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+The prio_tree.c code indexes vmas using 3 different indexes:
+ * heap_index = vm_pgoff + vm_size_in_pages : end_vm_pgoff
+ * radix_index = vm_pgoff : start_vm_pgoff
+ * size_index = vm_size_in_pages
+A regular radix-priority-search-tree indexes vmas using only heap_index and
+radix_index. The conditions for indexing are:
+ * ->heap_index >= ->left->heap_index &&
+ ->heap_index >= ->right->heap_index
+ * if (->heap_index == ->left->heap_index)
+ then ->radix_index < ->left->radix_index;
+ * if (->heap_index == ->right->heap_index)
+ then ->radix_index < ->right->radix_index;
+ * nodes are hashed to left or right subtree using radix_index
+ similar to a pure binary radix tree.
+A regular radix-priority-search-tree helps to store and query
+intervals (vmas). However, a regular radix-priority-search-tree is only
+suitable for storing vmas with different radix indices (vm_pgoff).
+Therefore, the prio_tree.c extends the regular radix-priority-search-tree
+to handle many vmas with the same vm_pgoff. Such vmas are handled in
+2 different ways: 1) All vmas with the same radix _and_ heap indices are
+linked using vm_set.list, 2) if there are many vmas with the same radix
+index, but different heap indices and if the regular radix-priority-search
+tree cannot index them all, we build an overflow-sub-tree that indexes such
+vmas using heap and size indices instead of heap and radix indices. For
+example, in the figure below some vmas with vm_pgoff = 0 (zero) are
+indexed by regular radix-priority-search-tree whereas others are pushed
+into an overflow-subtree. Note that all vmas in an overflow-sub-tree have
+the same vm_pgoff (radix_index) and if necessary we build different
+overflow-sub-trees to handle each possible radix_index. For example,
+in figure we have 3 overflow-sub-trees corresponding to radix indices
+0, 2, and 4.
+In the final tree the first few (prio_tree_root->index_bits) levels
+are indexed using heap and radix indices whereas the overflow-sub-trees below
+those levels (i.e. levels prio_tree_root->index_bits + 1 and higher) are
+indexed using heap and size indices. In overflow-sub-trees the size_index
+is used for hashing the nodes to appropriate places.
+Now, an example prio_tree:
+ vmas are represented [radix_index, size_index, heap_index]
+ i.e., [start_vm_pgoff, vm_size_in_pages, end_vm_pgoff]
+level prio_tree_root->index_bits = 3
+ 0 [0,7,7] |
+ / \ |
+ ------------------ ------------ | Regular
+ / \ | radix priority
+ 1 [1,6,7] [4,3,7] | search tree
+ / \ / \ |
+ ------- ----- ------ ----- | heap-and-radix
+ / \ / \ | indexed
+ 2 [0,6,6] [2,5,7] [5,2,7] [6,1,7] |
+ / \ / \ / \ / \ |
+ 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] |
+ / / / _
+ / / / _
+ 4 [0,4,4] [2,3,5] [4,1,5] |
+ / / / |
+ 5 [0,3,3] [2,2,4] [4,0,4] | Overflow-sub-trees
+ / / |
+ 6 [0,2,2] [2,1,3] | heap-and-size
+ / / | indexed
+ 7 [0,1,1] [2,0,2] |
+ / |
+ 8 [0,0,0] |
+Note that we use prio_tree_root->index_bits to optimize the height
+of the heap-and-radix indexed tree. Since prio_tree_root->index_bits is
+set according to the maximum end_vm_pgoff mapped, we are sure that all
+bits (in vm_pgoff) above prio_tree_root->index_bits are 0 (zero). Therefore,
+we only use the first prio_tree_root->index_bits as radix_index.
+Whenever index_bits is increased in prio_tree_expand, we shuffle the tree
+to make sure that the first prio_tree_root->index_bits levels of the tree
+is indexed properly using heap and radix indices.
+We do not optimize the height of overflow-sub-trees using index_bits.
+The reason is: there can be many such overflow-sub-trees and all of
+them have to be suffled whenever the index_bits increases. This may involve
+walking the whole prio_tree in prio_tree_insert->prio_tree_expand code
+path which is not desirable. Hence, we do not optimize the height of the
+heap-and-size indexed overflow-sub-trees using prio_tree->index_bits.
+Instead the overflow sub-trees are indexed using full BITS_PER_LONG bits
+of size_index. This may lead to skewed sub-trees because most of the
+higher significant bits of the size_index are likely to be be 0 (zero). In
+the example above, all 3 overflow-sub-trees are skewed. This may marginally
+affect the performance. However, processes rarely map many vmas with the
+same start_vm_pgoff but different end_vm_pgoffs. Therefore, we normally
+do not require overflow-sub-trees to index all vmas.
+From the above discussion it is clear that the maximum height of
+a prio_tree can be prio_tree_root->index_bits + BITS_PER_LONG.
+However, in most of the common cases we do not need overflow-sub-trees,
+so the tree height in the common cases will be prio_tree_root->index_bits.
+It is fair to mention here that the prio_tree_root->index_bits
+is increased on demand, however, the index_bits is not decreased when
+vmas are removed from the prio_tree. That's tricky to do. Hence, it's
+left as a home work problem.