Implement THROTTLED priority semantics.

net/base/network_throttle_manager_impl.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "net/base/network_throttle_manager_impl.h"
+
+#include <algorithm>
+
+#include "base/logging.h"
+#include "base/threading/thread_task_runner_handle.h"
+#include "base/time/default_tick_clock.h"
+
+namespace net {
+
+const size_t NetworkThrottleManagerImpl::kActiveRequestThrottlingLimit = 2;
+const int NetworkThrottleManagerImpl::kMedianLifetimeMultiple = 5;
+
+// Initial estimate based on the median in the
+// Net.RequestTime2.Success histogram, excluding cached results by eye.
+const int NetworkThrottleManagerImpl::kInitialMedianInMs = 400;
+
+// Set timers slightly further into the future than they need to be set, so
+// that the algorithm isn't vulnerable to timer round off errors triggering
+// the callback before the throttle would be considered aged out of the set.
+// Set to 17 to hanlde systems with |!base::TimeTicks::IsHighResolution()|.
+// Note that even if the timer goes off before it should, all that should cost
+// is a second task; this class does not rely on timer accuracy for its
+// correctness.
+const int kTimerFudgeInMs = 17;
+
+class NetworkThrottleManagerImpl::ThrottleImpl
+    : public NetworkThrottleManager::Throttle {
+ public:
+  // Allowed state transitions are BLOCKED -> OUTSTANDING -> AGED.
+  // Throttles may be created in the BLOCKED or OUTSTANDING states.
+  enum class State {
+    // Not allowed to proceed by manager.
+    BLOCKED,
+
+    // Allowed to proceed, counts as an "outstanding" request for
+    // manager accounting purposes.
+    OUTSTANDING,
+
+    // Old enough to not count as "outstanding" anymore for
+    // manager accounting purposes.
+    AGED
+  };
+
+  using ThrottleListQueuePointer =
+      NetworkThrottleManagerImpl::ThrottleList::iterator;
+
+  // Caller must arrange that |*delegate| and |*manager| outlive
+  // the ThrottleImpl class.
+  ThrottleImpl(bool blocked,
+               RequestPriority priority,
+               ThrottleDelegate* delegate,
+               NetworkThrottleManagerImpl* manager,
+               ThrottleListQueuePointer queue_pointer);
+
+  ~ThrottleImpl() override;
+
+  // Throttle:
+  bool IsBlocked() const override;
+  RequestPriority Priority() const override;
+  void SetPriority(RequestPriority priority) override;
+
+  State state() const { return state_; }
+
+  ThrottleListQueuePointer queue_pointer() const { return queue_pointer_; }
+  void set_queue_pointer(const ThrottleListQueuePointer& pointer) {
+    queue_pointer_ = pointer;
+  }
+
+  void set_start_time(base::TimeTicks start_time) { start_time_ = start_time; }
+  base::TimeTicks start_time() const { return start_time_; }
+
+  // Change the throttle's state to AGED.  The previous
+  // state must be OUTSTANDING.
+  void SetAged();
+
+  // Note that this call calls the delegate, and hence may result in
+  // re-entrant calls into the manager or ThrottleImpl.  The manager should
+  // not rely on any state other than its own existence being persistent
+  // across this call.
+  void NotifyUnblocked();
+
+ private:
+  State state_;
+  RequestPriority priority_;
+  ThrottleDelegate* const delegate_;
+  NetworkThrottleManagerImpl* const manager_;
+
+  base::TimeTicks start_time_;
+
+  // To allow deletion from the blocked queue (when the throttle is in the
+  // blocked queue).
+  ThrottleListQueuePointer queue_pointer_;
+
+  DISALLOW_COPY_AND_ASSIGN(ThrottleImpl);
+};
+
+NetworkThrottleManagerImpl::ThrottleImpl::ThrottleImpl(
+    bool blocked,
+    RequestPriority priority,
+    NetworkThrottleManager::ThrottleDelegate* delegate,
+    NetworkThrottleManagerImpl* manager,
+    ThrottleListQueuePointer queue_pointer)
+    : state_(blocked ? State::BLOCKED : State::OUTSTANDING),
+      priority_(priority),
+      delegate_(delegate),
+      manager_(manager),
+      queue_pointer_(queue_pointer) {
+  DCHECK(delegate);
+  if (!blocked)
+    start_time_ = manager->tick_clock_->NowTicks();
+}
+
+NetworkThrottleManagerImpl::ThrottleImpl::~ThrottleImpl() {
+  manager_->OnThrottleDestroyed(this);
+}
+
+bool NetworkThrottleManagerImpl::ThrottleImpl::IsBlocked() const {
+  return state_ == State::BLOCKED;
+}
+
+RequestPriority NetworkThrottleManagerImpl::ThrottleImpl::Priority() const {
+  return priority_;
+}
+
+void NetworkThrottleManagerImpl::ThrottleImpl::SetPriority(
+    RequestPriority new_priority) {
+  RequestPriority old_priority(priority_);
+  if (old_priority == new_priority)
+    return;
+  priority_ = new_priority;
+  manager_->OnThrottlePriorityChanged(this, old_priority, new_priority);
+}
+
+void NetworkThrottleManagerImpl::ThrottleImpl::SetAged() {
+  DCHECK_EQ(State::OUTSTANDING, state_);
+  state_ = State::AGED;
+}
+
+void NetworkThrottleManagerImpl::ThrottleImpl::NotifyUnblocked() {
+  // This method should only be called once, and only if the
+  // current state is blocked.
+  DCHECK_EQ(State::BLOCKED, state_);
+  state_ = State::OUTSTANDING;
+  delegate_->OnThrottleUnblocked(this);
+}
+
+NetworkThrottleManagerImpl::NetworkThrottleManagerImpl()
+    : lifetime_median_estimate_(PercentileEstimator::kMedianPercentile,
+                                kInitialMedianInMs),
+      outstanding_recomputation_timer_(false /* retain_user_task */,
+                                       false /* is_repeating */),
+      tick_clock_(new base::DefaultTickClock()),
+      weak_ptr_factory_(this) {
+  outstanding_recomputation_timer_.SetTaskRunner(
+      base::ThreadTaskRunnerHandle::Get());
+}
+
+NetworkThrottleManagerImpl::~NetworkThrottleManagerImpl() {}
+
+std::unique_ptr<NetworkThrottleManager::Throttle>
+NetworkThrottleManagerImpl::CreateThrottle(
+    NetworkThrottleManager::ThrottleDelegate* delegate,
+    RequestPriority priority,
+    bool ignore_limits) {
+  bool blocked =
+      (!ignore_limits && priority == THROTTLED &&
+       outstanding_throttles_.size() >= kActiveRequestThrottlingLimit);
+
+  std::unique_ptr<NetworkThrottleManagerImpl::ThrottleImpl> throttle(
+      new ThrottleImpl(blocked, priority, delegate, this,
+                       blocked_throttles_.end()));
+
+  ThrottleList& insert_list(blocked ? blocked_throttles_
+                                    : outstanding_throttles_);
+
+  throttle->set_queue_pointer(
+      insert_list.insert(insert_list.end(), throttle.get()));
+
+  // In case oustanding_throttles_ was empty, set up timer.
+  if (!blocked)
+    RecomputeOutstanding();
+
+  return std::move(throttle);
+}
+
+void NetworkThrottleManagerImpl::SetTickClockForTesting(
+    std::unique_ptr<base::TickClock> tick_clock) {
+  tick_clock_ = std::move(tick_clock);
+}
+
+bool NetworkThrottleManagerImpl::ConditionallyTriggerTimerForTesting() {
+  if (!outstanding_recomputation_timer_.IsRunning() ||
+      (tick_clock_->NowTicks() <
+       outstanding_recomputation_timer_.desired_run_time())) {
+    return false;
+  }
+
+  base::Closure timer_callback(outstanding_recomputation_timer_.user_task());
+  outstanding_recomputation_timer_.Stop();
+  timer_callback.Run();
+  return true;
+}
+
+void NetworkThrottleManagerImpl::OnThrottlePriorityChanged(
+    NetworkThrottleManagerImpl::ThrottleImpl* throttle,
+    RequestPriority old_priority,
+    RequestPriority new_priority) {
+  // The only case requiring a state change is if the priority change
+  // implies unblocking, which can only happen on a transition from blocked
+  // (implies THROTTLED) to non-THROTTLED.
+  if (throttle->IsBlocked() && new_priority != THROTTLED) {
+    // May result in re-entrant calls into this class.
+    UnblockThrottle(throttle);
+  }
+}
+
+void NetworkThrottleManagerImpl::OnThrottleDestroyed(ThrottleImpl* throttle) {
+  switch (throttle->state()) {
+    case ThrottleImpl::State::BLOCKED:
+      DCHECK(throttle->queue_pointer() != blocked_throttles_.end());
+      DCHECK(std::find(blocked_throttles_.begin(), blocked_throttles_.end(),
+                       throttle) == throttle->queue_pointer());

[mmenke, 2016/11/11 20:51:17]

If we want to do this, it seems like we could just do:

DCHECK_EQ(throttle, *throttle->queue_pointer());

Note that the double find after this block takes care of the second part of the
check.  Same for the OUTSTANDING case.

Alternatively/additionally, could add a similar DCHECK to set_queue_pointer (And
even queue_pointer(), for that matter), which seems a bit more comprehensive to
me, and ensure the checks pass on construction, which is generally more useful
than checking on destruction.

[Randy Smith (Not in Mondays), 2016/11/16 15:56:36]

Good point; done.
I looked into doing this, but figuring out the proper queue to look at in
set_queue_pointer() is tricky with the dance between NetworkThrottleManagerImpl
& ThrottleImpl about how the state is changed.  Which somewhat argues in favor
of breaking down the abstraction barrier, which I considered earlier in this CL,
but I'm afraid at this point I'm just going to leave it be.

[mmenke, 2016/11/16 16:02:08]

I don't think you need to look into a queue?

DCHECK_EQ(this, *queue_pointer_);?

+      blocked_throttles_.erase(throttle->queue_pointer());
+      break;
+    case ThrottleImpl::State::OUTSTANDING:
+      DCHECK(throttle->queue_pointer() != outstanding_throttles_.end());
+      DCHECK(std::find(outstanding_throttles_.begin(),
+                       outstanding_throttles_.end(),
+                       throttle) == throttle->queue_pointer());
+      outstanding_throttles_.erase(throttle->queue_pointer());
+    // Fall through
+    case ThrottleImpl::State::AGED:
+      DCHECK(!throttle->start_time().is_null());
+      lifetime_median_estimate_.AddSample(
+          (tick_clock_->NowTicks() - throttle->start_time())
+              .InMillisecondsRoundedUp());
+      break;
+  }
+
+  DCHECK(std::find(blocked_throttles_.begin(), blocked_throttles_.end(),
+                   throttle) == blocked_throttles_.end());
+  DCHECK(std::find(outstanding_throttles_.begin(), outstanding_throttles_.end(),
+                   throttle) == outstanding_throttles_.end());
+
+  // Unblock the throttles if there's some chance there's a throttle to
+  // unblock.
+  if (outstanding_throttles_.size() < kActiveRequestThrottlingLimit &&
+      !blocked_throttles_.empty()) {
+    // Via PostTask so there aren't upcalls from within destructors.
+    base::ThreadTaskRunnerHandle::Get()->PostTask(
+        FROM_HERE,
+        base::Bind(&NetworkThrottleManagerImpl::MaybeUnblockThrottles,
+                   weak_ptr_factory_.GetWeakPtr()));
+  }
+}
+
+void NetworkThrottleManagerImpl::RecomputeOutstanding() {
+  // Remove all throttles that have aged out of the outstanding set.
+  base::TimeTicks now(tick_clock_->NowTicks());
+  base::TimeDelta age_horizon(base::TimeDelta::FromMilliseconds((
+      kMedianLifetimeMultiple * lifetime_median_estimate_.current_estimate())));
+  while (!outstanding_throttles_.empty()) {
+    ThrottleImpl* throttle = *outstanding_throttles_.begin();
+    if (throttle->start_time() + age_horizon >= now)
+      break;
+
+    outstanding_throttles_.erase(outstanding_throttles_.begin());
+    throttle->SetAged();
+    throttle->set_queue_pointer(outstanding_throttles_.end());
+  }
+
+  if (outstanding_throttles_.empty())
+    return;
+
+  // If the timer is already running, be conservative and leave it alone;
+  // the time for which it would be set will only be later than when it's
+  // currently set.
+  // This addresses, e.g., situations where a RecomputeOutstanding() races
+  // with a running timer which would unblock blocked throttles.
+  if (outstanding_recomputation_timer_.IsRunning())
+    return;
+
+  ThrottleImpl* first_throttle(*outstanding_throttles_.begin());
+  DCHECK_GE(first_throttle->start_time() + age_horizon, now);
+  outstanding_recomputation_timer_.Start(
+      FROM_HERE, ((first_throttle->start_time() + age_horizon) - now +
+                  base::TimeDelta::FromMilliseconds(kTimerFudgeInMs)),
+      // Unretained use of |this| is safe because the timer is
+      // owned by this object, and will be torn down if this object
+      // is destroyed.
+      base::Bind(&NetworkThrottleManagerImpl::MaybeUnblockThrottles,
+                 base::Unretained(this)));
+}
+
+void NetworkThrottleManagerImpl::UnblockThrottle(ThrottleImpl* throttle) {
+  DCHECK(throttle->IsBlocked());
+
+  blocked_throttles_.erase(throttle->queue_pointer());
+  throttle->set_start_time(tick_clock_->NowTicks());
+  throttle->set_queue_pointer(
+      outstanding_throttles_.insert(outstanding_throttles_.end(), throttle));
+
+  // Called in case |*throttle| was added to a null set.
+  RecomputeOutstanding();
+
+  // May result in re-entrant calls into this class.
+  throttle->NotifyUnblocked();
+}
+
+void NetworkThrottleManagerImpl::MaybeUnblockThrottles() {
+  RecomputeOutstanding();
+
+  while (outstanding_throttles_.size() < kActiveRequestThrottlingLimit &&
+         !blocked_throttles_.empty()) {
+    // NOTE: This call may result in reentrant calls into
+    // NetworkThrottleManagerImpl; no state should be assumed to be
+    // persistent across this call.
+    UnblockThrottle(blocked_throttles_.front());
+  }
+}
+
+}  // namespace net