if chunk is zero, split the input into num_thread * threadpool_default_chunk_ratio chunks.

src/threadpool.c

@@ -36,6 +36,7 @@
 /* Local headers. */
 #include "atomic.h"
 #include "error.h"
+#include "minmax.h"
 
 #ifdef SWIFT_DEBUG_THREADPOOL
 /**
@@ -250,7 +251,8 @@ void threadpool_init(struct threadpool *tp, int num_threads) {
  * @param map_data The data on which the mapping function will be called.
  * @param N Number of elements in @c map_data.
  * @param stride Size, in bytes, of each element of @c map_data.
- * @param chunk Number of map data elements to pass to the function at a time.
+ * @param chunk Number of map data elements to pass to the function at a time,
+ *        or zero to choose the number automatically.
  * @param extra_data Addtitional pointer that will be passed to the mapping
  *        function, may contain additional data.
  */
@@ -269,7 +271,10 @@ void threadpool_map(struct threadpool *tp, threadpool_map_function map_function,
   tp->map_data_stride = stride;
   tp->map_data_size = N;
   tp->map_data_count = 0;
-  tp->map_data_chunk = chunk;
+  tp->map_data_chunk =
+      chunk ? chunk
+            : max((int)(N / (tp->num_threads * threadpool_default_chunk_ratio)),
+                  1);
   tp->map_function = map_function;
   tp->map_data = map_data;
   tp->map_extra_data = extra_data;

src/threadpool.h

@@ -30,6 +30,7 @@
 
 /* Local defines. */
 #define threadpool_log_initial_size 1000
+#define threadpool_default_chunk_ratio 5
 
 /* Function type for mappings. */
 typedef void (*threadpool_map_function)(void *map_data, int num_elements,