folly/folly/executors/Codel.h

/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
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 * See the License for the specific language governing permissions and
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 */

#pragma once

#include <atomic>
#include <chrono>

namespace folly {

/// CoDel (controlled delay) is an active queue management algorithm from
/// networking for battling bufferbloat.
///
/// Services also have queues (of requests, not packets) and suffer from
/// queueing delay when overloaded. This class adapts the codel algorithm for
/// services.
///
/// Codel is discussed in depth on the web [1,2], but a basic sketch of the
/// algorithm is this: if every request has experienced queueing delay greater
/// than the target (5ms) during the past interval (100ms), then we shed load.
///
/// We have adapted the codel algorithm. TCP sheds load by changing windows in
/// reaction to dropped packets. Codel in a network setting drops packets at
/// increasingly shorter intervals (100 / sqrt(n)) to achieve a linear change
/// in throughput. In our experience a different scheme works better for
/// services: when overloaded slough off requests that we dequeue which have
/// exceeded an alternate timeout (2 * target_delay).
///
/// So in summary, to use this class, calculate the time each request spent in
/// the queue and feed that delay to overloaded(), which will tell you whether
/// to expire this request.
///
/// You can also ask for an instantaneous load estimate and the minimum delay
/// observed during this interval.
///
///
/// 1. http://queue.acm.org/detail.cfm?id=2209336
/// 2. https://en.wikipedia.org/wiki/CoDel
class Codel { … };

} // namespace folly