Holger Hildebrandt | fa07499 | 2020-03-27 15:42:06 +0000 | [diff] [blame^] | 1 | /* |
| 2 | * |
| 3 | * Copyright 2017 gRPC authors. |
| 4 | * |
| 5 | * Licensed under the Apache License, Version 2.0 (the "License"); |
| 6 | * you may not use this file except in compliance with the License. |
| 7 | * You may obtain a copy of the License at |
| 8 | * |
| 9 | * http://www.apache.org/licenses/LICENSE-2.0 |
| 10 | * |
| 11 | * Unless required by applicable law or agreed to in writing, software |
| 12 | * distributed under the License is distributed on an "AS IS" BASIS, |
| 13 | * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| 14 | * See the License for the specific language governing permissions and |
| 15 | * limitations under the License. |
| 16 | * |
| 17 | */ |
| 18 | |
| 19 | package transport |
| 20 | |
| 21 | import ( |
| 22 | "sync" |
| 23 | "time" |
| 24 | ) |
| 25 | |
| 26 | const ( |
| 27 | // bdpLimit is the maximum value the flow control windows will be increased |
| 28 | // to. TCP typically limits this to 4MB, but some systems go up to 16MB. |
| 29 | // Since this is only a limit, it is safe to make it optimistic. |
| 30 | bdpLimit = (1 << 20) * 16 |
| 31 | // alpha is a constant factor used to keep a moving average |
| 32 | // of RTTs. |
| 33 | alpha = 0.9 |
| 34 | // If the current bdp sample is greater than or equal to |
| 35 | // our beta * our estimated bdp and the current bandwidth |
| 36 | // sample is the maximum bandwidth observed so far, we |
| 37 | // increase our bbp estimate by a factor of gamma. |
| 38 | beta = 0.66 |
| 39 | // To put our bdp to be smaller than or equal to twice the real BDP, |
| 40 | // we should multiply our current sample with 4/3, however to round things out |
| 41 | // we use 2 as the multiplication factor. |
| 42 | gamma = 2 |
| 43 | ) |
| 44 | |
| 45 | // Adding arbitrary data to ping so that its ack can be identified. |
| 46 | // Easter-egg: what does the ping message say? |
| 47 | var bdpPing = &ping{data: [8]byte{2, 4, 16, 16, 9, 14, 7, 7}} |
| 48 | |
| 49 | type bdpEstimator struct { |
| 50 | // sentAt is the time when the ping was sent. |
| 51 | sentAt time.Time |
| 52 | |
| 53 | mu sync.Mutex |
| 54 | // bdp is the current bdp estimate. |
| 55 | bdp uint32 |
| 56 | // sample is the number of bytes received in one measurement cycle. |
| 57 | sample uint32 |
| 58 | // bwMax is the maximum bandwidth noted so far (bytes/sec). |
| 59 | bwMax float64 |
| 60 | // bool to keep track of the beginning of a new measurement cycle. |
| 61 | isSent bool |
| 62 | // Callback to update the window sizes. |
| 63 | updateFlowControl func(n uint32) |
| 64 | // sampleCount is the number of samples taken so far. |
| 65 | sampleCount uint64 |
| 66 | // round trip time (seconds) |
| 67 | rtt float64 |
| 68 | } |
| 69 | |
| 70 | // timesnap registers the time bdp ping was sent out so that |
| 71 | // network rtt can be calculated when its ack is received. |
| 72 | // It is called (by controller) when the bdpPing is |
| 73 | // being written on the wire. |
| 74 | func (b *bdpEstimator) timesnap(d [8]byte) { |
| 75 | if bdpPing.data != d { |
| 76 | return |
| 77 | } |
| 78 | b.sentAt = time.Now() |
| 79 | } |
| 80 | |
| 81 | // add adds bytes to the current sample for calculating bdp. |
| 82 | // It returns true only if a ping must be sent. This can be used |
| 83 | // by the caller (handleData) to make decision about batching |
| 84 | // a window update with it. |
| 85 | func (b *bdpEstimator) add(n uint32) bool { |
| 86 | b.mu.Lock() |
| 87 | defer b.mu.Unlock() |
| 88 | if b.bdp == bdpLimit { |
| 89 | return false |
| 90 | } |
| 91 | if !b.isSent { |
| 92 | b.isSent = true |
| 93 | b.sample = n |
| 94 | b.sentAt = time.Time{} |
| 95 | b.sampleCount++ |
| 96 | return true |
| 97 | } |
| 98 | b.sample += n |
| 99 | return false |
| 100 | } |
| 101 | |
| 102 | // calculate is called when an ack for a bdp ping is received. |
| 103 | // Here we calculate the current bdp and bandwidth sample and |
| 104 | // decide if the flow control windows should go up. |
| 105 | func (b *bdpEstimator) calculate(d [8]byte) { |
| 106 | // Check if the ping acked for was the bdp ping. |
| 107 | if bdpPing.data != d { |
| 108 | return |
| 109 | } |
| 110 | b.mu.Lock() |
| 111 | rttSample := time.Since(b.sentAt).Seconds() |
| 112 | if b.sampleCount < 10 { |
| 113 | // Bootstrap rtt with an average of first 10 rtt samples. |
| 114 | b.rtt += (rttSample - b.rtt) / float64(b.sampleCount) |
| 115 | } else { |
| 116 | // Heed to the recent past more. |
| 117 | b.rtt += (rttSample - b.rtt) * float64(alpha) |
| 118 | } |
| 119 | b.isSent = false |
| 120 | // The number of bytes accumulated so far in the sample is smaller |
| 121 | // than or equal to 1.5 times the real BDP on a saturated connection. |
| 122 | bwCurrent := float64(b.sample) / (b.rtt * float64(1.5)) |
| 123 | if bwCurrent > b.bwMax { |
| 124 | b.bwMax = bwCurrent |
| 125 | } |
| 126 | // If the current sample (which is smaller than or equal to the 1.5 times the real BDP) is |
| 127 | // greater than or equal to 2/3rd our perceived bdp AND this is the maximum bandwidth seen so far, we |
| 128 | // should update our perception of the network BDP. |
| 129 | if float64(b.sample) >= beta*float64(b.bdp) && bwCurrent == b.bwMax && b.bdp != bdpLimit { |
| 130 | sampleFloat := float64(b.sample) |
| 131 | b.bdp = uint32(gamma * sampleFloat) |
| 132 | if b.bdp > bdpLimit { |
| 133 | b.bdp = bdpLimit |
| 134 | } |
| 135 | bdp := b.bdp |
| 136 | b.mu.Unlock() |
| 137 | b.updateFlowControl(bdp) |
| 138 | return |
| 139 | } |
| 140 | b.mu.Unlock() |
| 141 | } |