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z3/src/smt/smt_parallel.cpp

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/*++
Copyright (c) 2020 Microsoft Corporation
Module Name:
smt_parallel.cpp
Abstract:
Parallel SMT, portfolio loop specialized to SMT core.
Author:
nbjorner 2020-01-31
--*/
#include "util/scoped_ptr_vector.h"
#include "ast/ast_util.h"
#include "ast/ast_pp.h"
#include "ast/ast_ll_pp.h"
#include "ast/ast_translation.h"
#include "smt/smt_parallel.h"
#include "smt/smt_lookahead.h"
#ifdef SINGLE_THREAD
namespace smt {
lbool parallel::operator()(expr_ref_vector const& asms) {
return l_undef;
}
}
#else
#include <thread>
#include <cassert>
namespace smt {
void parallel::worker::run() {
ast_translation g2l(p.ctx.m, m); // global to local context -- MUST USE p.ctx.m, not ctx->m, AS GLOBAL MANAGER!!!
ast_translation l2g(m, p.ctx.m); // local to global context
while (m.inc()) { // inc: increase the limit and check if it is canceled, vs m.limit().is_canceled() is readonly. the .limit() is also not necessary (m.inc() etc provides a convenience wrapper)
vector<expr_ref_vector> cubes;
b.get_cubes(g2l, cubes);
if (cubes.empty())
return;
collect_shared_clauses(g2l);
for (auto& cube : cubes) {
if (!m.inc()) {
b.set_exception("context cancelled");
return;
}
IF_VERBOSE(1, verbose_stream() << "Worker " << id << " cube: " << mk_bounded_pp(mk_and(cube), m, 3) << "\n");
lbool r = check_cube(cube);
if (m.limit().is_canceled()) {
IF_VERBOSE(1, verbose_stream() << "Worker " << id << " context cancelled\n");
return;
}
switch (r) {
case l_undef: {
IF_VERBOSE(1, verbose_stream() << "Worker " << id << " found undef cube\n");
// return unprocessed cubes to the batch manager
// add a split literal to the batch manager.
// optionally process other cubes and delay sending back unprocessed cubes to batch manager.
vector<expr_ref_vector> returned_cubes;
returned_cubes.push_back(cube);
auto split_atoms = get_split_atoms();
b.return_cubes(l2g, returned_cubes, split_atoms);
update_max_thread_conflicts();
break;
}
case l_true: {
IF_VERBOSE(1, verbose_stream() << "Worker " << id << " found sat cube\n");
model_ref mdl;
ctx->get_model(mdl);
b.set_sat(l2g, *mdl);
return;
}
case l_false: {
// if unsat core only contains (external) assumptions (i.e. all the unsat core are asms), then unsat and return as this does NOT depend on cubes
// otherwise, extract lemmas that can be shared (units (and unsat core?)).
// share with batch manager.
// process next cube.
expr_ref_vector const& unsat_core = ctx->unsat_core();
IF_VERBOSE(1, verbose_stream() << "unsat core: " << unsat_core << "\n");
// If the unsat core only contains assumptions,
// unsatisfiability does not depend on the current cube and the entire problem is unsat.
if (all_of(unsat_core, [&](expr* e) { return asms.contains(e); })) {
IF_VERBOSE(1, verbose_stream() << "Worker " << id << " determined formula unsat\n");
b.set_unsat(l2g, unsat_core);
return;
}
for (expr* e : unsat_core)
if (asms.contains(e))
b.report_assumption_used(l2g, e); // report assumptions used in unsat core, so they can be used in final core
IF_VERBOSE(1, verbose_stream() << "Worker " << id << " found unsat cube\n");
b.collect_clause(l2g, id, mk_not(mk_and(unsat_core)));
break;
}
}
}
share_units(l2g);
}
}
parallel::worker::worker(unsigned id, parallel& p, expr_ref_vector const& _asms): id(id), p(p), b(p.m_batch_manager), m_smt_params(p.ctx.get_fparams()), asms(m) {
ast_translation g2l(p.ctx.m, m);
for (auto e : _asms)
asms.push_back(g2l(e));
IF_VERBOSE(1, verbose_stream() << "Worker " << id << " created with " << asms.size() << " assumptions\n");
m_smt_params.m_preprocess = false;
ctx = alloc(context, m, m_smt_params, p.ctx.get_params());
context::copy(p.ctx, *ctx, true);
ctx->set_random_seed(id + m_smt_params.m_random_seed);
m_max_thread_conflicts = ctx->get_fparams().m_threads_max_conflicts;
m_max_conflicts = ctx->get_fparams().m_max_conflicts;
}
void parallel::worker::share_units(ast_translation& l2g) {
// Collect new units learned locally by this worker and send to batch manager
ctx->pop_to_base_lvl();
unsigned sz = ctx->assigned_literals().size();
for (unsigned j = m_num_shared_units; j < sz; ++j) { // iterate only over new literals since last sync
literal lit = ctx->assigned_literals()[j];
expr_ref e(ctx->bool_var2expr(lit.var()), ctx->m); // turn literal into a Boolean expression
if (lit.sign())
e = m.mk_not(e); // negate if literal is negative
b.collect_clause(l2g, id, e);
}
m_num_shared_units = sz;
}
void parallel::batch_manager::collect_clause(ast_translation& l2g, unsigned source_worker_id, expr* clause) {
std::scoped_lock lock(mux);
expr* g_clause = l2g(clause);
if (!shared_clause_set.contains(g_clause)) {
shared_clause_set.insert(g_clause);
shared_clause sc{source_worker_id, expr_ref(g_clause, m)};
shared_clause_trail.push_back(sc);
}
}
void parallel::worker::collect_shared_clauses(ast_translation& g2l) {
expr_ref_vector new_clauses = b.return_shared_clauses(g2l, m_shared_clause_limit, id); // get new clauses from the batch manager
// iterate over new clauses and assert them in the local context
for (expr* e : new_clauses) {
expr_ref local_clause(e, g2l.to()); // e was already translated to the local context in the batch manager!!
ctx->assert_expr(local_clause); // assert the clause in the local context
IF_VERBOSE(1, verbose_stream() << "Worker " << id << " asserting shared clause: " << mk_bounded_pp(local_clause, m, 3) << "\n");
}
}
// get new clauses from the batch manager and assert them in the local context
expr_ref_vector parallel::batch_manager::return_shared_clauses(ast_translation& g2l, unsigned& worker_limit, unsigned worker_id) {
std::scoped_lock lock(mux);
expr_ref_vector result(g2l.to());
for (unsigned i = worker_limit; i < shared_clause_trail.size(); ++i) {
if (shared_clause_trail[i].source_worker_id == worker_id)
continue; // skip clauses from the requesting worker
result.push_back(g2l(shared_clause_trail[i].clause.get()));
}
worker_limit = shared_clause_trail.size(); // update the worker limit to the end of the current trail
return result;
}
lbool parallel::worker::check_cube(expr_ref_vector const& cube) {
IF_VERBOSE(1, verbose_stream() << "Worker " << id << " checking cube\n";);
for (auto& atom : cube)
asms.push_back(atom);
lbool r = l_undef;
ctx->get_fparams().m_max_conflicts = std::min(m_max_thread_conflicts, m_max_conflicts);
try {
r = ctx->check(asms.size(), asms.data());
}
catch (z3_error& err) {
b.set_exception(err.error_code());
}
catch (z3_exception& ex) {
b.set_exception(ex.what());
}
catch (...) {
b.set_exception("unknown exception");
}
asms.shrink(asms.size() - cube.size());
IF_VERBOSE(1, verbose_stream() << "Worker " << id << " DONE checking cube " << r << "\n";);
return r;
}
void parallel::batch_manager::get_cubes(ast_translation& g2l, vector<expr_ref_vector>& cubes) {
std::scoped_lock lock(mux);
if (m_cubes.size() == 1 && m_cubes[0].size() == 0) {
// special initialization: the first cube is emtpy, have the worker work on an empty cube.
cubes.push_back(expr_ref_vector(g2l.to()));
return;
}
for (unsigned i = 0; i < std::min(m_max_batch_size / p.num_threads, (unsigned)m_cubes.size()) && !m_cubes.empty(); ++i) {
auto& cube = m_cubes.back();
expr_ref_vector l_cube(g2l.to());
for (auto& e : cube) {
l_cube.push_back(g2l(e));
}
cubes.push_back(l_cube);
m_cubes.pop_back();
}
}
void parallel::batch_manager::set_sat(ast_translation& l2g, model& m) {
std::scoped_lock lock(mux);
if (m_state != state::is_running)
return;
m_state = state::is_sat;
p.ctx.set_model(m.translate(l2g));
cancel_workers();
}
void parallel::batch_manager::set_unsat(ast_translation& l2g, expr_ref_vector const& unsat_core) {
std::scoped_lock lock(mux);
if (m_state != state::is_running)
return;
m_state = state::is_unsat;
// every time we do a check_sat call, don't want to have old info coming from a prev check_sat call
// the unsat core gets reset internally in the context after each check_sat, so we assert this property here
// takeaway: each call to check_sat needs to have a fresh unsat core
SASSERT(p.ctx.m_unsat_core.empty());
for (expr* e : unsat_core)
p.ctx.m_unsat_core.push_back(l2g(e));
cancel_workers();
}
void parallel::batch_manager::set_exception(unsigned error_code) {
std::scoped_lock lock(mux);
if (m_state != state::is_running)
return;
m_state = state::is_exception_code;
m_exception_code = error_code;
cancel_workers();
}
void parallel::batch_manager::set_exception(std::string const& msg) {
std::scoped_lock lock(mux);
if (m_state != state::is_running || m.limit().is_canceled())
return;
m_state = state::is_exception_msg;
m_exception_msg = msg;
cancel_workers();
}
void parallel::batch_manager::report_assumption_used(ast_translation& l2g, expr* assumption) {
std::scoped_lock lock(mux);
p.m_assumptions_used.insert(l2g(assumption));
}
lbool parallel::batch_manager::get_result() const {
if (m.limit().is_canceled())
return l_undef; // the main context was cancelled, so we return undef.
switch (m_state) {
case state::is_running: // batch manager is still running, but all threads have processed their cubes, which means all cubes were unsat
if (!m_cubes.empty())
throw default_exception("inconsistent end state");
if (!p.m_assumptions_used.empty()) {
// collect unsat core from assumptions used, if any --> case when all cubes were unsat, but depend on nonempty asms, so we need to add these asms to final unsat core
SASSERT(p.ctx.m_unsat_core.empty());
for (auto a : p.m_assumptions_used)
p.ctx.m_unsat_core.push_back(a);
}
return l_false;
case state::is_unsat:
return l_false;
case state::is_sat:
return l_true;
case state::is_exception_msg:
throw default_exception(m_exception_msg.c_str());
case state::is_exception_code:
throw z3_error(m_exception_code);
default:
UNREACHABLE();
return l_undef;
}
}
/*
Batch manager maintains C_batch, A_batch.
C_batch - set of cubes
A_batch - set of split atoms.
return_cubes is called with C_batch A_batch C A.
C_worker - one or more cubes
A_worker - split atoms form the worker thread.
Assumption: A_worker does not occur in C_worker.
------------------------------------------------------------------------------------------------------------------------------------------------------------
Greedy strategy:
For each returned cube c from the worker, you split it on all split atoms not in it (i.e., A_batch \ atoms(c)), plus any new atoms from A_worker.
For each existing cube in the batch, you also split it on the new atoms from A_worker.
return_cubes C_batch A_batch C_worker A_worker:
C_batch <- { cube * 2^(A_worker u (A_batch \ atoms(cube)) | cube in C_worker } u
{ cube * 2^(A_worker \ A_batch) | cube in C_batch }
=
let C_batch' = C_batch u { cube * 2^(A_batch \ atoms(cube)) | cube in C_worker }
in { cube * 2^(A_worker \ A_batch) | cube in C_batch' }
A_batch <- A_batch u A_worker
------------------------------------------------------------------------------------------------------------------------------------------------------------
Frugal strategy: only split on worker cubes
case 1: thread returns no cubes, just atoms: just create 2^k cubes from all combinations of atoms so far.
return_cubes C_batch A_batch [[]] A_worker:
C_batch <- C_batch u 2^(A_worker u A_batch),
A_batch <- A_batch u A_worker
case 2: thread returns both cubes and atoms
Only the returned cubes get split by the newly discovered atoms (A_worker). Existing cubes are not touched.
return_cubes C_batch A_batch C_worker A_worker:
C_batch <- C_batch u { cube * 2^A_worker | cube in C_worker }.
A_batch <- A_batch u A_worker
This means:
Only the returned cubes get split by the newly discovered atoms (A_worker).
Existing cubes are not touched.
------------------------------------------------------------------------------------------------------------------------------------------------------------
Hybrid: Between Frugal and Greedy: (generalizes the first case of empty cube returned by worker) -- don't focus on this approach
i.e. Expand only the returned cubes, but allow them to be split on both new and old atoms not already in them.
C_batch <- C_batch u { cube * 2^(A_worker u (A_batch \ atoms(cube)) | cube in C_worker }
A_batch <- A_batch u A_worker
------------------------------------------------------------------------------------------------------------------------------------------------------------
Final thought (do this!): use greedy strategy by a policy when C_batch, A_batch, A_worker are "small". -- want to do this. switch to frugal strategy after reaching size limit
*/
// currenly, the code just implements the greedy strategy
void parallel::batch_manager::return_cubes(ast_translation& l2g, vector<expr_ref_vector>const& C_worker, expr_ref_vector const& A_worker) {
auto atom_in_cube = [&](expr_ref_vector const& cube, expr* atom) {
return any_of(cube, [&](expr* e) { return e == atom || (m.is_not(e, e) && e == atom); });
};
auto add_split_atom = [&](expr* atom, unsigned start) {
unsigned stop = m_cubes.size();
for (unsigned i = start; i < stop; ++i) {
m_cubes.push_back(m_cubes[i]);
m_cubes.back().push_back(m.mk_not(atom));
m_cubes[i].push_back(atom);
}
};
std::scoped_lock lock(mux);
unsigned max_cubes = 1000;
bool greedy_mode = (m_cubes.size() <= max_cubes);
unsigned a_worker_start_idx = 0;
//
// --- Phase 1: Greedy split of *existing* cubes on new A_worker atoms (greedy) ---
//
if (greedy_mode) {
for (; a_worker_start_idx < A_worker.size(); ++a_worker_start_idx) {
expr_ref g_atom(l2g(A_worker[a_worker_start_idx]), l2g.to());
if (m_split_atoms.contains(g_atom))
continue;
m_split_atoms.push_back(g_atom);
add_split_atom(g_atom, 0); // split all *existing* cubes
if (m_cubes.size() > max_cubes) {
greedy_mode = false;
++a_worker_start_idx; // start frugal from here
break;
}
}
}
unsigned initial_m_cubes_size = m_cubes.size(); // where to start processing the worker cubes after splitting the EXISTING cubes on the new worker atoms
// --- Phase 2: Process worker cubes (greedy) ---
for (auto& c : C_worker) {
expr_ref_vector g_cube(l2g.to());
for (auto& atom : c)
g_cube.push_back(l2g(atom));
unsigned start = m_cubes.size(); // update start after adding each cube so we only process the current cube being added
m_cubes.push_back(g_cube);
if (greedy_mode) {
// Split new cube on all existing m_split_atoms not in it
for (auto g_atom : m_split_atoms) {
if (!atom_in_cube(g_cube, g_atom)) {
add_split_atom(g_atom, start);
if (m_cubes.size() > max_cubes) {
greedy_mode = false;
break;
}
}
}
}
}
// --- Phase 3: Frugal fallback: only process NEW worker cubes with NEW atoms ---
if (!greedy_mode) {
for (unsigned i = a_worker_start_idx; i < A_worker.size(); ++i) {
expr_ref g_atom(l2g(A_worker[i]), l2g.to());
if (!m_split_atoms.contains(g_atom))
m_split_atoms.push_back(g_atom);
add_split_atom(g_atom, initial_m_cubes_size);
}
}
}
expr_ref_vector parallel::worker::get_split_atoms() {
unsigned k = 2;
auto candidates = ctx->m_pq_scores.get_heap();
std::sort(candidates.begin(), candidates.end(),
[](const auto& a, const auto& b) { return a.priority > b.priority; });
expr_ref_vector top_lits(m);
for (const auto& node: candidates) {
if (ctx->get_assignment(node.key) != l_undef)
continue;
expr* e = ctx->bool_var2expr(node.key);
if (!e)
continue;
top_lits.push_back(expr_ref(e, m));
if (top_lits.size() >= k)
break;
}
IF_VERBOSE(1, verbose_stream() << "top literals " << top_lits << " head size " << ctx->m_pq_scores.size() << " num vars " << ctx->get_num_bool_vars() << "\n");
return top_lits;
}
void parallel::batch_manager::initialize() {
m_state = state::is_running;
m_cubes.reset();
m_cubes.push_back(expr_ref_vector(m)); // push empty cube
m_split_atoms.reset();
}
lbool parallel::operator()(expr_ref_vector const& asms) {
ast_manager& m = ctx.m;
if (m.has_trace_stream())
throw default_exception("trace streams have to be off in parallel mode");
struct scoped_clear_table {
obj_hashtable<expr>& ht;
scoped_clear_table(obj_hashtable<expr>& ht) : ht(ht) {} // Constructor: Takes a reference to a hash table when the object is created and saves it.
~scoped_clear_table() { ht.reset(); } // Destructor: When the scoped_clear_table object goes out of scope, it automatically calls reset() on that hash table, clearing it
};
scoped_clear_table clear(m_assumptions_used); // creates a scoped_clear_table named clear, bound to m_assumptions_used
{
m_batch_manager.initialize();
m_workers.reset();
scoped_limits sl(m.limit());
flet<unsigned> _nt(ctx.m_fparams.m_threads, 1);
SASSERT(num_threads > 1);
for (unsigned i = 0; i < num_threads; ++i)
m_workers.push_back(alloc(worker, i, *this, asms)); // i.e. "new worker(i, *this, asms)"
// THIS WILL ALLOW YOU TO CANCEL ALL THE CHILD THREADS
// within the lexical scope of the code block, creates a data structure that allows you to push children
// objects to the limit object, so if someone cancels the parent object, the cancellation propagates to the children
// and that cancellation has the lifetime of the scope
// even if this code doesn't expliclty kill the main thread, still applies bc if you e.g. Ctrl+C the main thread, the children threads need to be cancelled
for (auto w : m_workers)
sl.push_child(&(w->limit()));
// Launch threads
vector<std::thread> threads(num_threads);
for (unsigned i = 0; i < num_threads; ++i) {
threads[i] = std::thread([&, i]() {
m_workers[i]->run();
});
}
// Wait for all threads to finish
for (auto& th : threads)
th.join();
for (auto w : m_workers)
w->collect_statistics(ctx.m_aux_stats);
}
m_workers.clear();
return m_batch_manager.get_result(); // i.e. all threads have finished all of their cubes -- so if state::is_running is still true, means the entire formula is unsat (otherwise a thread would have returned l_undef)
}
}
#endif
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