1 /* Copyright (c) 2016-2023. The SimGrid Team. All rights reserved. */
3 /* This program is free software; you can redistribute it and/or modify it
4 * under the terms of the license (GNU LGPL) which comes with this package. */
6 #ifndef SIMGRID_MC_CLOCK_VECTOR_HPP
7 #define SIMGRID_MC_CLOCK_VECTOR_HPP
9 #include "simgrid/forward.h"
12 #include <initializer_list>
14 #include <unordered_map>
16 namespace simgrid::mc {
19 * @brief A multi-dimensional vector used to keep track of
20 * happens-before relation between dependent events in an
21 * execution of DPOR, SDPOR, or ODPOR
23 * Clock vectors permit actors in a distributed system
24 * to determine whether two events occurred one after the other
25 * but they may not have); i.e. they permit actors to keep track of "time".
26 * A clever observation made in the original DPOR paper is that a
27 * transition-based "happens-before" relation can be computed for
28 * any particular trace `S` using clock vectors, effectively
29 * treating dependency like the passing of a message (the context
30 * in which vector clocks are typically used).
32 * Support, however, needs to be added to clock vectors since
33 * SimGrid permits the *creation* of new actors during execution.
34 * Since we don't know this size before-hand, we have to allow
35 * clock vectors to behave as if they were "infinitely" large. To
36 * do so, all newly mapped elements, if not assigned a value, are
37 * defaulted to `0`. This corresponds to the value this actor would
38 * have had regardless had its creation been known to have evnetually
39 * occurred: no actions taken by that actor had occurred prior, so
40 * there's no way the clock vector would have been updated. In other
41 * words, when comparing clock vectors of different sizes, it's equivalent
42 * to imagine both of the same size with elements absent in one or
43 * the other implicitly mapped to zero.
45 struct ClockVector final {
47 std::unordered_map<aid_t, uint32_t> contents;
50 ClockVector() = default;
51 ClockVector(const ClockVector&) = default;
52 ClockVector& operator=(ClockVector const&) = default;
53 ClockVector(ClockVector&&) = default;
54 ClockVector(std::initializer_list<std::pair<const aid_t, uint32_t>> init) : contents(std::move(init)) {}
57 * @brief The number of components in this
60 * A `ClockVector` implicitly maps the id of an actor
61 * it does not contain to a default value of `0`.
62 * Thus, a `ClockVector` is "lazy" in the sense
63 * that new actors are "automatically" mapped
64 * without needing to be explicitly added the clock
65 * vector when the actor is created. This means that
66 * comparison between clock vectors is possible
67 * even as actors become enabled and disabled
69 * @return uint32_t the number of elements in
72 size_t size() const { return this->contents.size(); }
74 uint32_t& operator[](aid_t aid)
76 // NOTE: The `operator[]` overload of
77 // unordered_map will create a new key-value
78 // pair if `tid` does not exist and will use
79 // a _default_ value for the value (0 in this case)
80 // which is precisely what we want here
81 return this->contents[aid];
85 * @brief Retrieves the value mapped to the given
86 * actor if it is contained in this clock vector
88 std::optional<uint32_t> get(aid_t aid) const
90 if (const auto iter = this->contents.find(aid); iter != this->contents.end())
91 return std::optional<uint32_t>{iter->second};
96 * @brief Computes a clock vector whose components
97 * are larger than the components of both of
98 * the given clock vectors
100 * The maximum of two clock vectors is definied to
101 * be the clock vector whose components are the maxmimum
102 * of the corresponding components of the arguments.
103 * Since the `ClockVector` class is "lazy", the two
104 * clock vectors given as arguments may not be of the same size.
105 * The resultant clock vector has components as follows:
107 * 1. For each actor that each clock vector maps, the
108 * resulting clock vector maps that thread to the maxmimum
109 * of the values mapped for the actor in each clock vector
111 * 2. For each actor that only a single clock vector maps,
112 * the resulting clock vector maps that thread to the
113 * value mapped by the lone clock vector
115 * The scheme is equivalent to assuming that an unmapped
116 * thread by any one clock vector is implicitly mapped to zero
118 * @param cv1 the first clock vector
119 * @param cv2 the second clock vector
120 * @return a clock vector whose components are at
121 * least as large as the corresponding components of each clock
122 * vector and whose size is large enough to contain the union
123 * of components of each clock vector
125 static ClockVector max(const ClockVector& cv1, const ClockVector& cv2);
128 } // namespace simgrid::mc