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BDP estimation and window update. (grpc#1310)
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,125 @@ | ||
| package transport | ||
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| import ( | ||
| "sync" | ||
| "time" | ||
| ) | ||
|
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||
| const ( | ||
| // bdpLimit is the maximum value the flow control windows | ||
| // will be increased to. | ||
| bdpLimit = (1 << 20) * 4 | ||
| // alpha is a constant factor used to keep a moving average | ||
| // of RTTs. | ||
| alpha = 0.9 | ||
| // If the current bdp sample is greater than or equal to | ||
| // our beta * our estimated bdp and the current bandwidth | ||
| // sample is the maximum bandwidth observed so far, we | ||
| // increase our bbp estimate by a factor of gamma. | ||
| beta = 0.66 | ||
| // To put our bdp to be smaller than or equal to twice the real BDP, | ||
| // we should multiply our current sample with 4/3, however to round things out | ||
| // we use 2 as the multiplication factor. | ||
| gamma = 2 | ||
| ) | ||
|
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| var ( | ||
| // Adding arbitrary data to ping so that its ack can be | ||
| // identified. | ||
| // Easter-egg: what does the ping message say? | ||
| bdpPing = &ping{data: [8]byte{2, 4, 16, 16, 9, 14, 7, 7}} | ||
| ) | ||
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| type bdpEstimator struct { | ||
| // sentAt is the time when the ping was sent. | ||
| sentAt time.Time | ||
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| mu sync.Mutex | ||
| // bdp is the current bdp estimate. | ||
| bdp uint32 | ||
| // sample is the number of bytes received in one measurement cycle. | ||
| sample uint32 | ||
| // bwMax is the maximum bandwidth noted so far (bytes/sec). | ||
| bwMax float64 | ||
| // bool to keep track of the begining of a new measurement cycle. | ||
| isSent bool | ||
| // Callback to update the window sizes. | ||
| updateFlowControl func(n uint32) | ||
| // sampleCount is the number of samples taken so far. | ||
| sampleCount uint64 | ||
| // round trip time (seconds) | ||
| rtt float64 | ||
| } | ||
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| // timesnap registers the time bdp ping was sent out so that | ||
| // network rtt can be calculated when its ack is recieved. | ||
| // It is called (by controller) when the bdpPing is | ||
| // being written on the wire. | ||
| func (b *bdpEstimator) timesnap(d [8]byte) { | ||
| if bdpPing.data != d { | ||
| return | ||
| } | ||
| b.sentAt = time.Now() | ||
| } | ||
|
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| // add adds bytes to the current sample for calculating bdp. | ||
| // It returns true only if a ping must be sent. This can be used | ||
| // by the caller (handleData) to make decision about batching | ||
| // a window update with it. | ||
| func (b *bdpEstimator) add(n uint32) bool { | ||
| b.mu.Lock() | ||
| defer b.mu.Unlock() | ||
| if b.bdp == bdpLimit { | ||
| return false | ||
| } | ||
| if !b.isSent { | ||
| b.isSent = true | ||
| b.sample = n | ||
| b.sentAt = time.Time{} | ||
| b.sampleCount++ | ||
| return true | ||
| } | ||
| b.sample += n | ||
| return false | ||
| } | ||
|
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||
| // calculate is called when an ack for a bdp ping is received. | ||
| // Here we calculate the current bdp and bandwidth sample and | ||
| // decide if the flow control windows should go up. | ||
| func (b *bdpEstimator) calculate(d [8]byte) { | ||
| // Check if the ping acked for was the bdp ping. | ||
| if bdpPing.data != d { | ||
| return | ||
| } | ||
| b.mu.Lock() | ||
| rttSample := time.Since(b.sentAt).Seconds() | ||
| if b.sampleCount < 10 { | ||
| // Bootstrap rtt with an average of first 10 rtt samples. | ||
| b.rtt += (rttSample - b.rtt) / float64(b.sampleCount) | ||
| } else { | ||
| // Heed to the recent past more. | ||
| b.rtt += (rttSample - b.rtt) * float64(alpha) | ||
| } | ||
| b.isSent = false | ||
| // The number of bytes accumalated so far in the sample is smaller | ||
| // than or equal to 1.5 times the real BDP on a saturated connection. | ||
| bwCurrent := float64(b.sample) / (b.rtt * float64(1.5)) | ||
| if bwCurrent > b.bwMax { | ||
| b.bwMax = bwCurrent | ||
| } | ||
| // If the current sample (which is smaller than or equal to the 1.5 times the real BDP) is | ||
| // greater than or equal to 2/3rd our perceived bdp AND this is the maximum bandwidth seen so far, we | ||
| // should update our perception of the network BDP. | ||
| if float64(b.sample) >= beta*float64(b.bdp) && bwCurrent == b.bwMax && b.bdp != bdpLimit { | ||
| sampleFloat := float64(b.sample) | ||
| b.bdp = uint32(gamma * sampleFloat) | ||
| if b.bdp > bdpLimit { | ||
| b.bdp = bdpLimit | ||
| } | ||
| bdp := b.bdp | ||
| b.mu.Unlock() | ||
| b.updateFlowControl(bdp) | ||
| return | ||
| } | ||
| b.mu.Unlock() | ||
| } |
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