Add tranquilizer mechanism to improve on token bucket mechanism

src/util/lib.rs

@@ -10,3 +10,4 @@ pub mod error;
 pub mod persister;
 pub mod time;
 pub mod token_bucket;
+pub mod tranquilizer;

src/util/tranquilizer.rs

@@ -0,0 +1,57 @@
+use std::collections::VecDeque;
+use std::time::{Duration, Instant};
+
+use tokio::time::sleep;
+
+/// A tranquilizer is a helper object that is used to make
+/// background operations not take up too much time.
+///
+/// Background operations are done in a loop that does the following:
+/// - do one step of the background process
+/// - tranquilize, i.e. wait some time to not overload the system
+///
+/// The tranquilizer observes how long the steps take, and keeps
+/// in memory a number of observations. The tranquilize operation
+/// simply sleeps k * avg(observed step times), where k is
+/// the tranquility factor. For instance with a tranquility of 2,
+/// the tranquilizer will sleep on average 2 units of time for every
+/// 1 unit of time spent doing the background task.
+pub struct Tranquilizer {
+	n_observations: usize,
+	observations: VecDeque<Duration>,
+	sum_observations: Duration,
+	last_step_begin: Instant,
+}
+
+impl Tranquilizer {
+	pub fn new(n_observations: usize) -> Self {
+		Self {
+			n_observations,
+			observations: VecDeque::with_capacity(n_observations + 1),
+			sum_observations: Duration::ZERO,
+			last_step_begin: Instant::now(),
+		}
+	}
+
+	pub async fn tranquilize(&mut self, tranquility: u32) {
+		let observation = Instant::now() - self.last_step_begin;
+
+		self.observations.push_back(observation);
+		self.sum_observations += observation;
+
+		while self.observations.len() > self.n_observations {
+			self.sum_observations -= self.observations.pop_front().unwrap();
+		}
+
+		if !self.observations.is_empty() {
+			let delay = (tranquility * self.sum_observations) / (self.observations.len() as u32);
+			sleep(delay).await;
+		}
+
+		self.reset();
+	}
+
+	pub fn reset(&mut self) {
+		self.last_step_begin = Instant::now();
+	}
+}