mirror of
https://github.com/ossrs/srs.git
synced 2025-03-09 15:49:59 +00:00
60 lines
3.1 KiB
Text
60 lines
3.1 KiB
Text
How the timeout heap works
|
|
|
|
As of version 1.5, the State Threads Library represents the queue of
|
|
sleeping threads using a heap data structure rather than a sorted
|
|
linked list. This improves performance when there is a large number
|
|
of sleeping threads, since insertion into a heap takes O(log N) time
|
|
while insertion into a sorted list takes O(N) time. For example, in
|
|
one test 1000 threads were created, each thread called st_usleep()
|
|
with a random time interval, and then all the threads where
|
|
immediately interrupted and joined before the sleeps had a chance to
|
|
finish. The whole process was repeated 1000 times, for a total of a
|
|
million sleep queue insertions and removals. With the old list-based
|
|
sleep queue, this test took 100 seconds; now it takes only 12 seconds.
|
|
|
|
Heap data structures are typically based on dynamically resized
|
|
arrays. However, since the existing ST code base was very nicely
|
|
structured around linking the thread objects into pointer-based lists
|
|
without the need for any auxiliary data structures, implementing the
|
|
heap using a similar nodes-and-pointers based approach seemed more
|
|
appropriate for ST than introducing a separate array.
|
|
|
|
Thus, the new ST timeout heap works by organizing the existing
|
|
_st_thread_t objects in a balanced binary tree, just as they were
|
|
previously organized into a doubly-linked, sorted list. The global
|
|
_ST_SLEEPQ variable, formerly a linked list head, is now simply a
|
|
pointer to the root of this tree, and the root node of the tree is the
|
|
thread with the earliest timeout. Each thread object has two child
|
|
pointers, "left" and "right", pointing to threads with later timeouts.
|
|
|
|
Each node in the tree is numbered with an integer index, corresponding
|
|
to the array index in an array-based heap, and the tree is kept fully
|
|
balanced and left-adjusted at all times. In other words, the tree
|
|
consists of any number of fully populated top levels, followed by a
|
|
single bottom level which may be partially populated, such that any
|
|
existing nodes form a contiguous block to the left and the spaces for
|
|
missing nodes form a contiguous block to the right. For example, if
|
|
there are nine threads waiting for a timeout, they are numbered and
|
|
arranged in a tree exactly as follows:
|
|
|
|
1
|
|
/ \
|
|
2 3
|
|
/ \ / \
|
|
4 5 6 7
|
|
/ \
|
|
8 9
|
|
|
|
Each node has either no children, only a left child, or both a left
|
|
and a right child. Children always time out later than their parents
|
|
(this is called the "heap invariant"), but when a node has two
|
|
children, their mutual order is unspecified - the left child may time
|
|
out before or after the right child. If a node is numbered N, its
|
|
left child is numbered 2N, and its right child is numbered 2N+1.
|
|
|
|
There is no pointer from a child to its parent; all pointers point
|
|
downward. Additions and deletions both work by starting at the root
|
|
and traversing the tree towards the leaves, going left or right
|
|
according to the binary digits forming the index of the destination
|
|
node. As nodes are added or deleted, existing nodes are rearranged to
|
|
maintain the heap invariant.
|