src/mc/checker/CommunicationDeterminismChecker.cpp

   1 /* Copyright (c) 2008-2020. The SimGrid Team. All rights reserved.          */
   2
   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. */
   5
   6 #include "src/mc/checker/CommunicationDeterminismChecker.hpp"
   7 #include "src/kernel/activity/MailboxImpl.hpp"
   8 #include "src/mc/Session.hpp"
   9 #include "src/mc/mc_api.hpp"
  10 #include "src/mc/mc_config.hpp"
  11 #include "src/mc/mc_exit.hpp"
  12 #include "src/mc/mc_private.hpp"
  13 #include "src/mc/mc_request.hpp"
  14 #include "src/mc/mc_smx.hpp"
  15
  16 #if HAVE_SMPI
  17 #include "smpi_request.hpp"
  18 #endif
  19
  20 #include <cstdint>
  21
  22 using mcapi = simgrid::mc::mc_api;
  23
  24 XBT_LOG_NEW_DEFAULT_SUBCATEGORY(mc_comm_determinism, mc, "Logging specific to MC communication determinism detection");
  25
  26 /********** Global variables **********/
  27
  28 std::vector<simgrid::mc::PatternCommunicationList> initial_communications_pattern;
  29 std::vector<std::vector<simgrid::mc::PatternCommunication*>> incomplete_communications_pattern;
  30
  31 /********** Static functions ***********/
  32
  33 static simgrid::mc::CommPatternDifference compare_comm_pattern(const simgrid::mc::PatternCommunication* comm1,
  34                                                                const simgrid::mc::PatternCommunication* comm2)
  35 {
  36   using simgrid::mc::CommPatternDifference;
  37   if (comm1->type != comm2->type)
  38     return CommPatternDifference::TYPE;
  39   if (comm1->rdv != comm2->rdv)
  40     return CommPatternDifference::RDV;
  41   if (comm1->src_proc != comm2->src_proc)
  42     return CommPatternDifference::SRC_PROC;
  43   if (comm1->dst_proc != comm2->dst_proc)
  44     return CommPatternDifference::DST_PROC;
  45   if (comm1->tag != comm2->tag)
  46     return CommPatternDifference::TAG;
  47   if (comm1->data.size() != comm2->data.size())
  48     return CommPatternDifference::DATA_SIZE;
  49   if (comm1->data != comm2->data)
  50     return CommPatternDifference::DATA;
  51   return CommPatternDifference::NONE;
  52 }
  53
  54 static void patterns_copy(std::vector<simgrid::mc::PatternCommunication*>& dest,
  55                              std::vector<simgrid::mc::PatternCommunication> const& source)
  56 {
  57   dest.clear();
  58   for (simgrid::mc::PatternCommunication const& comm : source) {
  59     auto* copy_comm = new simgrid::mc::PatternCommunication(comm.dup());
  60     dest.push_back(copy_comm);
  61   }
  62 }
  63
  64 static void restore_communications_pattern(simgrid::mc::State* state)
  65 {
  66   for (size_t i = 0; i < initial_communications_pattern.size(); i++)
  67     initial_communications_pattern[i].index_comm = state->communication_indices_[i];
  68
  69   for (unsigned long i = 0; i < mcapi::get().get_maxpid(); i++)
  70     patterns_copy(incomplete_communications_pattern[i], state->incomplete_comm_pattern_[i]);
  71 }
  72
  73 static char* print_determinism_result(simgrid::mc::CommPatternDifference diff, int process,
  74                                       const simgrid::mc::PatternCommunication* comm, unsigned int cursor)
  75 {
  76   char* type;
  77   char* res;
  78
  79   if (comm->type == simgrid::mc::PatternCommunicationType::send)
  80     type = bprintf("The send communications pattern of the process %d is different!", process - 1);
  81   else
  82     type = bprintf("The recv communications pattern of the process %d is different!", process - 1);
  83
  84   using simgrid::mc::CommPatternDifference;
  85   switch (diff) {
  86     case CommPatternDifference::TYPE:
  87       res = bprintf("%s Different type for communication #%u", type, cursor);
  88       break;
  89     case CommPatternDifference::RDV:
  90       res = bprintf("%s Different rdv for communication #%u", type, cursor);
  91       break;
  92     case CommPatternDifference::TAG:
  93       res = bprintf("%s Different tag for communication #%u", type, cursor);
  94       break;
  95     case CommPatternDifference::SRC_PROC:
  96       res = bprintf("%s Different source for communication #%u", type, cursor);
  97       break;
  98     case CommPatternDifference::DST_PROC:
  99       res = bprintf("%s Different destination for communication #%u", type, cursor);
 100       break;
 101     case CommPatternDifference::DATA_SIZE:
 102       res = bprintf("%s Different data size for communication #%u", type, cursor);
 103       break;
 104     case CommPatternDifference::DATA:
 105       res = bprintf("%s Different data for communication #%u", type, cursor);
 106       break;
 107     default:
 108       res = nullptr;
 109       break;
 110   }
 111
 112   return res;
 113 }
 114
 115 static void update_comm_pattern(simgrid::mc::PatternCommunication* comm_pattern,
 116                                 const simgrid::kernel::activity::CommImpl* comm_addr)
 117 {
 118   auto src_proc = mcapi::get().get_src_actor(comm_addr);
 119   auto dst_proc = mcapi::get().get_dst_actor(comm_addr);
 120   comm_pattern->src_proc = src_proc->get_pid();
 121   comm_pattern->dst_proc = dst_proc->get_pid();
 122   comm_pattern->src_host = mcapi::get().get_actor_host_name(src_proc);
 123   comm_pattern->dst_host = mcapi::get().get_actor_host_name(dst_proc);
 124
 125   if (comm_pattern->data.empty())
 126     comm_pattern->data = mcapi::get().get_pattern_comm_data(comm_addr);
 127 }
 128
 129 namespace simgrid {
 130 namespace mc {
 131
 132 void CommunicationDeterminismChecker::deterministic_comm_pattern(int process, const PatternCommunication* comm,
 133                                                                  int backtracking)
 134 {
 135   if (not backtracking) {
 136     PatternCommunicationList& list = initial_communications_pattern[process];
 137     CommPatternDifference diff     = compare_comm_pattern(list.list[list.index_comm].get(), comm);
 138
 139     if (diff != CommPatternDifference::NONE) {
 140       if (comm->type == PatternCommunicationType::send) {
 141         this->send_deterministic = false;
 142         if (this->send_diff != nullptr)
 143           xbt_free(this->send_diff);
 144         this->send_diff = print_determinism_result(diff, process, comm, list.index_comm + 1);
 145       } else {
 146         this->recv_deterministic = false;
 147         if (this->recv_diff != nullptr)
 148           xbt_free(this->recv_diff);
 149         this->recv_diff = print_determinism_result(diff, process, comm, list.index_comm + 1);
 150       }
 151       if (_sg_mc_send_determinism && not this->send_deterministic) {
 152         XBT_INFO("*********************************************************");
 153         XBT_INFO("***** Non-send-deterministic communications pattern *****");
 154         XBT_INFO("*********************************************************");
 155         XBT_INFO("%s", this->send_diff);
 156         xbt_free(this->send_diff);
 157         this->send_diff = nullptr;
 158         mcapi::get().log_state();
 159         mcapi::get().mc_exit(SIMGRID_MC_EXIT_NON_DETERMINISM);
 160       } else if (_sg_mc_comms_determinism && (not this->send_deterministic && not this->recv_deterministic)) {
 161         XBT_INFO("****************************************************");
 162         XBT_INFO("***** Non-deterministic communications pattern *****");
 163         XBT_INFO("****************************************************");
 164         if (this->send_diff) {
 165           XBT_INFO("%s", this->send_diff);
 166           xbt_free(this->send_diff);
 167           this->send_diff = nullptr;
 168         }
 169         if (this->recv_diff) {
 170           XBT_INFO("%s", this->recv_diff);
 171           xbt_free(this->recv_diff);
 172           this->recv_diff = nullptr;
 173         }
 174         mcapi::get().log_state();
 175         mcapi::get().mc_exit(SIMGRID_MC_EXIT_NON_DETERMINISM);
 176       }
 177     }
 178   }
 179 }
 180
 181 /********** Non Static functions ***********/
 182
 183 void CommunicationDeterminismChecker::get_comm_pattern(smx_simcall_t request, CallType call_type, int backtracking)
 184 {
 185   const smx_actor_t issuer                                     = mcapi::get().simcall_get_issuer(request);
 186   const mc::PatternCommunicationList& initial_pattern          = initial_communications_pattern[issuer->get_pid()];
 187   const std::vector<PatternCommunication*>& incomplete_pattern = incomplete_communications_pattern[issuer->get_pid()];
 188
 189   auto pattern   = std::make_unique<PatternCommunication>();
 190   pattern->index = initial_pattern.index_comm + incomplete_pattern.size();
 191
 192   if (call_type == CallType::SEND) {
 193     /* Create comm pattern */
 194     pattern->type      = PatternCommunicationType::send;
 195     pattern->comm_addr = mcapi::get().get_comm_isend_raw_addr(request);
 196     pattern->rdv      = mcapi::get().get_pattern_comm_rdv(pattern->comm_addr);
 197     pattern->src_proc = mcapi::get().get_pattern_comm_src_proc(pattern->comm_addr);
 198     pattern->src_host = mc_api::get().get_actor_host_name(issuer);
 199
 200 #if HAVE_SMPI
 201     pattern->tag = mcapi::get().get_smpi_request_tag(request, simgrid::simix::Simcall::COMM_ISEND);
 202 #endif
 203     pattern->data = mcapi::get().get_pattern_comm_data(pattern->comm_addr);
 204
 205 #if HAVE_SMPI
 206     auto send_detached = mcapi::get().check_send_request_detached(request);
 207     if (send_detached) {
 208       if (this->initial_communications_pattern_done) {
 209         /* Evaluate comm determinism */
 210         this->deterministic_comm_pattern(pattern->src_proc, pattern.get(), backtracking);
 211         initial_communications_pattern[pattern->src_proc].index_comm++;
 212       } else {
 213         /* Store comm pattern */
 214         initial_communications_pattern[pattern->src_proc].list.push_back(std::move(pattern));
 215       }
 216       return;
 217     }
 218 #endif
 219   } else if (call_type == CallType::RECV) {
 220     pattern->type = PatternCommunicationType::receive;
 221     pattern->comm_addr = mcapi::get().get_comm_isend_raw_addr(request);
 222
 223 #if HAVE_SMPI
 224     pattern->tag = mcapi::get().get_smpi_request_tag(request, simgrid::simix::Simcall::COMM_IRECV);
 225 #endif
 226     auto comm_addr = pattern->comm_addr;
 227     pattern->rdv = mcapi::get().get_pattern_comm_rdv(comm_addr);
 228     pattern->dst_proc = mcapi::get().get_pattern_comm_dst_proc(comm_addr);
 229     pattern->dst_host = mcapi::get().get_actor_host_name(issuer);
 230   } else
 231     xbt_die("Unexpected call_type %i", (int)call_type);
 232
 233   XBT_DEBUG("Insert incomplete comm pattern %p for process %ld", pattern.get(), issuer->get_pid());
 234   incomplete_communications_pattern[issuer->get_pid()].push_back(pattern.release());
 235 }
 236
 237 void CommunicationDeterminismChecker::complete_comm_pattern(const kernel::activity::CommImpl* comm_addr,
 238                                                             unsigned int issuer, int backtracking)
 239 {
 240   /* Complete comm pattern */
 241   std::vector<PatternCommunication*>& incomplete_pattern = incomplete_communications_pattern[issuer];
 242   auto current_comm_pattern =
 243       std::find_if(begin(incomplete_pattern), end(incomplete_pattern),
 244                    [&comm_addr](const PatternCommunication* comm) { return mcapi::get().comm_addr_equal(comm->comm_addr, comm_addr); });
 245   if (current_comm_pattern == std::end(incomplete_pattern))
 246     xbt_die("Corresponding communication not found!");
 247
 248   update_comm_pattern(*current_comm_pattern, comm_addr);
 249   std::unique_ptr<PatternCommunication> comm_pattern(*current_comm_pattern);
 250   XBT_DEBUG("Remove incomplete comm pattern for process %u at cursor %zd", issuer,
 251             std::distance(begin(incomplete_pattern), current_comm_pattern));
 252   incomplete_pattern.erase(current_comm_pattern);
 253
 254   if (this->initial_communications_pattern_done) {
 255     /* Evaluate comm determinism */
 256     this->deterministic_comm_pattern(issuer, comm_pattern.get(), backtracking);
 257     initial_communications_pattern[issuer].index_comm++;
 258   } else {
 259     /* Store comm pattern */
 260     initial_communications_pattern[issuer].list.push_back(std::move(comm_pattern));
 261   }
 262 }
 263
 264 CommunicationDeterminismChecker::CommunicationDeterminismChecker(Session& s) : Checker(s) {}
 265
 266 CommunicationDeterminismChecker::~CommunicationDeterminismChecker() = default;
 267
 268 RecordTrace CommunicationDeterminismChecker::get_record_trace() // override
 269 {
 270   RecordTrace res;
 271   for (auto const& state : stack_)
 272     res.push_back(state->get_transition());
 273   return res;
 274 }
 275
 276 std::vector<std::string> CommunicationDeterminismChecker::get_textual_trace() // override
 277 {
 278   std::vector<std::string> trace;
 279   for (auto const& state : stack_) {
 280     smx_simcall_t req = &state->executed_req_;
 281     trace.push_back(mcapi::get().request_to_string(req, state->transition_.argument_, RequestType::executed));
 282   }
 283   return trace;
 284 }
 285
 286 void CommunicationDeterminismChecker::log_state() // override
 287 {
 288   if (_sg_mc_comms_determinism) {
 289     if (this->send_deterministic && not this->recv_deterministic) {
 290       XBT_INFO("*******************************************************");
 291       XBT_INFO("**** Only-send-deterministic communication pattern ****");
 292       XBT_INFO("*******************************************************");
 293       XBT_INFO("%s", this->recv_diff);
 294     }
 295     if (not this->send_deterministic && this->recv_deterministic) {
 296       XBT_INFO("*******************************************************");
 297       XBT_INFO("**** Only-recv-deterministic communication pattern ****");
 298       XBT_INFO("*******************************************************");
 299       XBT_INFO("%s", this->send_diff);
 300     }
 301   }
 302   XBT_INFO("Expanded states = %lu", expanded_states_count_);
 303   XBT_INFO("Visited states = %lu", mcapi::get().mc_get_visited_states());
 304   XBT_INFO("Executed transitions = %lu", mcapi::get().mc_get_executed_trans());
 305   XBT_INFO("Send-deterministic : %s", this->send_deterministic ? "Yes" : "No");
 306   if (_sg_mc_comms_determinism)
 307     XBT_INFO("Recv-deterministic : %s", this->recv_deterministic ? "Yes" : "No");
 308 }
 309
 310 void CommunicationDeterminismChecker::prepare()
 311 {
 312   const auto maxpid = mcapi::get().get_maxpid();
 313
 314   initial_communications_pattern.resize(maxpid);
 315   incomplete_communications_pattern.resize(maxpid);
 316
 317   ++expanded_states_count_;
 318   auto initial_state = std::make_unique<State>(expanded_states_count_);
 319
 320   XBT_DEBUG("********* Start communication determinism verification *********");
 321
 322   /* Get an enabled actor and insert it in the interleave set of the initial state */
 323   auto actors = mcapi::get().get_actors();
 324   for (auto& actor : actors)
 325     if (mcapi::get().actor_is_enabled(actor.copy.get_buffer()->get_pid()))
 326       initial_state->add_interleaving_set(actor.copy.get_buffer());
 327
 328   stack_.push_back(std::move(initial_state));
 329 }
 330
 331 static inline bool all_communications_are_finished()
 332 {
 333   auto maxpid = mcapi::get().get_maxpid();
 334   for (size_t current_actor = 1; current_actor < maxpid; current_actor++) {
 335     if (not incomplete_communications_pattern[current_actor].empty()) {
 336       XBT_DEBUG("Some communications are not finished, cannot stop the exploration! State not visited.");
 337       return false;
 338     }
 339   }
 340   return true;
 341 }
 342
 343 void CommunicationDeterminismChecker::restoreState()
 344 {
 345   /* Intermediate backtracking */
 346   State* last_state = stack_.back().get();
 347   if (last_state->system_state_) {
 348     mc_api::get().restore_state(last_state->system_state_);
 349     restore_communications_pattern(last_state);
 350     return;
 351   }
 352
 353   /* Restore the initial state */
 354   mcapi::get().restore_initial_state();
 355
 356   unsigned long n = mcapi::get().get_maxpid();
 357   assert(n == incomplete_communications_pattern.size());
 358   assert(n == initial_communications_pattern.size());
 359   for (unsigned long j = 0; j < n; j++) {
 360     incomplete_communications_pattern[j].clear();
 361     initial_communications_pattern[j].index_comm = 0;
 362   }
 363
 364   /* Traverse the stack from the state at position start and re-execute the transitions */
 365   for (std::unique_ptr<simgrid::mc::State> const& state : stack_) {
 366     if (state == stack_.back())
 367       break;
 368
 369     int req_num                    = state->transition_.argument_;
 370     const s_smx_simcall* saved_req = &state->executed_req_;
 371     xbt_assert(saved_req);
 372
 373     /* because we got a copy of the executed request, we have to fetch the
 374        real one, pointed by the request field of the issuer process */
 375
 376     const smx_actor_t issuer = mcapi::get().simcall_get_issuer(saved_req);
 377     smx_simcall_t req        = &issuer->simcall_;
 378
 379     /* TODO : handle test and testany simcalls */
 380     CallType call = MC_get_call_type(req);
 381     mcapi::get().handle_simcall(state->transition_);
 382     handle_comm_pattern(call, req, req_num, 1);
 383     mcapi::get().mc_wait_for_requests();
 384
 385     /* Update statistics */
 386     mcapi::get().mc_inc_visited_states();
 387     mcapi::get().mc_inc_executed_trans();
 388   }
 389 }
 390
 391 void CommunicationDeterminismChecker::handle_comm_pattern(simgrid::mc::CallType call_type, smx_simcall_t req, int value, int backtracking)
 392 {
 393   using simgrid::mc::CallType;
 394   switch(call_type) {
 395     case CallType::NONE:
 396       break;
 397     case CallType::SEND:
 398     case CallType::RECV:
 399       get_comm_pattern(req, call_type, backtracking);
 400       break;
 401     case CallType::WAIT:
 402     case CallType::WAITANY: {
 403       simgrid::kernel::activity::CommImpl* comm_addr = nullptr;
 404       if (call_type == CallType::WAIT)
 405         comm_addr = mcapi::get().get_comm_wait_raw_addr(req);
 406       else
 407         comm_addr = mcapi::get().get_comm_waitany_raw_addr(req, value);
 408       auto simcall_issuer = mcapi::get().simcall_get_issuer(req);
 409       complete_comm_pattern(comm_addr, simcall_issuer->get_pid(), backtracking);
 410     } break;
 411   default:
 412     xbt_die("Unexpected call type %i", (int)call_type);
 413   }
 414 }
 415
 416 void CommunicationDeterminismChecker::real_run()
 417 {
 418   std::unique_ptr<VisitedState> visited_state = nullptr;
 419   smx_simcall_t req                           = nullptr;
 420
 421   while (not stack_.empty()) {
 422     /* Get current state */
 423     State* cur_state = stack_.back().get();
 424
 425     XBT_DEBUG("**************************************************");
 426     XBT_DEBUG("Exploration depth = %zu (state = %d, interleaved processes = %zu)", stack_.size(), cur_state->num_,
 427               cur_state->interleave_size());
 428
 429     /* Update statistics */
 430     mcapi::get().mc_inc_visited_states();
 431
 432     if (stack_.size() <= (std::size_t)_sg_mc_max_depth)
 433       req = mcapi::get().mc_state_choose_request(cur_state);
 434     else
 435       req = nullptr;
 436
 437     if (req != nullptr && visited_state == nullptr) {
 438       int req_num = cur_state->transition_.argument_;
 439
 440       XBT_DEBUG("Execute: %s", mcapi::get().request_to_string(req, req_num, RequestType::simix).c_str());
 441
 442       std::string req_str;
 443       if (dot_output != nullptr)
 444         req_str = mcapi::get().request_get_dot_output(req, req_num);
 445
 446       mcapi::get().mc_inc_executed_trans();
 447
 448       /* TODO : handle test and testany simcalls */
 449       CallType call = CallType::NONE;
 450       if (_sg_mc_comms_determinism || _sg_mc_send_determinism)
 451         call = MC_get_call_type(req);
 452
 453       /* Answer the request */
 454       mcapi::get().handle_simcall(cur_state->transition_);
 455       /* After this call req is no longer useful */
 456
 457       handle_comm_pattern(call, req, req_num, 0);
 458
 459       /* Wait for requests (schedules processes) */
 460       mcapi::get().mc_wait_for_requests();
 461
 462       /* Create the new expanded state */
 463       ++expanded_states_count_;
 464       auto next_state = std::make_unique<State>(expanded_states_count_);
 465
 466       /* If comm determinism verification, we cannot stop the exploration if some communications are not finished (at
 467        * least, data are transferred). These communications  are incomplete and they cannot be analyzed and compared
 468        * with the initial pattern. */
 469       bool compare_snapshots = this->initial_communications_pattern_done && all_communications_are_finished();
 470
 471       if (_sg_mc_max_visited_states != 0)
 472         visited_state = visited_states_.addVisitedState(expanded_states_count_, next_state.get(), compare_snapshots);
 473       else
 474         visited_state = nullptr;
 475
 476       if (visited_state == nullptr) {
 477         /* Get enabled actors and insert them in the interleave set of the next state */
 478         auto actors = mcapi::get().get_actors();
 479         for (auto& actor : actors)
 480           if (mcapi::get().actor_is_enabled(actor.copy.get_buffer()->get_pid()))
 481             next_state->add_interleaving_set(actor.copy.get_buffer());
 482
 483         if (dot_output != nullptr)
 484           fprintf(dot_output, "\"%d\" -> \"%d\" [%s];\n", cur_state->num_, next_state->num_, req_str.c_str());
 485
 486       } else if (dot_output != nullptr)
 487         fprintf(dot_output, "\"%d\" -> \"%d\" [%s];\n", cur_state->num_,
 488                 visited_state->original_num == -1 ? visited_state->num : visited_state->original_num, req_str.c_str());
 489
 490       stack_.push_back(std::move(next_state));
 491     } else {
 492       if (stack_.size() > (std::size_t)_sg_mc_max_depth)
 493         XBT_WARN("/!\\ Max depth reached! /!\\ ");
 494       else if (visited_state != nullptr)
 495         XBT_DEBUG("State already visited (equal to state %d), exploration stopped on this path.",
 496                   visited_state->original_num == -1 ? visited_state->num : visited_state->original_num);
 497       else
 498         XBT_DEBUG("There are no more processes to interleave. (depth %zu)", stack_.size());
 499
 500       this->initial_communications_pattern_done = true;
 501
 502       /* Trash the current state, no longer needed */
 503       XBT_DEBUG("Delete state %d at depth %zu", cur_state->num_, stack_.size());
 504       stack_.pop_back();
 505
 506       visited_state = nullptr;
 507
 508       /* Check for deadlocks */
 509       if (mcapi::get().mc_check_deadlock()) {
 510         mcapi::get().mc_show_deadlock();
 511         throw simgrid::mc::DeadlockError();
 512       }
 513
 514       while (not stack_.empty()) {
 515         std::unique_ptr<State> state(std::move(stack_.back()));
 516         stack_.pop_back();
 517         if (state->interleave_size() && stack_.size() < (std::size_t)_sg_mc_max_depth) {
 518           /* We found a back-tracking point, let's loop */
 519           XBT_DEBUG("Back-tracking to state %d at depth %zu", state->num_, stack_.size() + 1);
 520           stack_.push_back(std::move(state));
 521
 522           this->restoreState();
 523
 524           XBT_DEBUG("Back-tracking to state %d at depth %zu done", stack_.back()->num_, stack_.size());
 525
 526           break;
 527         } else {
 528           XBT_DEBUG("Delete state %d at depth %zu", state->num_, stack_.size() + 1);
 529         }
 530       }
 531     }
 532   }
 533
 534   mcapi::get().log_state();
 535 }
 536
 537 void CommunicationDeterminismChecker::run()
 538 {
 539   XBT_INFO("Check communication determinism");
 540   mcapi::get().session_initialize();
 541
 542   this->prepare();
 543   this->real_run();
 544 }
 545
 546 Checker* createCommunicationDeterminismChecker(Session& s)
 547 {
 548   return new CommunicationDeterminismChecker(s);
 549 }
 550
 551 } // namespace mc
 552 } // namespace simgrid