2021-05-19 07:09:03 -05:00
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// Copyright 2021 The go-ethereum Authors
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// This file is part of the go-ethereum library.
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//
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// The go-ethereum library is free software: you can redistribute it and/or modify
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// it under the terms of the GNU Lesser General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// The go-ethereum library is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU Lesser General Public License for more details.
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//
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// You should have received a copy of the GNU Lesser General Public License
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// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
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// Package msgrate allows estimating the throughput of peers for more balanced syncs.
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package msgrate
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import (
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"errors"
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"math"
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"sort"
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"sync"
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"time"
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"github.com/ethereum/go-ethereum/log"
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)
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// measurementImpact is the impact a single measurement has on a peer's final
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// capacity value. A value closer to 0 reacts slower to sudden network changes,
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// but it is also more stable against temporary hiccups. 0.1 worked well for
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// most of Ethereum's existence, so might as well go with it.
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const measurementImpact = 0.1
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// capacityOverestimation is the ratio of items to over-estimate when retrieving
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// a peer's capacity to avoid locking into a lower value due to never attempting
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// to fetch more than some local stable value.
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const capacityOverestimation = 1.01
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// qosTuningPeers is the number of best peers to tune round trip times based on.
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// An Ethereum node doesn't need hundreds of connections to operate correctly,
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// so instead of lowering our download speed to the median of potentially many
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// bad nodes, we can target a smaller set of vey good nodes. At worse this will
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// result in less nodes to sync from, but that's still better than some hogging
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// the pipeline.
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const qosTuningPeers = 5
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// rttMinEstimate is the minimal round trip time to target requests for. Since
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// every request entails a 2 way latency + bandwidth + serving database lookups,
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// it should be generous enough to permit meaningful work to be done on top of
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// the transmission costs.
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const rttMinEstimate = 2 * time.Second
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// rttMaxEstimate is the maximal round trip time to target requests for. Although
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// the expectation is that a well connected node will never reach this, certain
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// special connectivity ones might experience significant delays (e.g. satellite
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// uplink with 3s RTT). This value should be low enough to forbid stalling the
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// pipeline too long, but large enough to cover the worst of the worst links.
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const rttMaxEstimate = 20 * time.Second
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// rttPushdownFactor is a multiplier to attempt forcing quicker requests than
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// what the message rate tracker estimates. The reason is that message rate
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// tracking adapts queries to the RTT, but multiple RTT values can be perfectly
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// valid, they just result in higher packet sizes. Since smaller packets almost
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// always result in stabler download streams, this factor hones in on the lowest
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// RTT from all the functional ones.
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const rttPushdownFactor = 0.9
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// rttMinConfidence is the minimum value the roundtrip confidence factor may drop
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// to. Since the target timeouts are based on how confident the tracker is in the
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// true roundtrip, it's important to not allow too huge fluctuations.
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const rttMinConfidence = 0.1
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// ttlScaling is the multiplier that converts the estimated roundtrip time to a
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// timeout cap for network requests. The expectation is that peers' response time
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// will fluctuate around the estimated roundtrip, but depending in their load at
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// request time, it might be higher than anticipated. This scaling factor ensures
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// that we allow remote connections some slack but at the same time do enforce a
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// behavior similar to our median peers.
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const ttlScaling = 3
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// ttlLimit is the maximum timeout allowance to prevent reaching crazy numbers
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// if some unforeseen network events shappen. As much as we try to hone in on
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// the most optimal values, it doesn't make any sense to go above a threshold,
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// even if everything is slow and screwy.
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const ttlLimit = time.Minute
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// tuningConfidenceCap is the number of active peers above which to stop detuning
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// the confidence number. The idea here is that once we hone in on the capacity
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// of a meaningful number of peers, adding one more should ot have a significant
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// impact on things, so just ron with the originals.
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const tuningConfidenceCap = 10
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// tuningImpact is the influence that a new tuning target has on the previously
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// cached value. This number is mostly just an out-of-the-blue heuristic that
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// prevents the estimates from jumping around. There's no particular reason for
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// the current value.
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const tuningImpact = 0.25
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// Tracker estimates the throughput capacity of a peer with regard to each data
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// type it can deliver. The goal is to dynamically adjust request sizes to max
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// out network throughput without overloading either the peer or th elocal node.
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//
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// By tracking in real time the latencies and bandiwdths peers exhibit for each
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// packet type, it's possible to prevent overloading by detecting a slowdown on
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// one type when another type is pushed too hard.
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//
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// Similarly, real time measurements also help avoid overloading the local net
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// connection if our peers would otherwise be capable to deliver more, but the
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// local link is saturated. In that case, the live measurements will force us
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// to reduce request sizes until the throughput gets stable.
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//
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// Lastly, message rate measurements allows us to detect if a peer is unusually
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// slow compared to other peers, in which case we can decide to keep it around
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// or free up the slot so someone closer.
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//
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// Since throughput tracking and estimation adapts dynamically to live network
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// conditions, it's fine to have multiple trackers locally track the same peer
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// in different subsystem. The throughput will simply be distributed across the
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// two trackers if both are highly active.
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type Tracker struct {
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// capacity is the number of items retrievable per second of a given type.
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// It is analogous to bandwidth, but we deliberately avoided using bytes
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// as the unit, since serving nodes also spend a lot of time loading data
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// from disk, which is linear in the number of items, but mostly constant
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// in their sizes.
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//
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// Callers of course are free to use the item counter as a byte counter if
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// or when their protocol of choice if capped by bytes instead of items.
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// (eg. eth.getHeaders vs snap.getAccountRange).
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capacity map[uint64]float64
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// roundtrip is the latency a peer in general responds to data requests.
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// This number is not used inside the tracker, but is exposed to compare
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// peers to each other and filter out slow ones. Note however, it only
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// makes sense to compare RTTs if the caller caters request sizes for
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// each peer to target the same RTT. There's no need to make this number
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// the real networking RTT, we just need a number to compare peers with.
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roundtrip time.Duration
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lock sync.RWMutex
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}
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// NewTracker creates a new message rate tracker for a specific peer. An initial
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// RTT is needed to avoid a peer getting marked as an outlier compared to others
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// right after joining. It's suggested to use the median rtt across all peers to
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// init a new peer tracker.
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func NewTracker(caps map[uint64]float64, rtt time.Duration) *Tracker {
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if caps == nil {
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caps = make(map[uint64]float64)
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}
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return &Tracker{
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capacity: caps,
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roundtrip: rtt,
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}
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}
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// Capacity calculates the number of items the peer is estimated to be able to
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// retrieve within the allotted time slot. The method will round up any division
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// errors and will add an additional overestimation ratio on top. The reason for
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// overshooting the capacity is because certain message types might not increase
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// the load proportionally to the requested items, so fetching a bit more might
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// still take the same RTT. By forcefully overshooting by a small amount, we can
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// avoid locking into a lower-that-real capacity.
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func (t *Tracker) Capacity(kind uint64, targetRTT time.Duration) int {
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t.lock.RLock()
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defer t.lock.RUnlock()
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// Calculate the actual measured throughput
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throughput := t.capacity[kind] * float64(targetRTT) / float64(time.Second)
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// Return an overestimation to force the peer out of a stuck minima, adding
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// +1 in case the item count is too low for the overestimator to dent
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return roundCapacity(1 + capacityOverestimation*throughput)
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}
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// roundCapacity gives the integer value of a capacity.
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// The result fits int32, and is guaranteed to be positive.
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func roundCapacity(cap float64) int {
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const maxInt32 = float64(1<<31 - 1)
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return int(math.Min(maxInt32, math.Max(1, math.Ceil(cap))))
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}
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// Update modifies the peer's capacity values for a specific data type with a new
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// measurement. If the delivery is zero, the peer is assumed to have either timed
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// out or to not have the requested data, resulting in a slash to 0 capacity. This
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// avoids assigning the peer retrievals that it won't be able to honour.
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func (t *Tracker) Update(kind uint64, elapsed time.Duration, items int) {
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t.lock.Lock()
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defer t.lock.Unlock()
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// If nothing was delivered (timeout / unavailable data), reduce throughput
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// to minimum
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if items == 0 {
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t.capacity[kind] = 0
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return
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}
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// Otherwise update the throughput with a new measurement
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if elapsed <= 0 {
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elapsed = 1 // +1 (ns) to ensure non-zero divisor
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}
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measured := float64(items) / (float64(elapsed) / float64(time.Second))
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t.capacity[kind] = (1-measurementImpact)*(t.capacity[kind]) + measurementImpact*measured
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t.roundtrip = time.Duration((1-measurementImpact)*float64(t.roundtrip) + measurementImpact*float64(elapsed))
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}
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// Trackers is a set of message rate trackers across a number of peers with the
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// goal of aggregating certain measurements across the entire set for outlier
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// filtering and newly joining initialization.
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type Trackers struct {
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trackers map[string]*Tracker
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// roundtrip is the current best guess as to what is a stable round trip time
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// across the entire collection of connected peers. This is derived from the
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// various trackers added, but is used as a cache to avoid recomputing on each
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// network request. The value is updated once every RTT to avoid fluctuations
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// caused by hiccups or peer events.
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roundtrip time.Duration
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// confidence represents the probability that the estimated roundtrip value
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// is the real one across all our peers. The confidence value is used as an
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// impact factor of new measurements on old estimates. As our connectivity
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// stabilizes, this value gravitates towards 1, new measurements havinng
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// almost no impact. If there's a large peer churn and few peers, then new
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// measurements will impact it more. The confidence is increased with every
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// packet and dropped with every new connection.
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confidence float64
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// tuned is the time instance the tracker recalculated its cached roundtrip
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// value and confidence values. A cleaner way would be to have a heartbeat
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// goroutine do it regularly, but that requires a lot of maintenance to just
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// run every now and again.
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tuned time.Time
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// The fields below can be used to override certain default values. Their
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// purpose is to allow quicker tests. Don't use them in production.
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OverrideTTLLimit time.Duration
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log log.Logger
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lock sync.RWMutex
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}
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// NewTrackers creates an empty set of trackers to be filled with peers.
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func NewTrackers(log log.Logger) *Trackers {
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return &Trackers{
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trackers: make(map[string]*Tracker),
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roundtrip: rttMaxEstimate,
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confidence: 1,
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tuned: time.Now(),
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OverrideTTLLimit: ttlLimit,
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log: log,
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}
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}
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// Track inserts a new tracker into the set.
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func (t *Trackers) Track(id string, tracker *Tracker) error {
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t.lock.Lock()
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defer t.lock.Unlock()
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if _, ok := t.trackers[id]; ok {
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return errors.New("already tracking")
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}
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t.trackers[id] = tracker
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t.detune()
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return nil
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}
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// Untrack stops tracking a previously added peer.
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func (t *Trackers) Untrack(id string) error {
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t.lock.Lock()
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defer t.lock.Unlock()
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if _, ok := t.trackers[id]; !ok {
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return errors.New("not tracking")
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}
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delete(t.trackers, id)
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return nil
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}
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// MedianRoundTrip returns the median RTT across all known trackers. The purpose
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// of the median RTT is to initialize a new peer with sane statistics that it will
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// hopefully outperform. If it seriously underperforms, there's a risk of dropping
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// the peer, but that is ok as we're aiming for a strong median.
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func (t *Trackers) MedianRoundTrip() time.Duration {
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t.lock.RLock()
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defer t.lock.RUnlock()
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return t.medianRoundTrip()
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}
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// medianRoundTrip is the internal lockless version of MedianRoundTrip to be used
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// by the QoS tuner.
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func (t *Trackers) medianRoundTrip() time.Duration {
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// Gather all the currently measured round trip times
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rtts := make([]float64, 0, len(t.trackers))
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for _, tt := range t.trackers {
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tt.lock.RLock()
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rtts = append(rtts, float64(tt.roundtrip))
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tt.lock.RUnlock()
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}
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sort.Float64s(rtts)
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median := rttMaxEstimate
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if qosTuningPeers <= len(rtts) {
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median = time.Duration(rtts[qosTuningPeers/2]) // Median of our best few peers
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} else if len(rtts) > 0 {
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median = time.Duration(rtts[len(rtts)/2]) // Median of all out connected peers
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}
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// Restrict the RTT into some QoS defaults, irrelevant of true RTT
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if median < rttMinEstimate {
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median = rttMinEstimate
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}
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if median > rttMaxEstimate {
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median = rttMaxEstimate
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}
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return median
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}
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// MeanCapacities returns the capacities averaged across all the added trackers.
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// The purpos of the mean capacities are to initialize a new peer with some sane
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// starting values that it will hopefully outperform. If the mean overshoots, the
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// peer will be cut back to minimal capacity and given another chance.
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func (t *Trackers) MeanCapacities() map[uint64]float64 {
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t.lock.RLock()
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defer t.lock.RUnlock()
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return t.meanCapacities()
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}
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// meanCapacities is the internal lockless version of MeanCapacities used for
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// debug logging.
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func (t *Trackers) meanCapacities() map[uint64]float64 {
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capacities := make(map[uint64]float64)
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for _, tt := range t.trackers {
|
|
|
|
tt.lock.RLock()
|
|
|
|
for key, val := range tt.capacity {
|
|
|
|
capacities[key] += val
|
|
|
|
}
|
|
|
|
tt.lock.RUnlock()
|
|
|
|
}
|
|
|
|
for key, val := range capacities {
|
|
|
|
capacities[key] = val / float64(len(t.trackers))
|
|
|
|
}
|
|
|
|
return capacities
|
|
|
|
}
|
|
|
|
|
|
|
|
// TargetRoundTrip returns the current target round trip time for a request to
|
|
|
|
// complete in.The returned RTT is slightly under the estimated RTT. The reason
|
|
|
|
// is that message rate estimation is a 2 dimensional problem which is solvable
|
|
|
|
// for any RTT. The goal is to gravitate towards smaller RTTs instead of large
|
|
|
|
// messages, to result in a stabler download stream.
|
|
|
|
func (t *Trackers) TargetRoundTrip() time.Duration {
|
|
|
|
// Recalculate the internal caches if it's been a while
|
|
|
|
t.tune()
|
|
|
|
|
|
|
|
// Caches surely recent, return target roundtrip
|
|
|
|
t.lock.RLock()
|
|
|
|
defer t.lock.RUnlock()
|
|
|
|
|
|
|
|
return time.Duration(float64(t.roundtrip) * rttPushdownFactor)
|
|
|
|
}
|
|
|
|
|
|
|
|
// TargetTimeout returns the timeout allowance for a single request to finish
|
|
|
|
// under. The timeout is proportional to the roundtrip, but also takes into
|
|
|
|
// consideration the tracker's confidence in said roundtrip and scales it
|
|
|
|
// accordingly. The final value is capped to avoid runaway requests.
|
|
|
|
func (t *Trackers) TargetTimeout() time.Duration {
|
|
|
|
// Recalculate the internal caches if it's been a while
|
|
|
|
t.tune()
|
|
|
|
|
|
|
|
// Caches surely recent, return target timeout
|
|
|
|
t.lock.RLock()
|
|
|
|
defer t.lock.RUnlock()
|
|
|
|
|
|
|
|
return t.targetTimeout()
|
|
|
|
}
|
|
|
|
|
|
|
|
// targetTimeout is the internal lockless version of TargetTimeout to be used
|
|
|
|
// during QoS tuning.
|
|
|
|
func (t *Trackers) targetTimeout() time.Duration {
|
|
|
|
timeout := time.Duration(ttlScaling * float64(t.roundtrip) / t.confidence)
|
|
|
|
if timeout > t.OverrideTTLLimit {
|
|
|
|
timeout = t.OverrideTTLLimit
|
|
|
|
}
|
|
|
|
return timeout
|
|
|
|
}
|
|
|
|
|
|
|
|
// tune gathers the individual tracker statistics and updates the estimated
|
|
|
|
// request round trip time.
|
|
|
|
func (t *Trackers) tune() {
|
|
|
|
// Tune may be called concurrently all over the place, but we only want to
|
|
|
|
// periodically update and even then only once. First check if it was updated
|
|
|
|
// recently and abort if so.
|
|
|
|
t.lock.RLock()
|
|
|
|
dirty := time.Since(t.tuned) > t.roundtrip
|
|
|
|
t.lock.RUnlock()
|
|
|
|
if !dirty {
|
|
|
|
return
|
|
|
|
}
|
|
|
|
// If an update is needed, obtain a write lock but make sure we don't update
|
|
|
|
// it on all concurrent threads one by one.
|
|
|
|
t.lock.Lock()
|
|
|
|
defer t.lock.Unlock()
|
|
|
|
|
|
|
|
if dirty := time.Since(t.tuned) > t.roundtrip; !dirty {
|
|
|
|
return // A concurrent request beat us to the tuning
|
|
|
|
}
|
|
|
|
// First thread reaching the tuning point, update the estimates and return
|
|
|
|
t.roundtrip = time.Duration((1-tuningImpact)*float64(t.roundtrip) + tuningImpact*float64(t.medianRoundTrip()))
|
|
|
|
t.confidence = t.confidence + (1-t.confidence)/2
|
|
|
|
|
|
|
|
t.tuned = time.Now()
|
|
|
|
t.log.Debug("Recalculated msgrate QoS values", "rtt", t.roundtrip, "confidence", t.confidence, "ttl", t.targetTimeout(), "next", t.tuned.Add(t.roundtrip))
|
|
|
|
t.log.Trace("Debug dump of mean capacities", "caps", log.Lazy{Fn: t.meanCapacities})
|
|
|
|
}
|
|
|
|
|
|
|
|
// detune reduces the tracker's confidence in order to make fresh measurements
|
|
|
|
// have a larger impact on the estimates. It is meant to be used during new peer
|
|
|
|
// connections so they can have a proper impact on the estimates.
|
|
|
|
func (t *Trackers) detune() {
|
|
|
|
// If we have a single peer, confidence is always 1
|
|
|
|
if len(t.trackers) == 1 {
|
|
|
|
t.confidence = 1
|
|
|
|
return
|
|
|
|
}
|
|
|
|
// If we have a ton of peers, don't drop the confidence since there's enough
|
|
|
|
// remaining to retain the same throughput
|
|
|
|
if len(t.trackers) >= tuningConfidenceCap {
|
|
|
|
return
|
|
|
|
}
|
|
|
|
// Otherwise drop the confidence factor
|
|
|
|
peers := float64(len(t.trackers))
|
|
|
|
|
|
|
|
t.confidence = t.confidence * (peers - 1) / peers
|
|
|
|
if t.confidence < rttMinConfidence {
|
|
|
|
t.confidence = rttMinConfidence
|
|
|
|
}
|
|
|
|
t.log.Debug("Relaxed msgrate QoS values", "rtt", t.roundtrip, "confidence", t.confidence, "ttl", t.targetTimeout())
|
|
|
|
}
|
|
|
|
|
|
|
|
// Capacity is a helper function to access a specific tracker without having to
|
|
|
|
// track it explicitly outside.
|
2021-05-27 11:43:55 -05:00
|
|
|
func (t *Trackers) Capacity(id string, kind uint64, targetRTT time.Duration) int {
|
2021-05-19 07:09:03 -05:00
|
|
|
t.lock.RLock()
|
|
|
|
defer t.lock.RUnlock()
|
|
|
|
|
|
|
|
tracker := t.trackers[id]
|
|
|
|
if tracker == nil {
|
|
|
|
return 1 // Unregister race, don't return 0, it's a dangerous number
|
|
|
|
}
|
|
|
|
return tracker.Capacity(kind, targetRTT)
|
|
|
|
}
|
|
|
|
|
|
|
|
// Update is a helper function to access a specific tracker without having to
|
|
|
|
// track it explicitly outside.
|
|
|
|
func (t *Trackers) Update(id string, kind uint64, elapsed time.Duration, items int) {
|
|
|
|
t.lock.RLock()
|
|
|
|
defer t.lock.RUnlock()
|
|
|
|
|
|
|
|
if tracker := t.trackers[id]; tracker != nil {
|
|
|
|
tracker.Update(kind, elapsed, items)
|
|
|
|
}
|
|
|
|
}
|