associativity.rst

  1============================
  2NUMA resource associativity
  3============================
  4
  5Associativity represents the groupings of the various platform resources into
  6domains of substantially similar mean performance relative to resources outside
  7of that domain. Resources subsets of a given domain that exhibit better
  8performance relative to each other than relative to other resources subsets
  9are represented as being members of a sub-grouping domain. This performance
 10characteristic is presented in terms of NUMA node distance within the Linux kernel.
 11From the platform view, these groups are also referred to as domains.
 12
 13PAPR interface currently supports different ways of communicating these resource
 14grouping details to the OS. These are referred to as Form 0, Form 1 and Form2
 15associativity grouping. Form 0 is the oldest format and is now considered deprecated.
 16
 17Hypervisor indicates the type/form of associativity used via "ibm,architecture-vec-5 property".
 18Bit 0 of byte 5 in the "ibm,architecture-vec-5" property indicates usage of Form 0 or Form 1.
 19A value of 1 indicates the usage of Form 1 associativity. For Form 2 associativity
 20bit 2 of byte 5 in the "ibm,architecture-vec-5" property is used.
 21
 22Form 0
 23------
 24Form 0 associativity supports only two NUMA distances (LOCAL and REMOTE).
 25
 26Form 1
 27------
 28With Form 1 a combination of ibm,associativity-reference-points, and ibm,associativity
 29device tree properties are used to determine the NUMA distance between resource groups/domains.
 30
 31The “ibm,associativity” property contains a list of one or more numbers (domainID)
 32representing the resource’s platform grouping domains.
 33
 34The “ibm,associativity-reference-points” property contains a list of one or more numbers
 35(domainID index) that represents the 1 based ordinal in the associativity lists.
 36The list of domainID indexes represents an increasing hierarchy of resource grouping.
 37
 38ex:
 39{ primary domainID index, secondary domainID index, tertiary domainID index.. }
 40
 41Linux kernel uses the domainID at the primary domainID index as the NUMA node id.
 42Linux kernel computes NUMA distance between two domains by recursively comparing
 43if they belong to the same higher-level domains. For mismatch at every higher
 44level of the resource group, the kernel doubles the NUMA distance between the
 45comparing domains.
 46
 47Form 2
 48-------
 49Form 2 associativity format adds separate device tree properties representing NUMA node distance
 50thereby making the node distance computation flexible. Form 2 also allows flexible primary
 51domain numbering. With numa distance computation now detached from the index value in
 52"ibm,associativity-reference-points" property, Form 2 allows a large number of primary domain
 53ids at the same domainID index representing resource groups of different performance/latency
 54characteristics.
 55
 56Hypervisor indicates the usage of FORM2 associativity using bit 2 of byte 5 in the
 57"ibm,architecture-vec-5" property.
 58
 59"ibm,numa-lookup-index-table" property contains a list of one or more numbers representing
 60the domainIDs present in the system. The offset of the domainID in this property is
 61used as an index while computing numa distance information via "ibm,numa-distance-table".
 62
 63prop-encoded-array: The number N of the domainIDs encoded as with encode-int, followed by
 64N domainID encoded as with encode-int
 65
 66For ex:
 67"ibm,numa-lookup-index-table" =  {4, 0, 8, 250, 252}. The offset of domainID 8 (2) is used when
 68computing the distance of domain 8 from other domains present in the system. For the rest of
 69this document, this offset will be referred to as domain distance offset.
 70
 71"ibm,numa-distance-table" property contains a list of one or more numbers representing the NUMA
 72distance between resource groups/domains present in the system.
 73
 74prop-encoded-array: The number N of the distance values encoded as with encode-int, followed by
 75N distance values encoded as with encode-bytes. The max distance value we could encode is 255.
 76The number N must be equal to the square of m where m is the number of domainIDs in the
 77numa-lookup-index-table.
 78
 79For ex:
 80ibm,numa-lookup-index-table = <3 0 8 40>;
 81ibm,numa-distace-table = <9>, /bits/ 8 < 10  20  80 20  10 160 80 160  10>;
 82
 83::
 84
 85	  | 0    8   40
 86	--|------------
 87	  |
 88	0 | 10   20  80
 89	  |
 90	8 | 20   10  160
 91	  |
 92	40| 80   160  10
 93
 94A possible "ibm,associativity" property for resources in node 0, 8 and 40
 95
 96{ 3, 6, 7, 0 }
 97{ 3, 6, 9, 8 }
 98{ 3, 6, 7, 40}
 99
100With "ibm,associativity-reference-points"  { 0x3 }
101
102"ibm,lookup-index-table" helps in having a compact representation of distance matrix.
103Since domainID can be sparse, the matrix of distances can also be effectively sparse.
104With "ibm,lookup-index-table" we can achieve a compact representation of
105distance information.