| // 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 id := wr.StreamID(); id == 0 { |
| n = &ws.root |
| } else { |
| n = ws.nodes[id] |
| if n == nil { |
| // id is an idle or closed stream. wr should not be a HEADERS or |
| // DATA frame. However, wr can be a RST_STREAM. In this case, we |
| // push wr onto the root, rather than creating a new priorityNode, |
| // since RST_STREAM is tiny and the stream's priority is unknown |
| // anyway. See issue #17919. |
| 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) |
| } |