allocate verifiers up front, hibernate when not needed

ethcore/src/verification/queue/mod.rs

@@ -35,6 +35,9 @@ pub mod kind;
 const MIN_MEM_LIMIT: usize = 16384;
 const MIN_QUEUE_LIMIT: usize = 512;
 
+// maximum possible number of verification threads.
+const MAX_VERIFIERS: usize = 8;
+
 /// Type alias for block queue convenience.
 pub type BlockQueue = VerificationQueue<self::kind::Blocks>;
 
@@ -63,13 +66,26 @@ impl Default for Config {
 
 struct VerifierHandle {
 	deleting: Arc<AtomicBool>,
+	sleep: Arc<(Mutex<bool>, Condvar)>,
 	thread: JoinHandle<()>,
 }
 
 impl VerifierHandle {
+	// signal to the verifier thread that it should sleep.
+	fn sleep(&self) {
+		*self.sleep.0.lock() = true;
+	}
+
+	// signal to the verifier thread that it should wake up.
+	fn wake_up(&self) {
+		*self.sleep.0.lock() = false;
+		self.sleep.1.notify_all();
+	}
+
 	// signal to the verifier thread that it should conclude its
 	// operations.
 	fn conclude(&self) {
+		self.wake_up();
 		self.deleting.store(true, AtomicOrdering::Release);
 	}
 
@@ -115,7 +131,7 @@ pub struct VerificationQueue<K: Kind> {
 	engine: Arc<Engine>,
 	more_to_verify: Arc<SCondvar>,
 	verification: Arc<Verification<K>>,
-	verifiers: Mutex<Vec<VerifierHandle>>,
+	verifiers: Mutex<(Vec<VerifierHandle>, usize)>,
 	deleting: Arc<AtomicBool>,
 	ready_signal: Arc<QueueSignal>,
 	empty: Arc<SCondvar>,
@@ -212,31 +228,83 @@ impl<K: Kind> VerificationQueue<K> {
 		let empty = Arc::new(SCondvar::new());
 		let panic_handler = PanicHandler::new_in_arc();
 
-		let queue = VerificationQueue {
+		let max_verifiers = min(::num_cpus::get(), MAX_VERIFIERS);
+		let default_amount = max(::num_cpus::get(), 3) - 2;
+		let mut verifiers = Vec::with_capacity(max_verifiers);
+
+		debug!(target: "verification", "Allocating {} verifiers, {} initially active", max_verifiers, default_amount);
+
+		for i in 0..max_verifiers {
+			debug!(target: "verification", "Adding verification thread #{}", i);
+
+			let deleting = deleting.clone();
+			let panic_handler = panic_handler.clone();
+			let verification = verification.clone();
+			let engine = engine.clone();
+			let wait = more_to_verify.clone();
+			let ready = ready_signal.clone();
+			let empty = empty.clone();
+
+			// enable only the first few verifiers.
+			let sleep = if i < default_amount {
+				Arc::new((Mutex::new(false), Condvar::new()))
+			} else {
+				Arc::new((Mutex::new(true), Condvar::new()))
+			};
+
+			verifiers.push(VerifierHandle {
+				deleting: deleting.clone(),
+				sleep: sleep.clone(),
+				thread: thread::Builder::new()
+					.name(format!("Verifier #{}", i))
+					.spawn(move || {
+						panic_handler.catch_panic(move || {
+							VerificationQueue::verify(verification, engine, wait, ready, deleting, empty, sleep)
+						}).unwrap()
+					})
+					.expect("Failed to create verifier thread.")
+			});
+		}
+
+		VerificationQueue {
 			engine: engine,
 			panic_handler: panic_handler,
 			ready_signal: ready_signal,
 			more_to_verify: more_to_verify,
 			verification: verification,
-			verifiers: Mutex::new(Vec::with_capacity(::num_cpus::get())),
+			verifiers: Mutex::new((verifiers, default_amount)),
 			deleting: deleting,
 			processing: RwLock::new(HashSet::new()),
 			empty: empty,
 			ticks_since_adjustment: AtomicUsize::new(0),
 			max_queue_size: max(config.max_queue_size, MIN_QUEUE_LIMIT),
 			max_mem_use: max(config.max_mem_use, MIN_MEM_LIMIT),
-		};
-
-		let thread_count = max(::num_cpus::get(), 3) - 2;
-		for _ in 0..thread_count {
-			queue.add_verifier();
 		}
-
-		queue
 	}
 
-	fn verify(verification: Arc<Verification<K>>, engine: Arc<Engine>, wait: Arc<SCondvar>, ready: Arc<QueueSignal>, deleting: Arc<AtomicBool>, empty: Arc<SCondvar>) {
+	fn verify(
+		verification: Arc<Verification<K>>,
+		engine: Arc<Engine>,
+		wait: Arc<SCondvar>,
+		ready: Arc<QueueSignal>,
+		deleting: Arc<AtomicBool>,
+		empty: Arc<SCondvar>,
+		sleep: Arc<(Mutex<bool>, Condvar)>,
+	) {
 		while !deleting.load(AtomicOrdering::Acquire) {
+			{
+				let mut should_sleep = sleep.0.lock();
+				while *should_sleep {
+					trace!(target: "verification", "Verifier sleeping");
+					sleep.1.wait(&mut should_sleep);
+					trace!(target: "verification", "Verifier waking up");
+
+					if deleting.load(AtomicOrdering::Acquire) {
+						return;
+					}
+				}
+			}
+
 			{
 				let mut more_to_verify = verification.more_to_verify.lock().unwrap();
 
@@ -548,7 +616,7 @@ impl<K: Kind> VerificationQueue<K> {
 
 		// compute the average rate of verification per thread and determine
 		// how many are necessary to match the rate of draining.
-		let num_verifiers = self.verifiers.lock().len();
+		let num_verifiers = self.verifiers.lock().1;
 		let v_count_per = v_count as f64 / num_verifiers as f64;
 		let needed = if v_count < 20 {
 			1
@@ -559,63 +627,34 @@ impl<K: Kind> VerificationQueue<K> {
 		trace!(target: "verification", "v_rate_per={}, target={}, scaling to {} verifiers",
 			v_count_per, target, needed);
 
-		for _ in num_verifiers..needed {
-			self.add_verifier();
-		}
-
-		for _ in needed..num_verifiers {
-			self.remove_verifier();
-		}
+		self.scale_verifiers(needed);
 	}
 
-	// add a verifier thread if possible.
-	fn add_verifier(&self) {
+	// wake up or sleep verifiers to get as close to the target as
+	// possible, never going over the amount of initially allocated threads
+	// or below 1.
+	fn scale_verifiers(&self, target: usize) {
 		let mut verifiers = self.verifiers.lock();
-		let len = verifiers.len();
-		if len == ::num_cpus::get() {
-			return;
+		let &mut (ref mut verifiers, ref mut verifier_count) = &mut *verifiers;
+
+		let target = min(verifiers.capacity(), target);
+		let target = max(1, target);
+
+		debug!(target: "verification", "Scaling from {} to {} verifiers", verifier_count, target);
+
+		// scaling up
+		for i in *verifier_count..target {
+			debug!(target: "verification", "Waking up verifier {}", i);
+			verifiers[i].wake_up();
 		}
 
-		debug!(target: "verification", "Adding verification thread #{}", len);
-
-		let deleting = Arc::new(AtomicBool::new(false));
-		let panic_handler = self.panic_handler.clone();
-		let verification = self.verification.clone();
-		let engine = self.engine.clone();
-		let wait = self.more_to_verify.clone();
-		let ready = self.ready_signal.clone();
-		let empty = self.empty.clone();
-
-		verifiers.push(VerifierHandle {
-			deleting: deleting.clone(),
-			thread: thread::Builder::new()
-				.name(format!("Verifier #{}", len))
-				.spawn(move || {
-					panic_handler.catch_panic(move || {
-						VerificationQueue::verify(verification, engine, wait, ready, deleting, empty)
-					}).unwrap()
-				})
-				.expect("Failed to create verifier thread.")
-		});
-	}
-
-	// remove a verifier thread if possible.
-	fn remove_verifier(&self) {
-		let mut verifiers = self.verifiers.lock();
-		let len = verifiers.len();
-
-		// never remove the last thread.
-		if len == 1 {
-			return;
+		// scaling down.
+		for i in target..*verifier_count {
+			debug!(target: "verification", "Putting verifier {} to sleep", i);
+			verifiers[i].sleep();
 		}
 
-		debug!(target: "verification", "Removing verification thread #{}", len - 1);
-
-		if let Some(handle) = verifiers.pop() {
-			handle.conclude();
-			self.more_to_verify.notify_all(); // to ensure it's joinable immediately.
-			handle.join();
-		}
+		*verifier_count = target;
 	}
 }
 
@@ -631,7 +670,8 @@ impl<K: Kind> Drop for VerificationQueue<K> {
 		self.clear();
 		self.deleting.store(true, AtomicOrdering::Release);
 
-		let mut verifiers = self.verifiers.lock();
+		let mut verifiers = self.verifiers.get_mut();
+		let mut verifiers = &mut verifiers.0;
 
 		// first pass to signal conclusion. must be done before
 		// notify or deadlock possible.