1 /* Copyright (c) 2004-2020. 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 SURF_MAXMIN_HPP
7 #define SURF_MAXMIN_HPP
9 #include "simgrid/kernel/resource/Action.hpp"
10 #include "simgrid/s4u/Link.hpp"
11 #include "src/surf/surf_interface.hpp"
12 #include "xbt/asserts.h"
13 #include "xbt/mallocator.h"
15 #include <boost/intrusive/list.hpp>
25 /** @addtogroup SURF_lmm
27 * A linear maxmin solver to resolve inequations systems.
29 * Most SimGrid model rely on a "fluid/steady-state" modeling that simulate the sharing of resources between actions at
30 * relatively coarse-grain. Such sharing is generally done by solving a set of linear inequations. Let's take an
31 * 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$x_1\f$
32 * 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
33 * case, we need to enforce:
35 * \f[ x_1 + x_2 \leq C_A \f]
37 * 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
38 * 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:
40 * \f[ x_3 \leq C_{L_1} \f]
41 * \f[ x_3 + x_4 \leq C_{L_2} \f]
42 * \f[ x_4 \leq C_{L_3} \f]
44 * One could set every variable to 0 to make sure the constraints are satisfied but this would obviously not be very
45 * realistic. A possible objective is to try to maximize the minimum of the \f$x_i\f$ . This ensures that all the
46 * \f$x_i\f$ are positive and "as large as possible".
48 * This is called *max-min fairness* and is the most commonly used objective in SimGrid. Another possibility is to
49 * maximize \f$\sum_if(x_i)\f$, where \f$f\f$ is a strictly increasing concave function.
64 * A possible system could be:
65 * - three variables: `var1`, `var2`, `var3`
66 * - two constraints: `cons1`, `cons2`
67 * - four elements linking:
68 * - `elem1` linking `var1` and `cons1`
69 * - `elem2` linking `var2` and `cons1`
70 * - `elem3` linking `var2` and `cons2`
71 * - `elem4` linking `var3` and `cons2`
73 * And the corresponding inequations will be:
75 * var1.value <= var1.bound
76 * var2.value <= var2.bound
77 * var3.value <= var3.bound
78 * var1.weight * var1.value * elem1.value + var2.weight * var2.value * elem2.value <= cons1.bound
79 * var2.weight * var2.value * elem3.value + var3.weight * var3.value * elem4.value <= cons2.bound
81 * where `var1.value`, `var2.value` and `var3.value` are the unknown values.
83 * If a constraint is not shared, the sum is replaced by a max.
84 * For example, a third non-shared constraint `cons3` and the associated elements `elem5` and `elem6` could write as:
86 * max( var1.weight * var1.value * elem5.value , var3.weight * var3.value * elem6.value ) <= cons3.bound
88 * This is useful for the sharing of resources for various models.
89 * For instance, for the network model, each link is associated to a constraint and each communication to a variable.
91 * Implementation details
93 * For implementation reasons, we are interested in distinguishing variables that actually participate to the
94 * computation of constraints, and those who are part of the equations but are stuck to zero.
95 * We call enabled variables, those which var.weight is strictly positive. Zero-weight variables are called disabled
97 * Unfortunately this concept of enabled/disabled variables intersects with active/inactive variable.
98 * Semantically, the intent is similar, but the conditions under which a variable is active is slightly more strict
99 * than the conditions for it to be enabled.
100 * A variable is active only if its var.value is non-zero (and, by construction, its var.weight is non-zero).
101 * In general, variables remain disabled after their creation, which often models an initialization phase (e.g. first
102 * packet propagating in the network). Then, it is enabled by the corresponding model. Afterwards, the max-min solver
103 * (lmm_solve()) activates it when appropriate. It is possible that the variable is again disabled, e.g. to model the
104 * pausing of an action.
106 * Concurrency limit and maximum
108 * We call concurrency, the number of variables that can be enabled at any time for each constraint.
109 * From a model perspective, this "concurrency" often represents the number of actions that actually compete for one
111 * The LMM solver is able to limit the concurrency for each constraint, and to monitor its maximum value.
113 * One may want to limit the concurrency of constraints for essentially three reasons:
114 * - Keep LMM system in a size that can be solved (it does not react very well with tens of thousands of variables per
116 * - Stay within parameters where the fluid model is accurate enough.
117 * - Model serialization effects
119 * The concurrency limit can also be set to a negative value to disable concurrency limit. This can improve performance
122 * Overall, each constraint contains three fields related to concurrency:
123 * - concurrency_limit which is the limit enforced by the solver
124 * - concurrency_current which is the current concurrency
125 * - concurrency_maximum which is the observed maximum concurrency
127 * Variables also have one field related to concurrency: concurrency_share.
128 * In effect, in some cases, one variable is involved multiple times (i.e. two elements) in a constraint.
129 * For example, cross-traffic is modeled using 2 elements per constraint.
130 * concurrency_share formally corresponds to the maximum number of elements that associate the variable and any given
134 /** @{ @ingroup SURF_lmm */
138 * Elements can be seen as glue between constraint objects and variable objects.
139 * Basically, each variable will have a set of elements, one for each constraint where it is involved.
140 * Then, it is used to list all variables involved in constraint through constraint's xxx_element_set lists, or
141 * vice-versa list all constraints for a given variable.
143 class XBT_PUBLIC Element {
145 int get_concurrency() const;
146 void decrease_concurrency();
147 void increase_concurrency();
150 void make_inactive();
152 /* hookup to constraint */
153 boost::intrusive::list_member_hook<> enabled_element_set_hook;
154 boost::intrusive::list_member_hook<> disabled_element_set_hook;
155 boost::intrusive::list_member_hook<> active_element_set_hook;
157 Constraint* constraint;
160 // consumption_weight: impact of 1 byte or flop of your application onto the resource (in byte or flop)
161 // - if CPU, then probably 1.
162 // - If network, then 1 in forward direction and 0.05 backward for the ACKs
163 double consumption_weight;
166 class ConstraintLight {
168 double remaining_over_usage;
173 * @brief LMM constraint
174 * Each constraint contains several partially overlapping logical sets of elements:
175 * \li Disabled elements which variable's weight is zero. This variables are not at all processed by LMM, but eventually
176 * the corresponding action will enable it (at least this is the idea).
177 * \li Enabled elements which variable's weight is non-zero. They are utilized in some LMM functions.
178 * \li Active elements which variable's weight is non-zero (i.e. it is enabled) AND its element value is non-zero.
179 * LMM_solve iterates over active elements during resolution, dynamically making them active or inactive.
181 class XBT_PUBLIC Constraint {
183 Constraint() = delete;
184 Constraint(resource::Resource* id_value, double bound_value);
186 /** @brief Unshare a constraint. */
187 void unshare() { sharing_policy_ = s4u::Link::SharingPolicy::FATPIPE; }
189 /** @brief Check how a constraint is shared */
190 s4u::Link::SharingPolicy get_sharing_policy() const { return sharing_policy_; }
192 /** @brief Get the usage of the constraint after the last lmm solve */
193 double get_usage() const;
194 int get_variable_amount() const;
196 /** @brief Sets the concurrency limit for this constraint */
197 void set_concurrency_limit(int limit)
199 xbt_assert(limit < 0 || concurrency_maximum_ <= limit,
200 "New concurrency limit should be larger than observed concurrency maximum. Maybe you want to call"
201 " concurrency_maximum_reset() to reset the maximum?");
202 concurrency_limit_ = limit;
205 /** @brief Gets the concurrency limit for this constraint */
206 int get_concurrency_limit() const { return concurrency_limit_; }
209 * @brief Reset the concurrency maximum for a given variable (we will update the maximum to reflect constraint
212 void reset_concurrency_maximum() { concurrency_maximum_ = 0; }
214 /** @brief Get the concurrency maximum for a given constraint (which reflects constraint evolution). */
215 int get_concurrency_maximum() const
217 xbt_assert(concurrency_limit_ < 0 || concurrency_maximum_ <= concurrency_limit_,
218 "Very bad: maximum observed concurrency is higher than limit. This is a bug of SURF, please report it.");
219 return concurrency_maximum_;
222 int get_concurrency_slack() const
224 return concurrency_limit_ < 0 ? std::numeric_limits<int>::max() : concurrency_limit_ - concurrency_current_;
228 * @brief Get a var associated to a constraint
229 * @details Get the first variable of the next variable of elem if elem is not NULL
230 * @param elem A element of constraint of the constraint or NULL
231 * @return A variable associated to a constraint
233 Variable* get_variable(const Element** elem) const;
236 * @brief Get a var associated to a constraint
237 * @details Get the first variable of the next variable of elem if elem is not NULL
238 * @param elem A element of constraint of the constraint or NULL
239 * @param nextelem A element of constraint of the constraint or NULL, the one after elem
240 * @param numelem parameter representing the number of elements to go
241 * @return A variable associated to a constraint
243 Variable* get_variable_safe(const Element** elem, const Element** nextelem, int* numelem) const;
246 * @brief Get the data associated to a constraint
247 * @return The data associated to the constraint
249 resource::Resource* get_id() const { return id_; }
251 /* hookup to system */
252 boost::intrusive::list_member_hook<> constraint_set_hook_;
253 boost::intrusive::list_member_hook<> active_constraint_set_hook_;
254 boost::intrusive::list_member_hook<> modified_constraint_set_hook_;
255 boost::intrusive::list_member_hook<> saturated_constraint_set_hook_;
256 boost::intrusive::list<Element, boost::intrusive::member_hook<Element, boost::intrusive::list_member_hook<>,
257 &Element::enabled_element_set_hook>>
258 enabled_element_set_;
259 boost::intrusive::list<Element, boost::intrusive::member_hook<Element, boost::intrusive::list_member_hook<>,
260 &Element::disabled_element_set_hook>>
261 disabled_element_set_;
262 boost::intrusive::list<Element, boost::intrusive::member_hook<Element, boost::intrusive::list_member_hook<>,
263 &Element::active_element_set_hook>>
265 double remaining_ = 0.0;
268 // TODO MARTIN Check maximum value across resources at the end of simulation and give a warning is more than e.g. 500
269 int concurrency_current_ = 0; /* The current concurrency */
270 int concurrency_maximum_ = 0; /* The maximum number of (enabled and disabled) variables associated to the constraint
271 * at any given time (essentially for tracing)*/
273 s4u::Link::SharingPolicy sharing_policy_ = s4u::Link::SharingPolicy::SHARED;
274 int rank_; // Only used in debug messages to identify the constraint
275 double lambda_ = 0.0;
276 double new_lambda_ = 0.0;
277 ConstraintLight* cnst_light_ = nullptr;
280 static int next_rank_; // To give a separate rank_ to each constraint
281 int concurrency_limit_ = sg_concurrency_limit; /* The maximum number of variables that may be enabled at any time
282 * (stage variables if necessary) */
283 resource::Resource* id_;
287 * @brief LMM variable
289 * When something prevents us from enabling a variable, we "stage" the weight that we would have like to set, so that as
290 * soon as possible we enable the variable with desired weight
292 class XBT_PUBLIC Variable {
294 void initialize(resource::Action* id_value, double sharing_penalty, double bound_value, int number_of_constraints,
295 unsigned visited_value);
297 /** @brief Get the value of the variable after the last lmm solve */
298 double get_value() const { return value_; }
300 /** @brief Get the maximum value of the variable (-1.0 if no specified maximum) */
301 double get_bound() const { return bound_; }
304 * @brief Set the concurrent share of the variable
305 * @param value The new concurrency share
307 void set_concurrency_share(short int value) { concurrency_share_ = value; }
310 * @brief Get the numth constraint associated to the variable
311 * @param num The rank of constraint we want to get
312 * @return The numth constraint
314 Constraint* get_constraint(unsigned num) const { return num < cnsts_.size() ? cnsts_[num].constraint : nullptr; }
317 * @brief Get the weight of the numth constraint associated to the variable
318 * @param num The rank of constraint we want to get
319 * @return The numth constraint
321 double get_constraint_weight(unsigned num) const
323 return num < cnsts_.size() ? cnsts_[num].consumption_weight : 0.0;
326 /** @brief Get the number of constraint associated to a variable */
327 size_t get_number_of_constraint() const { return cnsts_.size(); }
329 /** @brief Get the data associated to a variable */
330 resource::Action* get_id() const { return id_; }
332 /** @brief Get the penalty of a variable */
333 double get_penalty() const { return sharing_penalty_; }
335 /** @brief Measure the minimum concurrency slack across all constraints where the given var is involved */
336 int get_min_concurrency_slack() const;
338 /** @brief Check if a variable can be enabled
339 * Make sure to set staged_penalty before, if your intent is only to check concurrency
341 bool can_enable() const { return staged_penalty_ > 0 && get_min_concurrency_slack() >= concurrency_share_; }
343 /* hookup to system */
344 boost::intrusive::list_member_hook<> variable_set_hook_;
345 boost::intrusive::list_member_hook<> saturated_variable_set_hook_;
347 std::vector<Element> cnsts_;
349 // sharing_penalty: variable's impact on the resource during the sharing
350 // if == 0, the variable is not considered by LMM
351 // on CPU, actions with N threads have a sharing of N
352 // on network, the actions with higher latency have a lesser sharing_penalty
353 double sharing_penalty_;
355 double staged_penalty_; /* If non-zero, variable is staged for addition as soon as maxconcurrency constraints will be
359 short int concurrency_share_; /* The maximum number of elements that variable will add to a constraint */
360 resource::Action* id_;
361 int rank_; // Only used in debug messages to identify the variable
362 unsigned visited_; /* used by System::update_modified_set() */
366 static int next_rank_; // To give a separate rank_ to each variable
369 inline void Element::make_active()
371 constraint->active_element_set_.push_front(*this);
373 inline void Element::make_inactive()
375 if (active_element_set_hook.is_linked())
376 xbt::intrusive_erase(constraint->active_element_set_, *this);
382 class XBT_PUBLIC System {
385 * @brief Create a new Linear MaxMim system
386 * @param selective_update whether we should do lazy updates
388 explicit System(bool selective_update);
389 /** @brief Free an existing Linear MaxMin system */
393 * @brief Create a new Linear MaxMin constraint
394 * @param id Data associated to the constraint (e.g.: a network link)
395 * @param bound_value The bound value of the constraint
397 Constraint* constraint_new(resource::Resource* id, double bound_value);
400 * @brief Create a new Linear MaxMin variable
401 * @param id Data associated to the variable (e.g.: a network communication)
402 * @param sharing_penalty The weight of the variable (0.0 if not used)
403 * @param bound The maximum value of the variable (-1.0 if no maximum value)
404 * @param number_of_constraints The maximum number of constraints to associate to the variable
406 Variable* variable_new(resource::Action* id, double sharing_penalty, double bound = -1.0,
407 size_t number_of_constraints = 1);
410 * @brief Free a variable
411 * @param var The variable to free
413 void variable_free(Variable * var);
415 /** @brief Free all variables */
416 void variable_free_all();
419 * @brief Associate a variable to a constraint with a coefficient
420 * @param cnst A constraint
421 * @param var A variable
422 * @param value The coefficient associated to the variable in the constraint
424 void expand(Constraint * cnst, Variable * var, double value);
427 * @brief Add value to the coefficient between a constraint and a variable or create one
428 * @param cnst A constraint
429 * @param var A variable
430 * @param value The value to add to the coefficient associated to the variable in the constraint
432 void expand_add(Constraint * cnst, Variable * var, double value);
434 /** @brief Update the bound of a variable */
435 void update_variable_bound(Variable * var, double bound);
437 /** @brief Update the sharing penalty of a variable */
438 void update_variable_penalty(Variable* var, double penalty);
440 /** @brief Update a constraint bound */
441 void update_constraint_bound(Constraint * cnst, double bound);
443 int constraint_used(const Constraint* cnst) const { return cnst->active_constraint_set_hook_.is_linked(); }
445 /** @brief Print the lmm system */
448 /** @brief Solve the lmm system */
451 /** @brief Solve the lmm system. May be specialized in subclasses. */
452 virtual void solve() { lmm_solve(); }
455 static void* variable_mallocator_new_f();
456 static void variable_mallocator_free_f(void* var);
458 void var_free(Variable * var);
459 void cnst_free(Constraint * cnst);
460 Variable* extract_variable()
462 if (variable_set.empty())
464 Variable* res = &variable_set.front();
465 variable_set.pop_front();
468 Constraint* extract_constraint()
470 if (constraint_set.empty())
472 Constraint* res = &constraint_set.front();
473 constraint_set.pop_front();
476 void insert_constraint(Constraint * cnst) { constraint_set.push_back(*cnst); }
477 void remove_variable(Variable * var)
479 if (var->variable_set_hook_.is_linked())
480 xbt::intrusive_erase(variable_set, *var);
481 if (var->saturated_variable_set_hook_.is_linked())
482 xbt::intrusive_erase(saturated_variable_set, *var);
484 void make_constraint_active(Constraint * cnst)
486 if (not cnst->active_constraint_set_hook_.is_linked())
487 active_constraint_set.push_back(*cnst);
489 void make_constraint_inactive(Constraint * cnst)
491 if (cnst->active_constraint_set_hook_.is_linked())
492 xbt::intrusive_erase(active_constraint_set, *cnst);
493 if (cnst->modified_constraint_set_hook_.is_linked())
494 xbt::intrusive_erase(modified_constraint_set, *cnst);
497 void enable_var(Variable * var);
498 void disable_var(Variable * var);
499 void on_disabled_var(Constraint * cnstr);
502 * @brief Update the value of element linking the constraint and the variable
503 * @param cnst A constraint
504 * @param var A variable
505 * @param value The new value
507 void update(Constraint * cnst, Variable * var, double value);
509 void update_modified_set(Constraint* cnst);
510 void update_modified_set_rec(const Constraint* cnst);
512 /** @brief Remove all constraints of the modified_constraint_set. */
513 void remove_all_modified_set();
514 void check_concurrency() const;
516 template <class CnstList> void lmm_solve(CnstList& cnst_list);
519 bool modified_ = false;
520 boost::intrusive::list<Variable, boost::intrusive::member_hook<Variable, boost::intrusive::list_member_hook<>,
521 &Variable::variable_set_hook_>>
523 boost::intrusive::list<Constraint, boost::intrusive::member_hook<Constraint, boost::intrusive::list_member_hook<>,
524 &Constraint::active_constraint_set_hook_>>
525 active_constraint_set;
526 boost::intrusive::list<Variable, boost::intrusive::member_hook<Variable, boost::intrusive::list_member_hook<>,
527 &Variable::saturated_variable_set_hook_>>
528 saturated_variable_set;
529 boost::intrusive::list<Constraint, boost::intrusive::member_hook<Constraint, boost::intrusive::list_member_hook<>,
530 &Constraint::saturated_constraint_set_hook_>>
531 saturated_constraint_set;
533 std::unique_ptr<resource::Action::ModifiedSet> modified_set_ = nullptr;
536 using dyn_light_t = std::vector<int>;
538 //Data used in lmm::solve
539 std::vector<ConstraintLight> cnst_light_vec;
540 dyn_light_t saturated_constraints;
542 bool selective_update_active; /* flag to update partially the system only selecting changed portions */
543 unsigned visited_counter_ = 1; /* used by System::update_modified_set() and System::remove_all_modified_set() to
544 * cleverly (un-)flag the constraints (more details in these functions) */
545 boost::intrusive::list<Constraint, boost::intrusive::member_hook<Constraint, boost::intrusive::list_member_hook<>,
546 &Constraint::constraint_set_hook_>>
548 boost::intrusive::list<Constraint, boost::intrusive::member_hook<Constraint, boost::intrusive::list_member_hook<>,
549 &Constraint::modified_constraint_set_hook_>>
550 modified_constraint_set;
551 xbt_mallocator_t variable_mallocator_ =
552 xbt_mallocator_new(65536, System::variable_mallocator_new_f, System::variable_mallocator_free_f, nullptr);
555 class XBT_PUBLIC FairBottleneck : public System {
557 using System::System;
558 void solve() final { bottleneck_solve(); }
561 void bottleneck_solve();
566 } // namespace kernel
567 } // namespace simgrid