Avoid writing length on each IO for async channel

metered-channel/src/bounded.rs

@@ -242,6 +242,11 @@ impl<T> MeteredReceiver<T> {
 
 	/// Get an updated accessor object for all metrics collected.
 	pub fn meter(&self) -> &Meter {
+		// For async_channel we can update channel length in the meter access
+		// to avoid more expensive updates on each RW operation
+		#[cfg(feature = "async_channel")]
+		self.meter.note_channel_len(self.len());
+
 		&self.meter
 	}
 
@@ -262,7 +267,6 @@ impl<T> MeteredReceiver<T> {
 			Err(err) => Err(err),
 		};
 
-		self.meter.note_channel_len(self.len());
 		result
 	}
 
@@ -275,7 +279,6 @@ impl<T> MeteredReceiver<T> {
 			Err(err) => Err(err.into()),
 		};
 
-		self.meter.note_channel_len(self.len());
 		result
 	}
 
@@ -288,7 +291,6 @@ impl<T> MeteredReceiver<T> {
 			Err(err) => Err(err),
 		};
 
-		self.meter.note_channel_len(self.len());
 		result
 	}
 
@@ -297,6 +299,12 @@ impl<T> MeteredReceiver<T> {
 	pub fn len(&self) -> usize {
 		self.inner.len()
 	}
+
+	#[cfg(feature = "futures_channel")]
+	/// Returns the current number of messages in the channel based on meter approximation
+	pub fn len(&self) -> usize {
+		self.meter.calculate_channel_len()
+	}
 }
 
 impl<T> futures::stream::FusedStream for MeteredReceiver<T> {
@@ -346,6 +354,10 @@ impl<T> MeteredSender<T> {
 
 	/// Get an updated accessor object for all metrics collected.
 	pub fn meter(&self) -> &Meter {
+		// For async_channel we can update channel length in the meter access
+		// to avoid more expensive updates on each RW operation
+		#[cfg(feature = "async_channel")]
+		self.meter.note_channel_len(self.len());
 		&self.meter
 	}
 
@@ -382,7 +394,6 @@ impl<T> MeteredSender<T> {
 			SendError::Closed(err.0.into())
 		});
 
-		self.meter.note_channel_len(self.len());
 		result
 	}
 
@@ -420,7 +431,6 @@ impl<T> MeteredSender<T> {
 			TrySendError::from(e)
 		});
 
-		self.meter.note_channel_len(self.len());
 		result
 	}
 
@@ -429,4 +439,10 @@ impl<T> MeteredSender<T> {
 	pub fn len(&self) -> usize {
 		self.inner.len()
 	}
+
+	#[cfg(feature = "futures_channel")]
+	/// Returns the current number of messages in the channel based on meter approximation
+	pub fn len(&self) -> usize {
+		self.meter.calculate_channel_len()
+	}
 }

metered-channel/src/lib.rs

@@ -42,6 +42,9 @@ use coarsetime::Instant as CoarseInstant;
 #[cfg(test)]
 mod tests;
 
+/// Defines the maximum number of time of flight values to be stored.
+const TOF_QUEUE_SIZE: usize = 100;
+
 /// A peek into the inner state of a meter.
 #[derive(Debug, Clone)]
 pub struct Meter {
@@ -54,7 +57,7 @@ pub struct Meter {
 	channel_len: Arc<AtomicUsize>,
 	// Number of times senders blocked while sending messages to a subsystem.
 	blocked: Arc<AtomicUsize>,
-	// Atomic ringbuffer of the last 50 time of flight values
+	// Atomic ringbuffer of the last `TOF_QUEUE_SIZE` time of flight values
 	tof: Arc<crossbeam_queue::ArrayQueue<CoarseDuration>>,
 }
 
@@ -66,7 +69,7 @@ impl std::default::Default for Meter {
 			#[cfg(feature = "async_channel")]
 			channel_len: Arc::new(AtomicUsize::new(0)),
 			blocked: Arc::new(AtomicUsize::new(0)),
-			tof: Arc::new(crossbeam_queue::ArrayQueue::new(100)),
+			tof: Arc::new(crossbeam_queue::ArrayQueue::new(TOF_QUEUE_SIZE)),
 		}
 	}
 }
@@ -140,6 +143,13 @@ impl Meter {
 	fn note_time_of_flight(&self, tof: CoarseDuration) {
 		let _ = self.tof.force_push(tof);
 	}
+
+	#[cfg(feature = "futures_channel")]
+	fn calculate_channel_len(&self) -> usize {
+		let sent = self.sent.load(Ordering::Relaxed);
+		let received = self.received.load(Ordering::Relaxed);
+		sent.saturating_sub(received) as usize
+	}
 }
 
 /// Determine if this instance shall be measured