40 - This also means that user input in QEMU command line does not change the NUMA distancing inside the guest for the pseries machine. |