1 /* Copyright (c) 2004-2023. 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_KERNEL_LMM_SYSTEM_HPP
7 #define SIMGRID_KERNEL_LMM_SYSTEM_HPP
9 #include "simgrid/kernel/resource/Action.hpp"
10 #include "simgrid/kernel/resource/Model.hpp"
11 #include "simgrid/s4u/Link.hpp"
12 #include "xbt/asserts.h"
14 #include "xbt/mallocator.h"
16 #include <boost/intrusive/list.hpp>
20 #include <string_view>
23 /* user-visible parameters */
24 XBT_PUBLIC_DATA double sg_precision_workamount;
25 XBT_PUBLIC_DATA double sg_precision_timing;
26 XBT_PUBLIC_DATA int sg_concurrency_limit;
28 namespace simgrid::kernel::lmm {
30 /** @addtogroup SURF_lmm
32 * A linear maxmin solver to resolve inequations systems.
34 * Most SimGrid model rely on a "fluid/steady-state" modeling that simulate the sharing of resources between actions at
35 * relatively coarse-grain. Such sharing is generally done by solving a set of linear inequations. Let's take an
36 * example and assume we have the variables \f$x_1\f$, \f$x_2\f$, \f$x_3\f$, and \f$x_4\f$ . Let's say that \f$x_1\f$
37 * and \f$x_2\f$ correspond to activities running and the same CPU \f$A\f$ whose capacity is \f$C_A\f$. In such a
38 * case, we need to enforce:
40 * \f[ x_1 + x_2 \leq C_A \f]
42 * Likewise, if \f$x_3\f$ (resp. \f$x_4\f$) corresponds to a network flow \f$F_3\f$ (resp. \f$F_4\f$) that goes through
43 * a set of links \f$L_1\f$ and \f$L_2\f$ (resp. \f$L_2\f$ and \f$L_3\f$), then we need to enforce:
45 * \f[ x_3 \leq C_{L_1} \f]
46 * \f[ x_3 + x_4 \leq C_{L_2} \f]
47 * \f[ x_4 \leq C_{L_3} \f]
49 * One could set every variable to 0 to make sure the constraints are satisfied but this would obviously not be very
50 * realistic. A possible objective is to try to maximize the minimum of the \f$x_i\f$ . This ensures that all the
51 * \f$x_i\f$ are positive and "as large as possible".
53 * This is called *max-min fairness* and is the most commonly used objective in SimGrid. Another possibility is to
54 * maximize \f$\sum_if(x_i)\f$, where \f$f\f$ is a strictly increasing concave function.
69 * A possible system could be:
70 * - three variables: `var1`, `var2`, `var3`
71 * - two constraints: `cons1`, `cons2`
72 * - four elements linking:
73 * - `elem1` linking `var1` and `cons1`
74 * - `elem2` linking `var2` and `cons1`
75 * - `elem3` linking `var2` and `cons2`
76 * - `elem4` linking `var3` and `cons2`
78 * And the corresponding inequations will be:
80 * var1.value <= var1.bound
81 * var2.value <= var2.bound
82 * var3.value <= var3.bound
83 * var1.weight * var1.value * elem1.value + var2.weight * var2.value * elem2.value <= cons1.bound
84 * var2.weight * var2.value * elem3.value + var3.weight * var3.value * elem4.value <= cons2.bound
86 * where `var1.value`, `var2.value` and `var3.value` are the unknown values.
88 * If a constraint is not shared, the sum is replaced by a max.
89 * For example, a third non-shared constraint `cons3` and the associated elements `elem5` and `elem6` could write as:
91 * max( var1.weight * var1.value * elem5.value , var3.weight * var3.value * elem6.value ) <= cons3.bound
93 * This is useful for the sharing of resources for various models.
94 * For instance, for the network model, each link is associated to a constraint and each communication to a variable.
96 * Implementation details
98 * For implementation reasons, we are interested in distinguishing variables that actually participate to the
99 * computation of constraints, and those who are part of the equations but are stuck to zero.
100 * We call enabled variables, those which var.weight is strictly positive. Zero-weight variables are called disabled
102 * Unfortunately this concept of enabled/disabled variables intersects with active/inactive variable.
103 * Semantically, the intent is similar, but the conditions under which a variable is active is slightly more strict
104 * than the conditions for it to be enabled.
105 * A variable is active only if its var.value is non-zero (and, by construction, its var.weight is non-zero).
106 * In general, variables remain disabled after their creation, which often models an initialization phase (e.g. first
107 * packet propagating in the network). Then, it is enabled by the corresponding model. Afterwards, the max-min solver
108 * (lmm_solve()) activates it when appropriate. It is possible that the variable is again disabled, e.g. to model the
109 * pausing of an action.
111 * Concurrency limit and maximum
113 * We call concurrency, the number of variables that can be enabled at any time for each constraint.
114 * From a model perspective, this "concurrency" often represents the number of actions that actually compete for one
116 * The LMM solver is able to limit the concurrency for each constraint, and to monitor its maximum value.
118 * One may want to limit the concurrency of constraints for essentially three reasons:
119 * - Keep LMM system in a size that can be solved (it does not react very well with tens of thousands of variables per
121 * - Stay within parameters where the fluid model is accurate enough.
122 * - Model serialization effects
124 * The concurrency limit can also be set to a negative value to disable concurrency limit. This can improve performance
127 * Overall, each constraint contains three fields related to concurrency:
128 * - concurrency_limit which is the limit enforced by the solver
129 * - concurrency_current which is the current concurrency
130 * - concurrency_maximum which is the observed maximum concurrency
132 * Variables consumes the concurrency_limit of each constraint they are using.
133 * Each pair variable/constrainst is linked by a *single* Element object. Through this
134 * object and the respective methods (get_concurrency(), increase_concurrency() and decrease_concurrency()),
135 * the variable changes the constraint's concurrency.
136 * The amount of concurrency slack taken by each variable is determined by the Element::get_concurrency() method.
137 * At the current state, each variable counts as 1 if its consumption weight is greater than 1.
140 /** @{ @ingroup SURF_lmm */
144 * Elements can be seen as glue between constraint objects and variable objects.
145 * Basically, each variable will have a set of elements, one for each constraint where it is involved.
146 * Then, it is used to list all variables involved in constraint through constraint's xxx_element_set lists, or
147 * vice-versa list all constraints for a given variable.
149 class XBT_PUBLIC Element {
151 // Use rule-of-three, and implicitely disable the move constructor which should be 'noexcept' according to C++ Core
153 Element(Constraint* constraint, Variable* variable, double cweight);
154 Element(const Element&) = default;
155 ~Element() = default;
158 * @brief Gets the "weight" of this element for concurrency checks.
160 * This is the amount taken by this variable of the constraint's concurrency slack
162 * @return 1 if consumption_weight greater than 1, 0 otherwise
164 int get_concurrency() const;
166 * @brief Decreases the constraint's concurrency
168 * Decreases the equivalent of get_concurrency() from the constraint related to this element
170 void decrease_concurrency();
172 * @brief Increase constraint concurrency
173 * @param check_limit Don't check constraint concurrency overflow right now
175 void increase_concurrency(bool check_limit = true);
178 void make_inactive();
180 /* hookup to constraint */
181 boost::intrusive::list_member_hook<> enabled_element_set_hook;
182 boost::intrusive::list_member_hook<> disabled_element_set_hook;
183 boost::intrusive::list_member_hook<> active_element_set_hook;
185 Constraint* constraint;
188 // consumption_weight: impact of 1 byte or flop of your application onto the resource (in byte or flop)
189 // - if CPU, then probably 1.
190 // - If network, then 1 in forward direction and 0.05 backward for the ACKs
191 double consumption_weight;
192 // maximum consumption weight (can be different from consumption_weight with subflows/ptasks)
193 double max_consumption_weight;
196 class ConstraintLight {
198 double remaining_over_usage;
203 * @brief LMM constraint
204 * Each constraint contains several partially overlapping logical sets of elements:
205 * \li Disabled elements which variable's weight is zero. This variables are not at all processed by LMM, but eventually
206 * the corresponding action will enable it (at least this is the idea).
207 * \li Enabled elements which variable's weight is non-zero. They are utilized in some LMM functions.
208 * \li Active elements which variable's weight is non-zero (i.e. it is enabled) AND its element value is non-zero.
209 * LMM_solve iterates over active elements during resolution, dynamically making them active or inactive.
211 class XBT_PUBLIC Constraint {
213 enum class SharingPolicy { WIFI = 3, NONLINEAR = 2, SHARED = 1, FATPIPE = 0 };
215 Constraint(resource::Resource* id_value, double bound_value);
217 /** @brief Unshare a constraint. */
218 void unshare() { sharing_policy_ = SharingPolicy::FATPIPE; }
220 /** @brief Set how a constraint is shared */
221 void set_sharing_policy(SharingPolicy policy, const s4u::NonLinearResourceCb& cb);
222 /** @brief Check how a constraint is shared */
223 SharingPolicy get_sharing_policy() const { return sharing_policy_; }
225 /** @brief Get the usage of the constraint after the last lmm solve */
226 double get_usage() const;
228 /** @brief Sets the concurrency limit for this constraint */
229 void set_concurrency_limit(int limit)
231 xbt_assert(limit < 0 || concurrency_maximum_ <= limit,
232 "New concurrency limit should be larger than observed concurrency maximum. Maybe you want to call"
233 " concurrency_maximum_reset() to reset the maximum?");
234 concurrency_limit_ = limit;
237 /** @brief Gets the concurrency limit for this constraint */
238 int get_concurrency_limit() const { return concurrency_limit_; }
241 * @brief Reset the concurrency maximum for a given variable (we will update the maximum to reflect constraint
244 void reset_concurrency_maximum() { concurrency_maximum_ = 0; }
246 /** @brief Get the concurrency maximum for a given constraint (which reflects constraint evolution). */
247 int get_concurrency_maximum() const
249 xbt_assert(concurrency_limit_ < 0 || concurrency_maximum_ <= concurrency_limit_,
250 "Very bad: maximum observed concurrency is higher than limit. This is a bug of SURF, please report it.");
251 return concurrency_maximum_;
255 * @brief Get constraint current concurrency slack
257 * This represents the "space" available for new variables in this contraint.
258 * A variable can be enabled and use this constraint if its get_concurrency() <= slack
260 * @return Constraint's slack
262 int get_concurrency_slack() const
264 return concurrency_limit_ < 0 ? std::numeric_limits<int>::max() : concurrency_limit_ - concurrency_current_;
268 * @brief Get a var associated to a constraint
269 * @details Get the first variable of the next variable of elem if elem is not NULL
270 * @param elem A element of constraint of the constraint or NULL
271 * @return A variable associated to a constraint
273 Variable* get_variable(const Element** elem) const;
276 * @brief Get a var associated to a constraint
277 * @details Get the first variable of the next variable of elem if elem is not NULL
278 * @param elem A element of constraint of the constraint or NULL
279 * @param nextelem A element of constraint of the constraint or NULL, the one after elem
280 * @param numelem parameter representing the number of elements to go
281 * @return A variable associated to a constraint
283 Variable* get_variable_safe(const Element** elem, const Element** nextelem, size_t* numelem) const;
286 * @brief Get the data associated to a constraint
287 * @return The data associated to the constraint
289 resource::Resource* get_id() const { return id_; }
291 /* hookup to system */
292 boost::intrusive::list_member_hook<> constraint_set_hook_;
293 boost::intrusive::list_member_hook<> active_constraint_set_hook_;
294 boost::intrusive::list_member_hook<> modified_constraint_set_hook_;
295 boost::intrusive::list_member_hook<> saturated_constraint_set_hook_;
296 boost::intrusive::list<Element, boost::intrusive::member_hook<Element, boost::intrusive::list_member_hook<>,
297 &Element::enabled_element_set_hook>>
298 enabled_element_set_;
299 boost::intrusive::list<Element, boost::intrusive::member_hook<Element, boost::intrusive::list_member_hook<>,
300 &Element::disabled_element_set_hook>>
301 disabled_element_set_;
302 boost::intrusive::list<Element, boost::intrusive::member_hook<Element, boost::intrusive::list_member_hook<>,
303 &Element::active_element_set_hook>>
305 double remaining_ = 0.0;
308 double dynamic_bound_ = 0.0; //!< dynamic bound for this constraint, defined by user's callback
309 // TODO MARTIN Check maximum value across resources at the end of simulation and give a warning is more than e.g. 500
310 int concurrency_current_ = 0; /* The current concurrency */
311 int concurrency_maximum_ = 0; /* The maximum number of (enabled and disabled) variables associated to the constraint
312 * at any given time (essentially for tracing)*/
314 SharingPolicy sharing_policy_ = SharingPolicy::SHARED;
315 int rank_; // Only used in debug messages to identify the constraint
316 double lambda_ = 0.0;
317 double new_lambda_ = 0.0;
318 ConstraintLight* cnst_light_ = nullptr;
319 s4u::NonLinearResourceCb dyn_constraint_cb_;
322 static int next_rank_; // To give a separate rank_ to each constraint
323 int concurrency_limit_ = sg_concurrency_limit; /* The maximum number of variables that may be enabled at any time
324 * (stage variables if necessary) */
325 resource::Resource* id_;
329 * @brief LMM variable
331 * When something prevents us from enabling a variable, we "stage" the weight that we would have like to set, so that as
332 * soon as possible we enable the variable with desired weight
334 class XBT_PUBLIC Variable {
336 void initialize(resource::Action* id_value, double sharing_penalty, double bound_value, size_t number_of_constraints,
337 unsigned visited_value);
339 /** @brief Get the value of the variable after the last lmm solve */
340 double get_value() const { return value_; }
342 /** @brief Get the maximum value of the variable (-1.0 if no specified maximum) */
343 double get_bound() const { return bound_; }
346 * @brief Get the numth constraint associated to the variable
347 * @param num The rank of constraint we want to get
348 * @return The numth constraint
350 Constraint* get_constraint(unsigned num) const { return num < cnsts_.size() ? cnsts_[num].constraint : nullptr; }
353 * @brief Get the weight of the numth constraint associated to the variable
354 * @param num The rank of constraint we want to get
355 * @return The numth constraint
357 double get_constraint_weight(unsigned num) const
359 return num < cnsts_.size() ? cnsts_[num].consumption_weight : 0.0;
362 /** @brief Get the number of constraint associated to a variable */
363 size_t get_number_of_constraint() const { return cnsts_.size(); }
365 /** @brief Get the data associated to a variable */
366 resource::Action* get_id() const { return id_; }
368 /** @brief Get the penalty of a variable */
369 double get_penalty() const { return sharing_penalty_; }
371 /** @brief Measure the minimum concurrency slack across all constraints where the given var is involved */
372 int get_min_concurrency_slack() const;
374 /** @brief Check if a variable can be enabled
375 * Make sure to set staged_penalty before, if your intent is only to check concurrency
377 bool can_enable() const { return staged_sharing_penalty_ > 0 && get_min_concurrency_slack() > 0; }
379 /* hookup to system */
380 boost::intrusive::list_member_hook<> variable_set_hook_;
381 boost::intrusive::list_member_hook<> saturated_variable_set_hook_;
383 std::vector<Element> cnsts_;
385 // sharing_penalty: variable's impact on the resource during the sharing
386 // if == 0, the variable is not considered by LMM
387 // on CPU, actions with N threads have a sharing of N
388 // on network, the actions with higher latency have a lesser sharing_penalty
389 double sharing_penalty_;
391 double staged_sharing_penalty_; /* If non-zero, variable is staged for addition as soon as maxconcurrency constraints
395 resource::Action* id_;
396 int rank_; // Only used in debug messages to identify the variable
397 unsigned visited_; /* used by System::update_modified_cnst_set() */
401 static int next_rank_; // To give a separate rank_ to each variable
404 inline void Element::make_active()
406 constraint->active_element_set_.push_front(*this);
408 inline void Element::make_inactive()
410 if (active_element_set_hook.is_linked())
411 xbt::intrusive_erase(constraint->active_element_set_, *this);
417 class XBT_PUBLIC System {
420 * @brief Creates a new System solver
422 * @param solver_name Name of the solver to be used
423 * @param selective_update Enables lazy updates
424 * @return pointer to System instance
426 static System* build(std::string_view solver_name, bool selective_update);
427 /** @brief Validates solver configuration */
428 static void validate_solver(const std::string& solver_name);
431 * @brief Create a new Linear MaxMim system
432 * @param selective_update whether we should do lazy updates
434 explicit System(bool selective_update);
435 /** @brief Free an existing Linear MaxMin system */
439 * @brief Create a new Linear MaxMin constraint
440 * @param id Data associated to the constraint (e.g.: a network link)
441 * @param bound_value The bound value of the constraint
443 Constraint* constraint_new(resource::Resource* id, double bound_value);
446 * @brief Create a new Linear MaxMin variable
447 * @param id Data associated to the variable (e.g.: a network communication)
448 * @param sharing_penalty The weight of the variable (0.0 if not used)
449 * @param bound The maximum value of the variable (-1.0 if no maximum value)
450 * @param number_of_constraints The maximum number of constraints to associate to the variable
452 Variable* variable_new(resource::Action* id, double sharing_penalty, double bound = -1.0,
453 size_t number_of_constraints = 1);
455 /** @brief Get the list of modified actions since last solve() */
456 resource::Action::ModifiedSet* get_modified_action_set() const;
459 * @brief Free a variable
460 * @param var The variable to free
462 void variable_free(Variable * var);
464 /** @brief Free all variables */
465 void variable_free_all();
468 * @brief Associate a variable to a constraint with a coefficient
469 * @param cnst A constraint
470 * @param var A variable
471 * @param value The coefficient associated to the variable in the constraint
472 * @param force_creation Force the creation of new element linking the variable to the constraint. Should be used only
473 * by the model ptask_L07 to cope with ptasks composed of flows running on the same resource (see
474 * https://framagit.org/simgrid/simgrid/-/issues/111)
476 void expand(Constraint* cnst, Variable* var, double value, bool force_creation = false);
478 /** @brief Update the bound of a variable */
479 void update_variable_bound(Variable * var, double bound);
481 /** @brief Update the sharing penalty of a variable */
482 void update_variable_penalty(Variable* var, double penalty);
484 /** @brief Update a constraint bound */
485 void update_constraint_bound(Constraint * cnst, double bound);
487 int constraint_used(const Constraint* cnst) const { return cnst->active_constraint_set_hook_.is_linked(); }
489 /** @brief Print the lmm system */
492 /** @brief Solve the lmm system. May be specialized in subclasses. */
496 static void* variable_mallocator_new_f();
497 static void variable_mallocator_free_f(void* var);
498 /** @brief Implements the solver. Must be specialized in subclasses. */
499 virtual void do_solve() = 0;
501 void var_free(Variable * var);
502 void cnst_free(Constraint * cnst);
503 Variable* extract_variable()
505 if (variable_set.empty())
507 Variable* res = &variable_set.front();
508 variable_set.pop_front();
511 Constraint* extract_constraint()
513 if (constraint_set.empty())
515 Constraint* res = &constraint_set.front();
516 constraint_set.pop_front();
519 void insert_constraint(Constraint * cnst) { constraint_set.push_back(*cnst); }
520 void remove_variable(Variable * var)
522 if (var->variable_set_hook_.is_linked())
523 xbt::intrusive_erase(variable_set, *var);
524 if (var->saturated_variable_set_hook_.is_linked())
525 xbt::intrusive_erase(saturated_variable_set, *var);
527 void make_constraint_active(Constraint * cnst)
529 if (not cnst->active_constraint_set_hook_.is_linked())
530 active_constraint_set.push_back(*cnst);
532 void make_constraint_inactive(Constraint * cnst)
534 if (cnst->active_constraint_set_hook_.is_linked())
535 xbt::intrusive_erase(active_constraint_set, *cnst);
536 if (cnst->modified_constraint_set_hook_.is_linked())
537 xbt::intrusive_erase(modified_constraint_set, *cnst);
540 void enable_var(Variable * var);
541 void disable_var(Variable * var);
542 void on_disabled_var(Constraint * cnstr);
543 void check_concurrency() const;
546 * @brief Auxiliary method to create a new Element which links a variable to a constraint
548 * @param cnst Constraint (resource)
549 * @param var Variable (action)
550 * @param consumption_weight how much of the resource is used for each unit of the action
551 * @return A reference to the new element
553 Element& expand_create_elem(Constraint* cnst, Variable* var, double consumption_weight);
555 * @brief Increments the element usage
557 * @param elem Element linking variable/action to resource
558 * @param cnst Constraint (resource)
559 * @param consumption_weight how much of the resource is used for each unit of the action
560 * @return elem itself
562 Element& expand_add_to_elem(Element& elem, const Constraint* cnst, double consumption_weight) const;
565 * @brief Update the value of element linking the constraint and the variable
566 * @param cnst A constraint
567 * @param var A variable
568 * @param value The new value
570 void update(Constraint * cnst, Variable * var, double value);
572 /** @brief Given a variable, update modified_constraint_set_ */
573 void update_modified_cnst_set_from_variable(const Variable* var);
574 void update_modified_cnst_set(Constraint* cnst);
575 void update_modified_cnst_set_rec(const Constraint* cnst);
576 /** @brief Remove all constraints of the modified_constraint_set. */
577 void remove_all_modified_cnst_set();
580 bool modified_ = false;
581 boost::intrusive::list<Variable, boost::intrusive::member_hook<Variable, boost::intrusive::list_member_hook<>,
582 &Variable::variable_set_hook_>>
584 boost::intrusive::list<Constraint, boost::intrusive::member_hook<Constraint, boost::intrusive::list_member_hook<>,
585 &Constraint::active_constraint_set_hook_>>
586 active_constraint_set;
587 boost::intrusive::list<Variable, boost::intrusive::member_hook<Variable, boost::intrusive::list_member_hook<>,
588 &Variable::saturated_variable_set_hook_>>
589 saturated_variable_set;
590 boost::intrusive::list<Constraint, boost::intrusive::member_hook<Constraint, boost::intrusive::list_member_hook<>,
591 &Constraint::saturated_constraint_set_hook_>>
592 saturated_constraint_set;
595 const bool selective_update_active; /* flag to update partially the system only selecting changed portions */
596 boost::intrusive::list<Constraint, boost::intrusive::member_hook<Constraint, boost::intrusive::list_member_hook<>,
597 &Constraint::modified_constraint_set_hook_>>
598 modified_constraint_set;
601 unsigned visited_counter_ =
602 1; /* used by System::update_modified_cnst_set() and System::remove_all_modified_cnst_set() to
603 * cleverly (un-)flag the constraints (more details in these functions) */
604 boost::intrusive::list<Constraint, boost::intrusive::member_hook<Constraint, boost::intrusive::list_member_hook<>,
605 &Constraint::constraint_set_hook_>>
607 xbt_mallocator_t variable_mallocator_ =
608 xbt_mallocator_new(65536, System::variable_mallocator_new_f, System::variable_mallocator_free_f, nullptr);
610 std::unique_ptr<resource::Action::ModifiedSet> modified_set_ = nullptr;
614 } // namespace simgrid::kernel::lmm