1 #ifndef _QUEUE_LENGTH_POLICY_H
2 #define _QUEUE_LENGTH_POLICY_H 1
9 // A class to estimate the future jitter. Used in QueueLengthPolicy (see below).
11 // There are many ways to estimate jitter; I've tested a few ones (and also
12 // some algorithms that don't explicitly model jitter) with different
13 // parameters on some real-life data in experiments/queue_drop_policy.cpp.
14 // This is one based on simple order statistics where I've added some margin in
15 // the number of starvation events; I believe that about one every hour would
16 // probably be acceptable, but this one typically goes lower than that, at the
17 // cost of 2–3 ms extra latency. (If the queue is hard-limited to one frame, it's
18 // possible to get ~10 ms further down, but this would mean framedrops every
19 // second or so.) The general strategy is: Take the 99.9-percentile jitter over
20 // last 5000 frames, multiply by two, and that's our worst-case jitter
21 // estimate. The fact that we're not using the max value means that we could
22 // actually even throw away very late frames immediately, which means we only
23 // get one user-visible event instead of seeing something both when the frame
24 // arrives late (duplicate frame) and then again when we drop.
27 static constexpr size_t history_length = 5000;
28 static constexpr double percentile = 0.999;
29 static constexpr double multiplier = 2.0;
32 void register_metrics(const std::vector<std::pair<std::string, std::string>> &labels);
33 void unregister_metrics(const std::vector<std::pair<std::string, std::string>> &labels);
38 expected_timestamp = std::chrono::steady_clock::time_point::min();
40 void frame_arrived(std::chrono::steady_clock::time_point now, int64_t frame_duration, size_t dropped_frames, bool verbose = false);
41 std::chrono::steady_clock::time_point get_expected_next_frame() const { return expected_timestamp; }
42 double estimate_max_jitter() const;
45 // A simple O(k) based algorithm for getting the k-th largest or
46 // smallest element from our window; we simply keep the multiset
47 // ordered (insertions and deletions are O(n) as always) and then
48 // iterate from one of the sides. If we had larger values of k,
49 // we could go for a more complicated setup with two sets or heaps
50 // (one increasing and one decreasing) that we keep balanced around
51 // the point, or it is possible to reimplement std::set with
52 // counts in each node. However, since k=5, we don't need this.
53 std::multiset<double> orders;
54 std::deque<std::multiset<double>::iterator> history;
56 std::chrono::steady_clock::time_point expected_timestamp = std::chrono::steady_clock::time_point::min();
57 int64_t last_duration = 0;
59 // Metrics. There are no direct summaries for jitter, since we already have latency summaries.
60 std::atomic<int64_t> metric_input_underestimated_jitter_frames{0};
61 std::atomic<double> metric_input_estimated_max_jitter_seconds{0.0 / 0.0};
64 // For any card that's not the master (where we pick out the frames as they
65 // come, as fast as we can process), there's going to be a queue. The question
66 // is when we should drop frames from that queue (apart from the obvious
67 // dropping if the 16-frame queue should become full), especially given that
68 // the frame rate could be lower or higher than the master (either subtly or
69 // dramatically). We have two (conflicting) demands:
71 // 1. We want to avoid starving the queue.
72 // 2. We don't want to add more delay than is needed.
74 // Our general strategy is to drop as many frames as we can (helping for #2)
75 // that we think is safe for #1 given jitter. To this end, we measure the
76 // deviation from the expected arrival time for all cards, and use that for
77 // continuous jitter estimation.
79 // We then drop everything from the queue that we're sure we won't need to
80 // serve the output in the time before the next frame arrives. Typically,
81 // this means the queue will contain 0 or 1 frames, although more is also
82 // possible if the jitter is very high.
83 class QueueLengthPolicy {
85 QueueLengthPolicy() {}
87 void register_metrics(const std::vector<std::pair<std::string, std::string>> &labels);
88 void unregister_metrics(const std::vector<std::pair<std::string, std::string>> &labels);
90 // Call after picking out a frame, so 0 means starvation.
91 // Note that the policy has no memory; everything is given in as parameters.
92 void update_policy(std::chrono::steady_clock::time_point now,
93 std::chrono::steady_clock::time_point expected_next_input_frame,
94 int64_t input_frame_duration,
95 int64_t master_frame_duration,
96 double max_input_card_jitter_seconds,
97 double max_master_card_jitter_seconds, bool verbose=false);
98 unsigned get_safe_queue_length() const { return safe_queue_length; }
101 unsigned safe_queue_length = 0; // Can never go below zero.
104 std::atomic<int64_t> metric_input_queue_safe_length_frames{1};
107 #endif // !defined(_QUEUE_LENGTH_POLICY_H)