05_parallel.c

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/* -*- C -*-
 * Serene programming language
 * Copyright (C) 2019-2026 Sameer Rahmani <[email protected]>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */
 
// Example 05 -- many fibers across many worker threads (M:N).
//
// The same scheduler runs on more than one OS thread by passing a worker count
// greater than one to `srn_sched_run`. The fibers are distributed across the
// workers, which steal work from one another to stay busy, so the fibers run in
// parallel rather than merely interleaved. The run returns once every worker is
// idle on an empty queue.
//
// Because the fibers run on different threads at the same time, any state they
// share has to be synchronised. This example keeps it simple with a single
// atomic counter that every fiber increments once. After the run the total
// equals the fiber count, with no lost updates.
 
#include <stdatomic.h>
#include <stdio.h>
 
#include <serene/runtime.h>
 
#define FIBER_COUNT  1000
#define WORKER_COUNT 4
 
static atomic_int total;
 
static srn_fiber_result_t increment(srn_context_t *ctx, void *arg) {
  (void)ctx;
  (void)arg;
  // The one piece of shared state. An atomic add is safe from any worker.
  atomic_fetch_add(&total, 1);
  return nullptr;
}
 
int main(void) {
  SERENE_RUNTIME_INIT_WITH_SCHED(engine, sched, nullptr);
  srn_context_t *ctx = srn_context_make(engine);
 
  atomic_store(&total, 0);
  for (int i = 0; i < FIBER_COUNT; i++) {
    (void)srn_fiber_spawn(ctx, increment, nullptr);
  }
 
  printf("running %d fibers across %d workers\n", FIBER_COUNT, WORKER_COUNT);
  srn_sched_run(sched, WORKER_COUNT);
 
  int counted = atomic_load(&total);
  printf(
    "counted %d (expected %d): %s\n", counted, FIBER_COUNT,
    counted == FIBER_COUNT ? "all fibers ran" : "lost an update"
  );
 
  srn_context_release(ctx);
  SERENE_RUNTIME_SHUTDOWN(engine);
  return 0;
}