some english notes on the purpose of batch sizes and the beginning of a worker thread implementation

inc/genetic.h

@@ -3,13 +3,17 @@
 namespace genetic {
 
 template <class T> struct Strategy {
-  // The recommended number of threads is <= number of cores on your pc.
-  // Set this to -1 use the default value (number of cores - 1)
-  int num_threads;     // Number of worker threads that will be evaluating cell fitness
+  int num_threads; // Number of worker threads that will be evaluating cell
+                   // fitness.
+  int num_retries; // Number of times worker threads will try to grab work pool
+                   // lock before sleeping
+  int batch_size;  // Number of cells a worker thread tries to evaluate in a row
+                   // before locking the pool again. 1 tends to be fine
   int num_cells;   // Size of the population pool
   int num_generations; // Number of times (epochs) to run the algorithm
   bool test_all; // Sets whether or not every cell is tested every generation
-  float test_chance;   // Chance to test any given cell's fitness. Relevant only if test_all is false.
+  float test_chance; // Chance to test any given cell's fitness. Relevant only
+                     // if test_all is false.
 
   // User defined functions
   T (*make_default_cell)();

src/genetic.cpp

@@ -1,5 +1,6 @@
 #include "genetic.h"
 #include "pthread.h"
+#include <algorithm>
 #include <queue>
 #include <vector>
 
@@ -12,11 +13,103 @@ template <class T> struct CellEntry {
 };
 
 template <class T> struct WorkEntry {
-  const std::vector<CellEntry<T>> &cur;
-  std::vector<CellEntry<T>> &next;
-  int cur_i;
+  const CellEntry<T> &cur;
+  float &score;
 };
 
+static pthread_mutex_t data_mutex = PTHREAD_MUTEX_INITIALIZER;
+
+static pthread_mutex_t ready_mutex = PTHREAD_MUTEX_INITIALIZER;
+static pthread_cond_t ready_cond = PTHREAD_COND_INITIALIZER;
+
+static pthread_mutex_t gen_complete_mutex = PTHREAD_MUTEX_INITIALIZER;
+static pthread_cond_t gen_complete_cond = PTHREAD_COND_INITIALIZER;
+
+static pthread_mutex_t run_complete_mutex = PTHREAD_MUTEX_INITIALIZER;
+static pthread_cond_t run_complete_cond = PTHREAD_COND_INITIALIZER;
+
+/*  Thoughts on this approach
+ * The ideal implementation of a worker thread has them operating at maximum
+ * load with as little synchronization overhead as possible. i.e. The ideal
+ * worker thread
+ *     1. Never waits for new work
+ *     2. Never spends time synchronizing with other worker threads
+ *
+ * Never is impossible, but we want to get as close as we can.
+ *
+ * There are two extreme situations to consider
+ *     1. Fitness functions with highly variable computation times
+ *     2. Fitness functions with identical computation times.
+ *
+ * Most applications that use this library will fall into the second
+ * category.
+ *
+ * In the highly-variable computation time case, it's useful for worker threads
+ * to operate on 1 work entry at a time. Imagine a scenario with 2 threads, each
+ * of which claims half the work to do. If thread A completes all of its work
+ * quickly, it goes to sleep while thread B slogs away on its harder-to-compute
+ * fitness jobs. However, if both threads only claim 1 work entry at a time,
+ * thread A can immediately claim new jobs after it completes its current one.
+ * Thread B can toil away, but little time is lost since thread A remains
+ * productive.
+ *
+ * In the highly consistent computation time case, it's ideal for each
+ * thread to claim an equal share of the jobs (as this minimizes time spent
+ * synchronizing access to the job pool). Give each thread its set of work once
+ * and let them have at it instead of each thread constantly locking/waiting
+ * on the job queue.
+ *
+ * I take a hybrid approach. Users can specify a "batch size". Worker threads
+ * will bite off jobs in chunks and complete them before locking
+ * the job pool again. The user to choose a batch size close to 1 if
+ * their fitness function compute time is highly variable, and closer to
+ * num_cells / num_threads if computation time is consistent. Users should
+ * experiment with a batch size that works well for their problem.
+ *
+ * Worth mentioning this optimization work is irrelevant once computation time
+ * >>> synchronization time.
+ *
+ * There might be room for dynamic batch size modification, but I don't expect
+ * to pursue this feature until the library is more mature (and I've run out of
+ * cooler things to do).
+ *
+ */
+template <class T>
+void worker(std::queue<WorkEntry<T>> &fitness_queue, int batch_size,
+            int num_retries) {
+  int retries = 0;
+  std::vector<WorkEntry<T>> batch;
+  bool gen_is_finished;
+  while (true) {
+    gen_is_finished = false;
+    if (pthread_mutex_trylock(&data_mutex)) {
+      retries = 0;
+      for (int i = 0; i < batch_size; i++) {
+        if (fitness_queue.empty()) {
+          gen_is_finished = true;
+          break;
+        }
+        batch.push_back(fitness_queue.front());
+        fitness_queue.pop();
+      }
+      pthread_mutex_unlock(&data_mutex);
+    } else {
+      retries++;
+    }
+
+    if (gen_is_finished) {
+      pthread_cond_signal(&gen_complete_cond, &gen_complete_mutex);
+    }
+
+    if (retries > num_retries) {
+      pthread_mutex_lock(&ready_mutex);
+      pthread_cond_wait(&ready_cond, &ready_mutex);
+      retries = 0;
+    }
+  }
+  pthread_mutex_lock(&data_mutex);
+}
+
 // Definitions
 template <class T> Stats<T> run(Strategy<T> strat) {
   Stats<T> stats;