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// Copyright 2016 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package http2
import (
"fmt"
"math"
"sort"
)
// RFC 7540, Section 5.3.5: the default weight is 16.
const priorityDefaultWeight = 15 // 16 = 15 + 1
// PriorityWriteSchedulerConfig configures a priorityWriteScheduler.
type PriorityWriteSchedulerConfig struct {
// MaxClosedNodesInTree controls the maximum number of closed streams to
// retain in the priority tree. Setting this to zero saves a small amount
// of memory at the cost of performance.
//
// See RFC 7540, Section 5.3.4:
// "It is possible for a stream to become closed while prioritization
// information ... is in transit. ... This potentially creates suboptimal
// prioritization, since the stream could be given a priority that is
// different from what is intended. To avoid these problems, an endpoint
// SHOULD retain stream prioritization state for a period after streams
// become closed. The longer state is retained, the lower the chance that
// streams are assigned incorrect or default priority values."
MaxClosedNodesInTree int
// MaxIdleNodesInTree controls the maximum number of idle streams to
// retain in the priority tree. Setting this to zero saves a small amount
// of memory at the cost of performance.
//
// See RFC 7540, Section 5.3.4:
// Similarly, streams that are in the "idle" state can be assigned
// priority or become a parent of other streams. This allows for the
// creation of a grouping node in the dependency tree, which enables
// more flexible expressions of priority. Idle streams begin with a
// default priority (Section 5.3.5).
MaxIdleNodesInTree int
// ThrottleOutOfOrderWrites enables write throttling to help ensure that
// data is delivered in priority order. This works around a race where
// stream B depends on stream A and both streams are about to call Write
// to queue DATA frames. If B wins the race, a naive scheduler would eagerly
// write as much data from B as possible, but this is suboptimal because A
// is a higher-priority stream. With throttling enabled, we write a small
// amount of data from B to minimize the amount of bandwidth that B can
// steal from A.
ThrottleOutOfOrderWrites bool
}
// NewPriorityWriteScheduler constructs a WriteScheduler that schedules
// frames by following HTTP/2 priorities as described in RFC 7540 Section 5.3.
// If cfg is nil, default options are used.
func NewPriorityWriteScheduler(cfg *PriorityWriteSchedulerConfig) WriteScheduler {
if cfg == nil {
// For justification of these defaults, see:
// https://docs.google.com/document/d/1oLhNg1skaWD4_DtaoCxdSRN5erEXrH-KnLrMwEpOtFY
cfg = &PriorityWriteSchedulerConfig{
MaxClosedNodesInTree: 10,
MaxIdleNodesInTree: 10,
ThrottleOutOfOrderWrites: false,
}
}
ws := &priorityWriteScheduler{
nodes: make(map[uint32]*priorityNode),
maxClosedNodesInTree: cfg.MaxClosedNodesInTree,
maxIdleNodesInTree: cfg.MaxIdleNodesInTree,
enableWriteThrottle: cfg.ThrottleOutOfOrderWrites,
}
ws.nodes[0] = &ws.root
if cfg.ThrottleOutOfOrderWrites {
ws.writeThrottleLimit = 1024
} else {
ws.writeThrottleLimit = math.MaxInt32
}
return ws
}
type priorityNodeState int
const (
priorityNodeOpen priorityNodeState = iota
priorityNodeClosed
priorityNodeIdle
)
// priorityNode is a node in an HTTP/2 priority tree.
// Each node is associated with a single stream ID.
// See RFC 7540, Section 5.3.
type priorityNode struct {
q writeQueue // queue of pending frames to write
id uint32 // id of the stream, or 0 for the root of the tree
weight uint8 // the actual weight is weight+1, so the value is in [1,256]
state priorityNodeState // open | closed | idle
bytes int64 // number of bytes written by this node, or 0 if closed
subtreeBytes int64 // sum(node.bytes) of all nodes in this subtree
// These links form the priority tree.
parent *priorityNode
kids *priorityNode // start of the kids list
prev, next *priorityNode // doubly-linked list of siblings
}
func (n *priorityNode) setParent(parent *priorityNode) {
if n == parent {
panic("setParent to self")
}
if n.parent == parent {
return
}
// Unlink from current parent.
if parent := n.parent; parent != nil {
if n.prev == nil {
parent.kids = n.next
} else {
n.prev.next = n.next
}
if n.next != nil {
n.next.prev = n.prev
}
}
// Link to new parent.
// If parent=nil, remove n from the tree.
// Always insert at the head of parent.kids (this is assumed by walkReadyInOrder).
n.parent = parent
if parent == nil {
n.next = nil
n.prev = nil
} else {
n.next = parent.kids
n.prev = nil
if n.next != nil {
n.next.prev = n
}
parent.kids = n
}
}
func (n *priorityNode) addBytes(b int64) {
n.bytes += b
for ; n != nil; n = n.parent {
n.subtreeBytes += b
}
}
// walkReadyInOrder iterates over the tree in priority order, calling f for each node
// with a non-empty write queue. When f returns true, this function returns true and the
// walk halts. tmp is used as scratch space for sorting.
//
// f(n, openParent) takes two arguments: the node to visit, n, and a bool that is true
// if any ancestor p of n is still open (ignoring the root node).
func (n *priorityNode) walkReadyInOrder(openParent bool, tmp *[]*priorityNode, f func(*priorityNode, bool) bool) bool {
if !n.q.empty() && f(n, openParent) {
return true
}
if n.kids == nil {
return false
}
// Don't consider the root "open" when updating openParent since
// we can't send data frames on the root stream (only control frames).
if n.id != 0 {
openParent = openParent || (n.state == priorityNodeOpen)
}
// Common case: only one kid or all kids have the same weight.
// Some clients don't use weights; other clients (like web browsers)
// use mostly-linear priority trees.
w := n.kids.weight
needSort := false
for k := n.kids.next; k != nil; k = k.next {
if k.weight != w {
needSort = true
break
}
}
if !needSort {
for k := n.kids; k != nil; k = k.next {
if k.walkReadyInOrder(openParent, tmp, f) {
return true
}
}
return false
}
// Uncommon case: sort the child nodes. We remove the kids from the parent,
// then re-insert after sorting so we can reuse tmp for future sort calls.
*tmp = (*tmp)[:0]
for n.kids != nil {
*tmp = append(*tmp, n.kids)
n.kids.setParent(nil)
}
sort.Sort(sortPriorityNodeSiblings(*tmp))
for i := len(*tmp) - 1; i >= 0; i-- {
(*tmp)[i].setParent(n) // setParent inserts at the head of n.kids
}
for k := n.kids; k != nil; k = k.next {
if k.walkReadyInOrder(openParent, tmp, f) {
return true
}
}
return false
}
type sortPriorityNodeSiblings []*priorityNode
func (z sortPriorityNodeSiblings) Len() int { return len(z) }
func (z sortPriorityNodeSiblings) Swap(i, k int) { z[i], z[k] = z[k], z[i] }
func (z sortPriorityNodeSiblings) Less(i, k int) bool {
// Prefer the subtree that has sent fewer bytes relative to its weight.
// See sections 5.3.2 and 5.3.4.
wi, bi := float64(z[i].weight+1), float64(z[i].subtreeBytes)
wk, bk := float64(z[k].weight+1), float64(z[k].subtreeBytes)
if bi == 0 && bk == 0 {
return wi >= wk
}
if bk == 0 {
return false
}
return bi/bk <= wi/wk
}
type priorityWriteScheduler struct {
// root is the root of the priority tree, where root.id = 0.
// The root queues control frames that are not associated with any stream.
root priorityNode
// nodes maps stream ids to priority tree nodes.
nodes map[uint32]*priorityNode
// maxID is the maximum stream id in nodes.
maxID uint32
// lists of nodes that have been closed or are idle, but are kept in
// the tree for improved prioritization. When the lengths exceed either
// maxClosedNodesInTree or maxIdleNodesInTree, old nodes are discarded.
closedNodes, idleNodes []*priorityNode
// From the config.
maxClosedNodesInTree int
maxIdleNodesInTree int
writeThrottleLimit int32
enableWriteThrottle bool
// tmp is scratch space for priorityNode.walkReadyInOrder to reduce allocations.
tmp []*priorityNode
// pool of empty queues for reuse.
queuePool writeQueuePool
}
func (ws *priorityWriteScheduler) OpenStream(streamID uint32, options OpenStreamOptions) {
// The stream may be currently idle but cannot be opened or closed.
if curr := ws.nodes[streamID]; curr != nil {
if curr.state != priorityNodeIdle {
panic(fmt.Sprintf("stream %d already opened", streamID))
}
curr.state = priorityNodeOpen
return
}
// RFC 7540, Section 5.3.5:
// "All streams are initially assigned a non-exclusive dependency on stream 0x0.
// Pushed streams initially depend on their associated stream. In both cases,
// streams are assigned a default weight of 16."
parent := ws.nodes[options.PusherID]
if parent == nil {
parent = &ws.root
}
n := &priorityNode{
q: *ws.queuePool.get(),
id: streamID,
weight: priorityDefaultWeight,
state: priorityNodeOpen,
}
n.setParent(parent)
ws.nodes[streamID] = n
if streamID > ws.maxID {
ws.maxID = streamID
}
}
func (ws *priorityWriteScheduler) CloseStream(streamID uint32) {
if streamID == 0 {
panic("violation of WriteScheduler interface: cannot close stream 0")
}
if ws.nodes[streamID] == nil {
panic(fmt.Sprintf("violation of WriteScheduler interface: unknown stream %d", streamID))
}
if ws.nodes[streamID].state != priorityNodeOpen {
panic(fmt.Sprintf("violation of WriteScheduler interface: stream %d already closed", streamID))
}
n := ws.nodes[streamID]
n.state = priorityNodeClosed
n.addBytes(-n.bytes)
q := n.q
ws.queuePool.put(&q)
n.q.s = nil
if ws.maxClosedNodesInTree > 0 {
ws.addClosedOrIdleNode(&ws.closedNodes, ws.maxClosedNodesInTree, n)
} else {
ws.removeNode(n)
}
}
func (ws *priorityWriteScheduler) AdjustStream(streamID uint32, priority PriorityParam) {
if streamID == 0 {
panic("adjustPriority on root")
}
// If streamID does not exist, there are two cases:
// - A closed stream that has been removed (this will have ID <= maxID)
// - An idle stream that is being used for "grouping" (this will have ID > maxID)
n := ws.nodes[streamID]
if n == nil {
if streamID <= ws.maxID || ws.maxIdleNodesInTree == 0 {
return
}
ws.maxID = streamID
n = &priorityNode{
q: *ws.queuePool.get(),
id: streamID,
weight: priorityDefaultWeight,
state: priorityNodeIdle,
}
n.setParent(&ws.root)
ws.nodes[streamID] = n
ws.addClosedOrIdleNode(&ws.idleNodes, ws.maxIdleNodesInTree, n)
}
// Section 5.3.1: A dependency on a stream that is not currently in the tree
// results in that stream being given a default priority (Section 5.3.5).
parent := ws.nodes[priority.StreamDep]
if parent == nil {
n.setParent(&ws.root)
n.weight = priorityDefaultWeight
return
}
// Ignore if the client tries to make a node its own parent.
if n == parent {
return
}
// Section 5.3.3:
// "If a stream is made dependent on one of its own dependencies, the
// formerly dependent stream is first moved to be dependent on the
// reprioritized stream's previous parent. The moved dependency retains
// its weight."
//
// That is: if parent depends on n, move parent to depend on n.parent.
for x := parent.parent; x != nil; x = x.parent {
if x == n {
parent.setParent(n.parent)
break
}
}
// Section 5.3.3: The exclusive flag causes the stream to become the sole
// dependency of its parent stream, causing other dependencies to become
// dependent on the exclusive stream.
if priority.Exclusive {
k := parent.kids
for k != nil {
next := k.next
if k != n {
k.setParent(n)
}
k = next
}
}
n.setParent(parent)
n.weight = priority.Weight
}
func (ws *priorityWriteScheduler) Push(wr FrameWriteRequest) {
var n *priorityNode
if wr.isControl() {
n = &ws.root
} else {
id := wr.StreamID()
n = ws.nodes[id]
if n == nil {
// id is an idle or closed stream. wr should not be a HEADERS or
// DATA frame. In other case, we push wr onto the root, rather
// than creating a new priorityNode.
if wr.DataSize() > 0 {
panic("add DATA on non-open stream")
}
n = &ws.root
}
}
n.q.push(wr)
}
func (ws *priorityWriteScheduler) Pop() (wr FrameWriteRequest, ok bool) {
ws.root.walkReadyInOrder(false, &ws.tmp, func(n *priorityNode, openParent bool) bool {
limit := int32(math.MaxInt32)
if openParent {
limit = ws.writeThrottleLimit
}
wr, ok = n.q.consume(limit)
if !ok {
return false
}
n.addBytes(int64(wr.DataSize()))
// If B depends on A and B continuously has data available but A
// does not, gradually increase the throttling limit to allow B to
// steal more and more bandwidth from A.
if openParent {
ws.writeThrottleLimit += 1024
if ws.writeThrottleLimit < 0 {
ws.writeThrottleLimit = math.MaxInt32
}
} else if ws.enableWriteThrottle {
ws.writeThrottleLimit = 1024
}
return true
})
return wr, ok
}
func (ws *priorityWriteScheduler) addClosedOrIdleNode(list *[]*priorityNode, maxSize int, n *priorityNode) {
if maxSize == 0 {
return
}
if len(*list) == maxSize {
// Remove the oldest node, then shift left.
ws.removeNode((*list)[0])
x := (*list)[1:]
copy(*list, x)
*list = (*list)[:len(x)]
}
*list = append(*list, n)
}
func (ws *priorityWriteScheduler) removeNode(n *priorityNode) {
for k := n.kids; k != nil; k = k.next {
k.setParent(n.parent)
}
n.setParent(nil)
delete(ws.nodes, n.id)
}