feat(multipath): add back path congestion metrics #3669
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There was a problem hiding this comment. This is metrics backend code. I'll have a much easier time with this PR if we don't try and do anything like that here.
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| Original file line number | Diff line number | Diff line change |
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| @@ -0,0 +1,157 @@ | ||
| //! Tracker for stateful metrics for connections and paths. | ||
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| use std::time::Duration; | ||
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| use quinn_proto::PathId; | ||
| use rustc_hash::FxHashMap; | ||
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| use crate::{magicsock::transports, metrics::MagicsockMetrics}; | ||
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| #[derive(Debug, Default)] | ||
| pub(super) struct MetricsTracker { | ||
| transport_summary: TransportSummary, | ||
| path_rtt_variance: FxHashMap<PathId, RttVariance>, | ||
| } | ||
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| impl MetricsTracker { | ||
| pub(super) fn add_path(&mut self, path_id: PathId, remote: &transports::Addr) { | ||
| self.transport_summary.add_path(remote); | ||
| self.path_rtt_variance.insert(path_id, Default::default()); | ||
| } | ||
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| pub(super) fn remove_path(&mut self, path_id: &PathId) { | ||
| self.path_rtt_variance.remove(path_id); | ||
| } | ||
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| pub(super) fn record_periodic( | ||
| &mut self, | ||
| metrics: &MagicsockMetrics, | ||
| conn: &quinn::Connection, | ||
| path_remotes: &FxHashMap<PathId, transports::Addr>, | ||
| selected_path: Option<transports::Addr>, | ||
| ) { | ||
| for (path_id, remote) in path_remotes.iter() { | ||
| let Some(stats) = conn.path_stats(*path_id) else { | ||
| continue; | ||
| }; | ||
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| let loss_rate = if stats.sent_packets == 0 { | ||
| 0.0 | ||
| } else { | ||
| stats.lost_packets as f64 / stats.sent_packets as f64 | ||
| }; | ||
| metrics.path_packet_loss_rate.observe(loss_rate); | ||
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| if Some(remote) == selected_path.as_ref() { | ||
| metrics | ||
| .connection_latency_ms | ||
| .observe(stats.rtt.as_millis() as f64); | ||
| } | ||
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| if let Some(rtt_variance) = self.path_rtt_variance.get_mut(path_id) { | ||
| rtt_variance.add_rtt_sample(stats.rtt); | ||
| if let Some(variance) = rtt_variance.rtt_variance() { | ||
| metrics | ||
| .path_rtt_variance_ms | ||
| .observe(variance.as_millis() as f64); | ||
| } | ||
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| let quality = rtt_variance.quality_score(loss_rate); | ||
| metrics.path_quality_score.observe(quality); | ||
| }; | ||
| } | ||
| } | ||
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| pub(super) fn record_closed(&self, metrics: &MagicsockMetrics) { | ||
| metrics.num_conns_closed.inc(); | ||
| match self.transport_summary { | ||
| TransportSummary::IpOnly => { | ||
| metrics.num_conns_transport_ip_only.inc(); | ||
| } | ||
| TransportSummary::RelayOnly => { | ||
| metrics.num_conns_transport_relay_only.inc(); | ||
| } | ||
| TransportSummary::IpAndRelay => { | ||
| metrics.num_conns_transport_ip_and_relay.inc(); | ||
| } | ||
| TransportSummary::None => {} | ||
| } | ||
| } | ||
| } | ||
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| /// Tracks RTT variance over time, as a congestion marker. | ||
| #[derive(Debug, Default)] | ||
| struct RttVariance { | ||
| /// Rolling window of recent latency measurements (stores up to 8 samples). | ||
| samples: [Option<Duration>; 8], | ||
| /// Index for next sample insertion (circular buffer). | ||
| index: usize, | ||
| } | ||
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| impl RttVariance { | ||
| fn add_rtt_sample(&mut self, rtt: Duration) { | ||
| self.samples[self.index] = Some(rtt); | ||
| self.index = (self.index + 1) % self.samples.len(); | ||
| } | ||
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| /// Calculate RTT variance as a congestion indicator. | ||
| /// Higher variance suggests congestion or unstable path. | ||
| fn rtt_variance(&self) -> Option<Duration> { | ||
| let samples: Vec<Duration> = self.samples.iter().filter_map(|&s| s).collect(); | ||
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| if samples.len() < 2 { | ||
| return None; | ||
| } | ||
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| let mean = samples.iter().sum::<Duration>() / samples.len() as u32; | ||
| let variance: f64 = samples | ||
| .iter() | ||
| .map(|&s| { | ||
| let diff = s.as_secs_f64() - mean.as_secs_f64(); | ||
| diff * diff | ||
| }) | ||
| .sum::<f64>() | ||
| / samples.len() as f64; | ||
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| Some(Duration::from_secs_f64(variance.sqrt())) | ||
| } | ||
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| /// Path quality score (0.0 = worst, 1.0 = best). | ||
| /// Factors in packet loss and RTT variance. | ||
| fn quality_score(&self, packet_loss: f64) -> f64 { | ||
| let loss_penalty = (1.0 - packet_loss).clamp(0.0, 1.0); | ||
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| // Penalize high RTT variance | ||
| let variance_penalty = match self.rtt_variance() { | ||
| Some(var) if var.as_millis() > 50 => 0.7, | ||
| Some(var) if var.as_millis() > 20 => 0.85, | ||
| Some(_) => 1.0, | ||
| None => 1.0, | ||
| }; | ||
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| loss_penalty * variance_penalty | ||
| } | ||
| } | ||
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| /// Used for metrics tracking. | ||
| #[derive(Debug, Clone, Copy, Default)] | ||
| enum TransportSummary { | ||
| #[default] | ||
| None, | ||
| IpOnly, | ||
| RelayOnly, | ||
| IpAndRelay, | ||
| } | ||
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| impl TransportSummary { | ||
| fn add_path(&mut self, addr: &transports::Addr) { | ||
| use transports::Addr; | ||
| *self = match (*self, addr) { | ||
| (TransportSummary::None | TransportSummary::IpOnly, Addr::Ip(_)) => Self::IpOnly, | ||
| (TransportSummary::None | TransportSummary::RelayOnly, Addr::Relay(_, _)) => { | ||
| Self::RelayOnly | ||
| } | ||
| _ => Self::IpAndRelay, | ||
| } | ||
| } | ||
| } |
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Soo, I'm going to argue that this is fundamentally doing metrics wrong. Metrics increment counters or record historgrams at discreet points. If you are recording something on a timer you were supposed to provide the metrics that can compute the thing, and the metrics backend can compute it on the fly over the range it wants, using sampling intervals it wants etc etc.
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What would be discreet points to record connection latency at? Open/close path events?