| // Copyright (c) 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. |
| |
| #ifndef QUICHE_QUIC_CORE_CONGESTION_CONTROL_WINDOWED_FILTER_H_ |
| #define QUICHE_QUIC_CORE_CONGESTION_CONTROL_WINDOWED_FILTER_H_ |
| |
| // Implements Kathleen Nichols' algorithm for tracking the minimum (or maximum) |
| // estimate of a stream of samples over some fixed time interval. (E.g., |
| // the minimum RTT over the past five minutes.) The algorithm keeps track of |
| // the best, second best, and third best min (or max) estimates, maintaining an |
| // invariant that the measurement time of the n'th best >= n-1'th best. |
| |
| // The algorithm works as follows. On a reset, all three estimates are set to |
| // the same sample. The second best estimate is then recorded in the second |
| // quarter of the window, and a third best estimate is recorded in the second |
| // half of the window, bounding the worst case error when the true min is |
| // monotonically increasing (or true max is monotonically decreasing) over the |
| // window. |
| // |
| // A new best sample replaces all three estimates, since the new best is lower |
| // (or higher) than everything else in the window and it is the most recent. |
| // The window thus effectively gets reset on every new min. The same property |
| // holds true for second best and third best estimates. Specifically, when a |
| // sample arrives that is better than the second best but not better than the |
| // best, it replaces the second and third best estimates but not the best |
| // estimate. Similarly, a sample that is better than the third best estimate |
| // but not the other estimates replaces only the third best estimate. |
| // |
| // Finally, when the best expires, it is replaced by the second best, which in |
| // turn is replaced by the third best. The newest sample replaces the third |
| // best. |
| |
| #include "quic/core/quic_time.h" |
| |
| namespace quic { |
| |
| // Compares two values and returns true if the first is less than or equal |
| // to the second. |
| template <class T> |
| struct QUIC_EXPORT_PRIVATE MinFilter { |
| bool operator()(const T& lhs, const T& rhs) const { return lhs <= rhs; } |
| }; |
| |
| // Compares two values and returns true if the first is greater than or equal |
| // to the second. |
| template <class T> |
| struct QUIC_EXPORT_PRIVATE MaxFilter { |
| bool operator()(const T& lhs, const T& rhs) const { return lhs >= rhs; } |
| }; |
| |
| // Use the following to construct a windowed filter object of type T. |
| // For example, a min filter using QuicTime as the time type: |
| // WindowedFilter<T, MinFilter<T>, QuicTime, QuicTime::Delta> ObjectName; |
| // A max filter using 64-bit integers as the time type: |
| // WindowedFilter<T, MaxFilter<T>, uint64_t, int64_t> ObjectName; |
| // Specifically, this template takes four arguments: |
| // 1. T -- type of the measurement that is being filtered. |
| // 2. Compare -- MinFilter<T> or MaxFilter<T>, depending on the type of filter |
| // desired. |
| // 3. TimeT -- the type used to represent timestamps. |
| // 4. TimeDeltaT -- the type used to represent continuous time intervals between |
| // two timestamps. Has to be the type of (a - b) if both |a| and |b| are |
| // of type TimeT. |
| template <class T, class Compare, typename TimeT, typename TimeDeltaT> |
| class QUIC_EXPORT_PRIVATE WindowedFilter { |
| public: |
| // |window_length| is the period after which a best estimate expires. |
| // |zero_value| is used as the uninitialized value for objects of T. |
| // Importantly, |zero_value| should be an invalid value for a true sample. |
| WindowedFilter(TimeDeltaT window_length, T zero_value, TimeT zero_time) |
| : window_length_(window_length), |
| zero_value_(zero_value), |
| zero_time_(zero_time), |
| estimates_{Sample(zero_value_, zero_time), |
| Sample(zero_value_, zero_time), |
| Sample(zero_value_, zero_time)} {} |
| |
| // Changes the window length. Does not update any current samples. |
| void SetWindowLength(TimeDeltaT window_length) { |
| window_length_ = window_length; |
| } |
| |
| // Updates best estimates with |sample|, and expires and updates best |
| // estimates as necessary. |
| void Update(T new_sample, TimeT new_time) { |
| // Reset all estimates if they have not yet been initialized, if new sample |
| // is a new best, or if the newest recorded estimate is too old. |
| if (estimates_[0].sample == zero_value_ || |
| Compare()(new_sample, estimates_[0].sample) || |
| new_time - estimates_[2].time > window_length_) { |
| Reset(new_sample, new_time); |
| return; |
| } |
| |
| if (Compare()(new_sample, estimates_[1].sample)) { |
| estimates_[1] = Sample(new_sample, new_time); |
| estimates_[2] = estimates_[1]; |
| } else if (Compare()(new_sample, estimates_[2].sample)) { |
| estimates_[2] = Sample(new_sample, new_time); |
| } |
| |
| // Expire and update estimates as necessary. |
| if (new_time - estimates_[0].time > window_length_) { |
| // The best estimate hasn't been updated for an entire window, so promote |
| // second and third best estimates. |
| estimates_[0] = estimates_[1]; |
| estimates_[1] = estimates_[2]; |
| estimates_[2] = Sample(new_sample, new_time); |
| // Need to iterate one more time. Check if the new best estimate is |
| // outside the window as well, since it may also have been recorded a |
| // long time ago. Don't need to iterate once more since we cover that |
| // case at the beginning of the method. |
| if (new_time - estimates_[0].time > window_length_) { |
| estimates_[0] = estimates_[1]; |
| estimates_[1] = estimates_[2]; |
| } |
| return; |
| } |
| if (estimates_[1].sample == estimates_[0].sample && |
| new_time - estimates_[1].time > window_length_ >> 2) { |
| // A quarter of the window has passed without a better sample, so the |
| // second-best estimate is taken from the second quarter of the window. |
| estimates_[2] = estimates_[1] = Sample(new_sample, new_time); |
| return; |
| } |
| |
| if (estimates_[2].sample == estimates_[1].sample && |
| new_time - estimates_[2].time > window_length_ >> 1) { |
| // We've passed a half of the window without a better estimate, so take |
| // a third-best estimate from the second half of the window. |
| estimates_[2] = Sample(new_sample, new_time); |
| } |
| } |
| |
| // Resets all estimates to new sample. |
| void Reset(T new_sample, TimeT new_time) { |
| estimates_[0] = estimates_[1] = estimates_[2] = |
| Sample(new_sample, new_time); |
| } |
| |
| void Clear() { Reset(zero_value_, zero_time_); } |
| |
| T GetBest() const { return estimates_[0].sample; } |
| T GetSecondBest() const { return estimates_[1].sample; } |
| T GetThirdBest() const { return estimates_[2].sample; } |
| |
| private: |
| struct QUIC_EXPORT_PRIVATE Sample { |
| T sample; |
| TimeT time; |
| Sample(T init_sample, TimeT init_time) |
| : sample(init_sample), time(init_time) {} |
| }; |
| |
| TimeDeltaT window_length_; // Time length of window. |
| T zero_value_; // Uninitialized value of T. |
| TimeT zero_time_; // Uninitialized value of TimeT. |
| Sample estimates_[3]; // Best estimate is element 0. |
| }; |
| |
| } // namespace quic |
| |
| #endif // QUICHE_QUIC_CORE_CONGESTION_CONTROL_WINDOWED_FILTER_H_ |