Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)
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FreeBSD/Linux Kernel Cross Reference
sys/Documentation/prio_tree.txt
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- FREEBSD - FREEBSD-13-STABLE - FREEBSD-13-0 - FREEBSD-12-STABLE - FREEBSD-12-0 - FREEBSD-11-STABLE - FREEBSD-11-0 - FREEBSD-10-STABLE - FREEBSD-10-0 - FREEBSD-9-STABLE - FREEBSD-9-0 - FREEBSD-8-STABLE - FREEBSD-8-0 - FREEBSD-7-STABLE - FREEBSD-7-0 - FREEBSD-6-STABLE - FREEBSD-6-0 - FREEBSD-5-STABLE - FREEBSD-5-0 - FREEBSD-4-STABLE - FREEBSD-3-STABLE - FREEBSD22 - l41 - OPENBSD - linux-2.6 - MK84 - PLAN9 - xnu-8792
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1 The prio_tree.c code indexes vmas using 3 different indexes: 2 * heap_index = vm_pgoff + vm_size_in_pages : end_vm_pgoff 3 * radix_index = vm_pgoff : start_vm_pgoff 4 * size_index = vm_size_in_pages 5 6 A regular radix-priority-search-tree indexes vmas using only heap_index and 7 radix_index. The conditions for indexing are: 8 * ->heap_index >= ->left->heap_index && 9 ->heap_index >= ->right->heap_index 10 * if (->heap_index == ->left->heap_index) 11 then ->radix_index < ->left->radix_index; 12 * if (->heap_index == ->right->heap_index) 13 then ->radix_index < ->right->radix_index; 14 * nodes are hashed to left or right subtree using radix_index 15 similar to a pure binary radix tree. 16 17 A regular radix-priority-search-tree helps to store and query 18 intervals (vmas). However, a regular radix-priority-search-tree is only 19 suitable for storing vmas with different radix indices (vm_pgoff). 20 21 Therefore, the prio_tree.c extends the regular radix-priority-search-tree 22 to handle many vmas with the same vm_pgoff. Such vmas are handled in 23 2 different ways: 1) All vmas with the same radix _and_ heap indices are 24 linked using vm_set.list, 2) if there are many vmas with the same radix 25 index, but different heap indices and if the regular radix-priority-search 26 tree cannot index them all, we build an overflow-sub-tree that indexes such 27 vmas using heap and size indices instead of heap and radix indices. For 28 example, in the figure below some vmas with vm_pgoff = 0 (zero) are 29 indexed by regular radix-priority-search-tree whereas others are pushed 30 into an overflow-subtree. Note that all vmas in an overflow-sub-tree have 31 the same vm_pgoff (radix_index) and if necessary we build different 32 overflow-sub-trees to handle each possible radix_index. For example, 33 in figure we have 3 overflow-sub-trees corresponding to radix indices 34 0, 2, and 4. 35 36 In the final tree the first few (prio_tree_root->index_bits) levels 37 are indexed using heap and radix indices whereas the overflow-sub-trees below 38 those levels (i.e. levels prio_tree_root->index_bits + 1 and higher) are 39 indexed using heap and size indices. In overflow-sub-trees the size_index 40 is used for hashing the nodes to appropriate places. 41 42 Now, an example prio_tree: 43 44 vmas are represented [radix_index, size_index, heap_index] 45 i.e., [start_vm_pgoff, vm_size_in_pages, end_vm_pgoff] 46 47 level prio_tree_root->index_bits = 3 48 ----- 49 _ 50 0 [0,7,7] | 51 / \ | 52 ------------------ ------------ | Regular 53 / \ | radix priority 54 1 [1,6,7] [4,3,7] | search tree 55 / \ / \ | 56 ------- ----- ------ ----- | heap-and-radix 57 / \ / \ | indexed 58 2 [0,6,6] [2,5,7] [5,2,7] [6,1,7] | 59 / \ / \ / \ / \ | 60 3 [0,5,5] [1,5,6] [2,4,6] [3,4,7] [4,2,6] [5,1,6] [6,0,6] [7,0,7] | 61 / / / _ 62 / / / _ 63 4 [0,4,4] [2,3,5] [4,1,5] | 64 / / / | 65 5 [0,3,3] [2,2,4] [4,0,4] | Overflow-sub-trees 66 / / | 67 6 [0,2,2] [2,1,3] | heap-and-size 68 / / | indexed 69 7 [0,1,1] [2,0,2] | 70 / | 71 8 [0,0,0] | 72 _ 73 74 Note that we use prio_tree_root->index_bits to optimize the height 75 of the heap-and-radix indexed tree. Since prio_tree_root->index_bits is 76 set according to the maximum end_vm_pgoff mapped, we are sure that all 77 bits (in vm_pgoff) above prio_tree_root->index_bits are 0 (zero). Therefore, 78 we only use the first prio_tree_root->index_bits as radix_index. 79 Whenever index_bits is increased in prio_tree_expand, we shuffle the tree 80 to make sure that the first prio_tree_root->index_bits levels of the tree 81 is indexed properly using heap and radix indices. 82 83 We do not optimize the height of overflow-sub-trees using index_bits. 84 The reason is: there can be many such overflow-sub-trees and all of 85 them have to be suffled whenever the index_bits increases. This may involve 86 walking the whole prio_tree in prio_tree_insert->prio_tree_expand code 87 path which is not desirable. Hence, we do not optimize the height of the 88 heap-and-size indexed overflow-sub-trees using prio_tree->index_bits. 89 Instead the overflow sub-trees are indexed using full BITS_PER_LONG bits 90 of size_index. This may lead to skewed sub-trees because most of the 91 higher significant bits of the size_index are likely to be 0 (zero). In 92 the example above, all 3 overflow-sub-trees are skewed. This may marginally 93 affect the performance. However, processes rarely map many vmas with the 94 same start_vm_pgoff but different end_vm_pgoffs. Therefore, we normally 95 do not require overflow-sub-trees to index all vmas. 96 97 From the above discussion it is clear that the maximum height of 98 a prio_tree can be prio_tree_root->index_bits + BITS_PER_LONG. 99 However, in most of the common cases we do not need overflow-sub-trees, 100 so the tree height in the common cases will be prio_tree_root->index_bits. 101 102 It is fair to mention here that the prio_tree_root->index_bits 103 is increased on demand, however, the index_bits is not decreased when 104 vmas are removed from the prio_tree. That's tricky to do. Hence, it's 105 left as a home work problem. 106 107
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