NAME¶
vm_map_entry_resize_free —
vm map free
space algorithm
SYNOPSIS¶
#include <sys/param.h>
#include <vm/vm.h>
#include <vm/vm_map.h>
void
vm_map_entry_resize_free(
vm_map_t
map,
vm_map_entry_t
entry);
DESCRIPTION¶
This manual page describes the
vm_map_entry fields used in
the VM map free space algorithm, how to maintain consistency of these
variables, and the
vm_map_entry_resize_free() function.
VM map entries are organized as both a doubly-linked list
(
prev and
next pointers) and as a
binary search tree (
left and
right
pointers). The search tree is organized as a Sleator and Tarjan splay tree,
also known as a “self-adjusting tree”.
struct vm_map_entry {
struct vm_map_entry *prev;
struct vm_map_entry *next;
struct vm_map_entry *left;
struct vm_map_entry *right;
vm_offset_t start;
vm_offset_t end;
vm_offset_t avail_ssize;
vm_size_t adj_free;
vm_size_t max_free;
...
};
The free space algorithm adds two fields to
struct
vm_map_entry:
adj_free and
max_free. The
adj_free field is
the amount of free address space adjacent to and immediately following (higher
address) the map entry. This field is unused in the map header. Note that
adj_free depends on the linked list, not the splay tree
and that
adj_free can be computed as:
entry->adj_free = (entry->next == &map->header ?
map->max_offset : entry->next->start) - entry->end;
The
max_free field is the maximum amount of contiguous
free space in the entry's subtree. Note that
max_free
depends on the splay tree, not the linked list and that
max_free is computed by taking the maximum of its own
adj_free and the
max_free of its
left and right subtrees. Again,
max_free is unused in
the map header.
These fields allow for an
O(
log n)
implementation of
vm_map_findspace(). Using
max_free, we can immediately test for a sufficiently
large free region in an entire subtree. This makes it possible to find a
first-fit free region of a given size in one pass down the tree, so
O(
log n) amortized using splay trees.
When a free region changes size, we must update
adj_free
and
max_free in the preceding map entry and propagate
max_free up the tree. This is handled in
vm_map_entry_link() and
vm_map_entry_unlink() for the cases of inserting and
deleting an entry. Note that
vm_map_entry_link() updates
both the new entry and the previous entry, and that
vm_map_entry_unlink() updates the previous entry. Also note
that
max_free is not actually propagated up the tree.
Instead, that entry is first splayed to the root and then the change is made
there. This is a common technique in splay trees and is also how map entries
are linked and unlinked into the tree.
The
vm_map_entry_resize_free() function updates the free space
variables in the given
entry and propagates those values
up the tree. This function should be called whenever a map entry is resized
in-place, that is, by modifying its
start or
end values. Note that if you change
end, then you should resize that entry, but if you
change
start, then you should resize the previous entry.
The map must be locked before calling this function, and again, propagating
max_free is performed by splaying that entry to the
root.
EXAMPLES¶
Consider adding a map entry with
vm_map_insert().
ret = vm_map_insert(map, object, offset, start, end, prot,
max_prot, cow);
In this case, no further action is required to maintain consistency of the free
space variables. The
vm_map_insert() function calls
vm_map_entry_link() which updates both the new entry and the
previous entry. The same would be true for
vm_map_delete()
and for calling
vm_map_entry_link() or
vm_map_entry_unlink() directly.
Now consider resizing an entry in-place without a call to
vm_map_entry_link() or
vm_map_entry_unlink().
entry->start = new_start;
if (entry->prev != &map->header)
vm_map_entry_resize_free(map, entry->prev);
In this case, resetting
start changes the amount of free
space following the previous entry, so we use
vm_map_entry_resize_free() to update the previous entry.
Finally, suppose we change an entry's
end address.
entry->end = new_end;
vm_map_entry_resize_free(map, entry);
Here, we call
vm_map_entry_resize_free() on the entry itself.
SEE ALSO¶
vm_map(9),
vm_map_findspace(9)
Daniel D. Sleator and
Robert E. Tarjan, Self-Adjusting
Binary Search Trees, JACM, vol.
32(3), pp. 652-686, July
1985.
HISTORY¶
Splay trees were added to the VM map in
FreeBSD 5.0, and
the
O(
log n) tree-based free space
algorithm was added in
FreeBSD 5.3.
AUTHORS¶
The tree-based free space algorithm and this manual page were written by
Mark W. Krentel
⟨krentel@dreamscape.com⟩.