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[CP-SAT] more work on hints; speed up memory allocation in LNS
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@lperron
lperron committed Dec 17, 2024
1 parent 05fccb4 commit 5394b6c
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Showing 6 changed files with 75 additions and 80 deletions.
1 change: 1 addition & 0 deletions ortools/sat/CMakeLists.txt
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Original file line number Diff line number Diff line change
Expand Up @@ -53,6 +53,7 @@ if(BUILD_TESTING)
${_FILE_NAME}
LINK_LIBRARIES
benchmark::benchmark
fuzztest::fuzztest
GTest::gmock
GTest::gtest_main
)
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8 changes: 4 additions & 4 deletions ortools/sat/cp_model_lns.h
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Expand Up @@ -47,9 +47,10 @@ struct Neighborhood {
static constexpr int kDefaultArenaSizePerVariable = 128;

explicit Neighborhood(int num_variables_hint = 10)
: arena_buffer(kDefaultArenaSizePerVariable * num_variables_hint),
arena(std::make_unique<google::protobuf::Arena>(arena_buffer.data(),
arena_buffer.size())),
: arena(std::make_unique<google::protobuf::Arena>(
google::protobuf::ArenaOptions(
{.start_block_size = static_cast<size_t>(
kDefaultArenaSizePerVariable * num_variables_hint)}))),
delta(*google::protobuf::Arena::Create<CpModelProto>(arena.get())) {}

// True if neighborhood generator was able to generate a neighborhood.
Expand All @@ -66,7 +67,6 @@ struct Neighborhood {
// The delta will contains all variables from the initial model, potentially
// with updated domains.
// It can contains new variables and new constraints, and solution hinting.
std::vector<char> arena_buffer;
std::unique_ptr<google::protobuf::Arena> arena;
CpModelProto& delta;

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50 changes: 16 additions & 34 deletions ortools/sat/cp_model_presolve.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -9609,16 +9609,17 @@ void CpModelPresolver::DetectDuplicateConstraintsWithDifferentEnforcements(
if (context_->VariableWithCostIsUniqueAndRemovable(a) &&
context_->VariableWithCostIsUniqueAndRemovable(b)) {
// Both these case should be presolved before, but it is easy to deal with
// if we encounter them here in some corner cases.
// if we encounter them here in some corner cases. And the code after
// 'continue' uses this, in particular to update the hint.
bool skip = false;
if (RefIsPositive(a) == context_->ObjectiveCoeff(PositiveRef(a)) > 0) {
context_->UpdateRuleStats("duplicate: dual fixing enforcement.");
if (!context_->SetLiteralToFalse(a)) return;
if (!context_->SetLiteralAndHintToFalse(a)) return;
skip = true;
}
if (RefIsPositive(b) == context_->ObjectiveCoeff(PositiveRef(b)) > 0) {
context_->UpdateRuleStats("duplicate: dual fixing enforcement.");
if (!context_->SetLiteralToFalse(b)) return;
if (!context_->SetLiteralAndHintToFalse(b)) return;
skip = true;
}
if (skip) continue;
Expand All @@ -9643,6 +9644,16 @@ void CpModelPresolver::DetectDuplicateConstraintsWithDifferentEnforcements(
// Sign is correct, i.e. ignoring the constraint is expensive.
// The two enforcement can be made equivalent.
context_->UpdateRuleStats("duplicate: dual equivalence of enforcement");
// If `a` and `b` hints are different then the whole hint satisfies
// the enforced constraint. We can thus change them to true (this cannot
// increase the objective value thanks to the `skip` test above -- the
// objective domain is non-constraining, but this only guarantees that
// singleton variables can freely *decrease* the objective).
if (context_->LiteralSolutionHint(a) !=
context_->LiteralSolutionHint(b)) {
context_->UpdateLiteralSolutionHint(a, true);
context_->UpdateLiteralSolutionHint(b, true);
}
context_->StoreBooleanEqualityRelation(a, b);

// We can also remove duplicate constraint now. It will be done later but
Expand Down Expand Up @@ -14364,38 +14375,9 @@ bool CpModelPresolver::MaybeRemoveFixedVariables(
}

// TODO(user): Right now the copy do not remove fixed variable from the
// objective, so we do that here so that these variable should not appear
// anymore. Fix that.
// objective, but ReadObjectiveFromProto() does it. Maybe we should just not
// copy them in the first place.
if (context_->working_model->has_objective()) {
auto* objective = context_->working_model->mutable_objective();
auto* mutable_vars = objective->mutable_vars();
auto* mutable_coeffs = objective->mutable_coeffs();
const int old_size = objective->vars().size();
int64_t offset = 0;
int new_size = 0;
for (int i = 0; i < old_size; ++i) {
const int var = objective->vars(i);
const int64_t coeff = objective->coeffs(i);
if (context_->IsFixed(var)) {
offset += context_->FixedValue(var) * coeff;
continue;
}
mutable_vars->Set(new_size, var);
mutable_coeffs->Set(new_size, coeff);
++new_size;
}
mutable_vars->Truncate(new_size);
mutable_coeffs->Truncate(new_size);

// Tricky: The objective domain is on the linear objective expression
// without the offset, so we need to shift it if there is one.
objective->set_offset(objective->offset() + offset);
if (!objective->domain().empty()) {
Domain obj_domain =
ReadDomainFromProto(*objective).AdditionWith(Domain(-offset));
FillDomainInProto(obj_domain, objective);
}

context_->ReadObjectiveFromProto();
if (!context_->CanonicalizeObjective()) return false;
if (!PropagateObjective()) return false;
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60 changes: 30 additions & 30 deletions ortools/sat/cp_model_solver.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -464,7 +464,7 @@ std::string CpModelStats(const CpModelProto& model_proto) {

if (num_constants > 0) {
const std::string temp =
absl::StrCat(" - ", num_constants, " constants in {",
absl::StrCat(" - ", FormatCounter(num_constants), " constants in {",
absl::StrJoin(constant_values, ","), "} \n");
absl::StrAppend(&result, Summarize(temp));
}
Expand Down Expand Up @@ -503,62 +503,62 @@ std::string CpModelStats(const CpModelProto& model_proto) {
FormatCounter(name_to_num_complex_domain[c_name]), ")");
}
if (name == "kInterval" && num_fixed_intervals > 0) {
absl::StrAppend(&constraints.back(), " (#fixed: ", num_fixed_intervals,
")");
} else if (name == "kNoOverlap2D") {
absl::StrAppend(&constraints.back(),
" (#rectangles: ", no_overlap_2d_num_rectangles);
" (#fixed: ", FormatCounter(num_fixed_intervals), ")");
} else if (name == "kNoOverlap2D") {
absl::StrAppend(&constraints.back(), " (#rectangles: ",
FormatCounter(no_overlap_2d_num_rectangles));
if (no_overlap_2d_num_optional_rectangles > 0) {
absl::StrAppend(&constraints.back(),
", #optional: ", no_overlap_2d_num_optional_rectangles);
absl::StrAppend(&constraints.back(), ", #optional: ",
FormatCounter(no_overlap_2d_num_optional_rectangles));
}
if (no_overlap_2d_num_linear_areas > 0) {
absl::StrAppend(&constraints.back(),
", #linear_areas: ", no_overlap_2d_num_linear_areas);
absl::StrAppend(&constraints.back(), ", #linear_areas: ",
FormatCounter(no_overlap_2d_num_linear_areas));
}
if (no_overlap_2d_num_quadratic_areas > 0) {
absl::StrAppend(&constraints.back(), ", #quadratic_areas: ",
no_overlap_2d_num_quadratic_areas);
FormatCounter(no_overlap_2d_num_quadratic_areas));
}
if (no_overlap_2d_num_fixed_rectangles > 0) {
absl::StrAppend(&constraints.back(), ", #fixed_rectangles: ",
no_overlap_2d_num_fixed_rectangles);
FormatCounter(no_overlap_2d_num_fixed_rectangles));
}
absl::StrAppend(&constraints.back(), ")");
} else if (name == "kCumulative") {
absl::StrAppend(&constraints.back(),
" (#intervals: ", cumulative_num_intervals);
absl::StrAppend(&constraints.back(), " (#intervals: ",
FormatCounter(cumulative_num_intervals));
if (cumulative_num_optional_intervals > 0) {
absl::StrAppend(&constraints.back(),
", #optional: ", cumulative_num_optional_intervals);
absl::StrAppend(&constraints.back(), ", #optional: ",
FormatCounter(cumulative_num_optional_intervals));
}
if (cumulative_num_variable_sizes > 0) {
absl::StrAppend(&constraints.back(),
", #variable_sizes: ", cumulative_num_variable_sizes);
absl::StrAppend(&constraints.back(), ", #variable_sizes: ",
FormatCounter(cumulative_num_variable_sizes));
}
if (cumulative_num_variable_demands > 0) {
absl::StrAppend(&constraints.back(), ", #variable_demands: ",
cumulative_num_variable_demands);
}
if (cumulative_num_fixed_intervals > 0) {
absl::StrAppend(&constraints.back(),
", #fixed_intervals: ", cumulative_num_fixed_intervals);
absl::StrAppend(&constraints.back(), ", #fixed_intervals: ",
FormatCounter(cumulative_num_fixed_intervals));
}
absl::StrAppend(&constraints.back(), ")");
} else if (name == "kNoOverlap") {
absl::StrAppend(&constraints.back(),
" (#intervals: ", no_overlap_num_intervals);
absl::StrAppend(&constraints.back(), " (#intervals: ",
FormatCounter(no_overlap_num_intervals));
if (no_overlap_num_optional_intervals > 0) {
absl::StrAppend(&constraints.back(),
", #optional: ", no_overlap_num_optional_intervals);
absl::StrAppend(&constraints.back(), ", #optional: ",
FormatCounter(no_overlap_num_optional_intervals));
}
if (no_overlap_num_variable_sizes > 0) {
absl::StrAppend(&constraints.back(),
", #variable_sizes: ", no_overlap_num_variable_sizes);
absl::StrAppend(&constraints.back(), ", #variable_sizes: ",
FormatCounter(no_overlap_num_variable_sizes));
}
if (no_overlap_num_fixed_intervals > 0) {
absl::StrAppend(&constraints.back(),
", #fixed_intervals: ", no_overlap_num_fixed_intervals);
absl::StrAppend(&constraints.back(), ", #fixed_intervals: ",
FormatCounter(no_overlap_num_fixed_intervals));
}
absl::StrAppend(&constraints.back(), ")");
}
Expand Down Expand Up @@ -1282,13 +1282,13 @@ class LnsSolver : public SubSolver {
shared_->time_limit->UpdateLocalLimit(local_time_limit);

// Presolve and solve the LNS fragment.
int64_t buffer_size;
size_t buffer_size;
{
absl::MutexLock l(&next_arena_size_mutex_);
buffer_size = next_arena_size_;
}
auto arena_buffer = std::make_unique<char[]>(buffer_size);
google::protobuf::Arena arena(arena_buffer.get(), buffer_size);
google::protobuf::Arena arena(
google::protobuf::ArenaOptions({.start_block_size = buffer_size}));
CpModelProto& lns_fragment =
*google::protobuf::Arena::Create<CpModelProto>(&arena);
CpModelProto& mapping_proto =
Expand Down
32 changes: 22 additions & 10 deletions ortools/sat/presolve_context.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -1951,29 +1951,41 @@ void PresolveContext::ReadObjectiveFromProto() {
// summing an objective partial sum. Because of the model validation, this
// shouldn't overflow, and we make sure it stays this way.
objective_overflow_detection_ = std::abs(objective_integer_before_offset_);
int64_t fixed_terms = 0;

objective_map_.clear();
for (int i = 0; i < obj.vars_size(); ++i) {
const int ref = obj.vars(i);
const int64_t var_max_magnitude =
std::max(std::abs(MinOf(ref)), std::abs(MaxOf(ref)));
int var = obj.vars(i);
int64_t coeff = obj.coeffs(i);

// Skipping var fixed to zero allow to avoid some overflow in situation
// were we can deal with it.
if (var_max_magnitude == 0) continue;
// TODO(user): There should be no negative reference here !
if (!RefIsPositive(var)) {
var = NegatedRef(var);
coeff = -coeff;
}

const int64_t coeff = obj.coeffs(i);
objective_overflow_detection_ += var_max_magnitude * std::abs(coeff);
// We remove fixed terms as we read the objective. This can help a lot on
// LNS problems with a large proportions of fixed terms.
if (IsFixed(var)) {
fixed_terms += FixedValue(var) * coeff;
continue;
}

const int var = PositiveRef(ref);
objective_map_[var] += RefIsPositive(ref) ? coeff : -coeff;
const int64_t var_max_magnitude =
std::max(std::abs(MinOf(var)), std::abs(MaxOf(var)));
objective_overflow_detection_ += var_max_magnitude * std::abs(coeff);

objective_map_[var] += RefIsPositive(var) ? coeff : -coeff;
if (objective_map_[var] == 0) {
RemoveVariableFromObjective(var);
} else {
var_to_constraints_[var].insert(kObjectiveConstraint);
}
}

if (fixed_terms != 0) {
AddToObjectiveOffset(fixed_terms);
}
}

bool PresolveContext::CanonicalizeOneObjectiveVariable(int var) {
Expand Down
4 changes: 2 additions & 2 deletions ortools/sat/presolve_context.h
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Original file line number Diff line number Diff line change
Expand Up @@ -543,9 +543,9 @@ class PresolveContext {
return it == objective_map_.end() ? 0 : it->second;
}

// Returns true if the variables in the objective with a positive (resp.
// Returns false if the variables in the objective with a positive (resp.
// negative) coefficient can freely decrease (resp. increase) within their
// domain (if we ignore the other constraints).
// domain (if we ignore the other constraints). Otherwise, returns true.
bool ObjectiveDomainIsConstraining() const {
return objective_domain_is_constraining_;
}
Expand Down

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