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delete stale code

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this was rewritten and put into its own crate
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Bryan Stitt 2023-06-16 10:02:14 -07:00
parent 7707729f77
commit 7c876ac561
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web3_proxy/src/peak_ewma.rs
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//! Code from [tower](https://github.com/tower-rs/tower/blob/3f31ffd2cf15f1e905142e5f43ab39ac995c22ed/tower/src/load/peak_ewma.rs)
//! Measures load using the PeakEWMA response latency.
//! TODO: refactor to work with our code
use std::task::{Context, Poll};
use std::{
sync::{Arc, Mutex},
time::Duration,
};
use tokio::time::Instant;
use tower_service::Service;
use tracing::trace;
/// Measures the load of the underlying service using Peak-EWMA load measurement.
///
/// [`PeakEwma`] implements [`Load`] with the [`Cost`] metric that estimates the amount of
/// pending work to an endpoint. Work is calculated by multiplying the
/// exponentially-weighted moving average (EWMA) of response latencies by the number of
/// pending requests. The Peak-EWMA algorithm is designed to be especially sensitive to
/// worst-case latencies. Over time, the peak latency value decays towards the moving
/// average of latencies to the endpoint.
///
/// When no latency information has been measured for an endpoint, an arbitrary default
/// RTT of 1 second is used to prevent the endpoint from being overloaded before a
/// meaningful baseline can be established..
///
/// ## Note
///
/// This is derived from [Finagle][finagle], which is distributed under the Apache V2
/// license. Copyright 2017, Twitter Inc.
///
/// [finagle]:
/// https://github.com/twitter/finagle/blob/9cc08d15216497bb03a1cafda96b7266cfbbcff1/finagle-core/src/main/scala/com/twitter/finagle/loadbalancer/PeakEwma.scala
#[derive(Debug)]
pub struct PeakEwma<S, C = CompleteOnResponse> {
service: S,
decay_ns: f64,
rtt_estimate: Arc<Mutex<RttEstimate>>,
completion: C,
}
#[cfg(feature = "discover")]
pin_project! {
/// Wraps a `D`-typed stream of discovered services with `PeakEwma`.
#[cfg_attr(docsrs, doc(cfg(feature = "discover")))]
#[derive(Debug)]
pub struct PeakEwmaDiscover<D, C = CompleteOnResponse> {
#[pin]
discover: D,
decay_ns: f64,
default_rtt: Duration,
completion: C,
}
}
/// Represents the relative cost of communicating with a service.
///
/// The underlying value estimates the amount of pending work to a service: the Peak-EWMA
/// latency estimate multiplied by the number of pending requests.
#[derive(Copy, Clone, Debug, PartialEq, PartialOrd)]
pub struct Cost(f64);
/// Tracks an in-flight request and updates the RTT-estimate on Drop.
#[derive(Debug)]
pub struct Handle {
sent_at: Instant,
decay_ns: f64,
rtt_estimate: Arc<Mutex<RttEstimate>>,
}
/// Holds the current RTT estimate and the last time this value was updated.
#[derive(Debug)]
struct RttEstimate {
update_at: Instant,
rtt_ns: f64,
}
const NANOS_PER_MILLI: f64 = 1_000_000.0;
// ===== impl PeakEwma =====
impl<S, C> PeakEwma<S, C> {
/// Wraps an `S`-typed service so that its load is tracked by the EWMA of its peak latency.
pub fn new(service: S, default_rtt: Duration, decay_ns: f64, completion: C) -> Self {
debug_assert!(decay_ns > 0.0, "decay_ns must be positive");
Self {
service,
decay_ns,
rtt_estimate: Arc::new(Mutex::new(RttEstimate::new(nanos(default_rtt)))),
completion,
}
}
fn handle(&self) -> Handle {
Handle {
decay_ns: self.decay_ns,
sent_at: Instant::now(),
rtt_estimate: self.rtt_estimate.clone(),
}
}
}
impl<S, C, Request> Service<Request> for PeakEwma<S, C>
where
S: Service<Request>,
C: TrackCompletion<Handle, S::Response>,
{
type Response = C::Output;
type Error = S::Error;
type Future = TrackCompletionFuture<S::Future, C, Handle>;
fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
self.service.poll_ready(cx)
}
fn call(&mut self, req: Request) -> Self::Future {
TrackCompletionFuture::new(
self.completion.clone(),
self.handle(),
self.service.call(req),
)
}
}
impl<S, C> Load for PeakEwma<S, C> {
type Metric = Cost;
fn load(&self) -> Self::Metric {
let pending = Arc::strong_count(&self.rtt_estimate) as u32 - 1;
// Update the RTT estimate to account for decay since the last update.
// If an estimate has not been established, a default is provided
let estimate = self.update_estimate();
let cost = Cost(estimate * f64::from(pending + 1));
trace!(
"load estimate={:.0}ms pending={} cost={:?}",
estimate / NANOS_PER_MILLI,
pending,
cost,
);
cost
}
}
impl<S, C> PeakEwma<S, C> {
fn update_estimate(&self) -> f64 {
let mut rtt = self.rtt_estimate.lock().expect("peak ewma prior_estimate");
rtt.decay(self.decay_ns)
}
}
// ===== impl PeakEwmaDiscover =====
#[cfg(feature = "discover")]
impl<D, C> PeakEwmaDiscover<D, C> {
/// Wraps a `D`-typed [`Discover`] so that services have a [`PeakEwma`] load metric.
///
/// The provided `default_rtt` is used as the default RTT estimate for newly
/// added services.
///
/// They `decay` value determines over what time period a RTT estimate should
/// decay.
pub fn new<Request>(discover: D, default_rtt: Duration, decay: Duration, completion: C) -> Self
where
D: Discover,
D::Service: Service<Request>,
C: TrackCompletion<Handle, <D::Service as Service<Request>>::Response>,
{
PeakEwmaDiscover {
discover,
decay_ns: nanos(decay),
default_rtt,
completion,
}
}
}
#[cfg(feature = "discover")]
impl<D, C> Stream for PeakEwmaDiscover<D, C>
where
D: Discover,
C: Clone,
{
type Item = Result<Change<D::Key, PeakEwma<D::Service, C>>, D::Error>;
fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
let this = self.project();
let change = match ready!(this.discover.poll_discover(cx)).transpose()? {
None => return Poll::Ready(None),
Some(Change::Remove(k)) => Change::Remove(k),
Some(Change::Insert(k, svc)) => {
let peak_ewma = PeakEwma::new(
svc,
*this.default_rtt,
*this.decay_ns,
this.completion.clone(),
);
Change::Insert(k, peak_ewma)
}
};
Poll::Ready(Some(Ok(change)))
}
}
// ===== impl RttEstimate =====
impl RttEstimate {
fn new(rtt_ns: f64) -> Self {
debug_assert!(0.0 < rtt_ns, "rtt must be positive");
Self {
rtt_ns,
update_at: Instant::now(),
}
}
/// Decays the RTT estimate with a decay period of `decay_ns`.
fn decay(&mut self, decay_ns: f64) -> f64 {
// Updates with a 0 duration so that the estimate decays towards 0.
let now = Instant::now();
self.update(now, now, decay_ns)
}
/// Updates the Peak-EWMA RTT estimate.
///
/// The elapsed time from `sent_at` to `recv_at` is added
fn update(&mut self, sent_at: Instant, recv_at: Instant, decay_ns: f64) -> f64 {
debug_assert!(
sent_at <= recv_at,
"recv_at={:?} after sent_at={:?}",
recv_at,
sent_at
);
let rtt = nanos(recv_at.saturating_duration_since(sent_at));
let now = Instant::now();
debug_assert!(
self.update_at <= now,
"update_at={:?} in the future",
self.update_at
);
self.rtt_ns = if self.rtt_ns < rtt {
// For Peak-EWMA, always use the worst-case (peak) value as the estimate for
// subsequent requests.
trace!(
"update peak rtt={}ms prior={}ms",
rtt / NANOS_PER_MILLI,
self.rtt_ns / NANOS_PER_MILLI,
);
rtt
} else {
// When an RTT is observed that is less than the estimated RTT, we decay the
// prior estimate according to how much time has elapsed since the last
// update. The inverse of the decay is used to scale the estimate towards the
// observed RTT value.
let elapsed = nanos(now.saturating_duration_since(self.update_at));
let decay = (-elapsed / decay_ns).exp();
let recency = 1.0 - decay;
let next_estimate = (self.rtt_ns * decay) + (rtt * recency);
trace!(
"update rtt={:03.0}ms decay={:06.0}ns; next={:03.0}ms",
rtt / NANOS_PER_MILLI,
self.rtt_ns - next_estimate,
next_estimate / NANOS_PER_MILLI,
);
next_estimate
};
self.update_at = now;
self.rtt_ns
}
}
// ===== impl Handle =====
impl Drop for Handle {
fn drop(&mut self) {
let recv_at = Instant::now();
if let Ok(mut rtt) = self.rtt_estimate.lock() {
rtt.update(self.sent_at, recv_at, self.decay_ns);
}
}
}
// ===== impl Cost =====
// Utility that converts durations to nanos in f64.
//
// Due to a lossy transformation, the maximum value that can be represented is ~585 years,
// which, I hope, is more than enough to represent request latencies.
fn nanos(d: Duration) -> f64 {
const NANOS_PER_SEC: u64 = 1_000_000_000;
let n = f64::from(d.subsec_nanos());
let s = d.as_secs().saturating_mul(NANOS_PER_SEC) as f64;
n + s
}
#[cfg(test)]
mod tests {
use futures_util::future;
use std::time::Duration;
use tokio::time;
use tokio_test::{assert_ready, assert_ready_ok, task};
use super::*;
struct Svc;
impl Service<()> for Svc {
type Response = ();
type Error = ();
type Future = future::Ready<Result<(), ()>>;
fn poll_ready(&mut self, _: &mut Context<'_>) -> Poll<Result<(), ()>> {
Poll::Ready(Ok(()))
}
fn call(&mut self, (): ()) -> Self::Future {
future::ok(())
}
}
/// The default RTT estimate decays, so that new nodes are considered if the
/// default RTT is too high.
#[tokio::test]
async fn default_decay() {
time::pause();
let svc = PeakEwma::new(
Svc,
Duration::from_millis(10),
NANOS_PER_MILLI * 1_000.0,
CompleteOnResponse,
);
let Cost(load) = svc.load();
assert_eq!(load, 10.0 * NANOS_PER_MILLI);
time::advance(Duration::from_millis(100)).await;
let Cost(load) = svc.load();
assert!(9.0 * NANOS_PER_MILLI < load && load < 10.0 * NANOS_PER_MILLI);
time::advance(Duration::from_millis(100)).await;
let Cost(load) = svc.load();
assert!(8.0 * NANOS_PER_MILLI < load && load < 9.0 * NANOS_PER_MILLI);
}
// The default RTT estimate decays, so that new nodes are considered if the default RTT is too
// high.
#[tokio::test]
async fn compound_decay() {
time::pause();
let mut svc = PeakEwma::new(
Svc,
Duration::from_millis(20),
NANOS_PER_MILLI * 1_000.0,
CompleteOnResponse,
);
assert_eq!(svc.load(), Cost(20.0 * NANOS_PER_MILLI));
time::advance(Duration::from_millis(100)).await;
let mut rsp0 = task::spawn(svc.call(()));
assert!(svc.load() > Cost(20.0 * NANOS_PER_MILLI));
time::advance(Duration::from_millis(100)).await;
let mut rsp1 = task::spawn(svc.call(()));
assert!(svc.load() > Cost(40.0 * NANOS_PER_MILLI));
time::advance(Duration::from_millis(100)).await;
let () = assert_ready_ok!(rsp0.poll());
assert_eq!(svc.load(), Cost(400_000_000.0));
time::advance(Duration::from_millis(100)).await;
let () = assert_ready_ok!(rsp1.poll());
assert_eq!(svc.load(), Cost(200_000_000.0));
// Check that values decay as time elapses
time::advance(Duration::from_secs(1)).await;
assert!(svc.load() < Cost(100_000_000.0));
time::advance(Duration::from_secs(10)).await;
assert!(svc.load() < Cost(100_000.0));
}
#[test]
fn nanos() {
assert_eq!(super::nanos(Duration::new(0, 0)), 0.0);
assert_eq!(super::nanos(Duration::new(0, 123)), 123.0);
assert_eq!(super::nanos(Duration::new(1, 23)), 1_000_000_023.0);
assert_eq!(
super::nanos(Duration::new(::std::u64::MAX, 999_999_999)),
18446744074709553000.0
);
}
}