API to specify number of threads, from threadpool, to use for the task

include/pthreadpool.h

@@ -160,6 +160,25 @@ pthreadpool_t pthreadpool_create(size_t threads_count);
  */
 size_t pthreadpool_get_threads_count(pthreadpool_t threadpool);
 
+/*
+ * API to enable doing work with fewer threads than available in
+ * threadpool.
+ * Purpose of this is to ameliorate some perf degradation observed
+ * due to OS mapping a given set of threads to fewer cores.
+ *
+ * @param  num_threads  num threads to use for the subsequent tasks
+ *    submitted.
+ */
+void pthreadpool_set_num_threads_to_use(size_t num_threads);
+
+/*
+ * Query current setting of the number of threads to use
+ *
+ * @returns  The number of threads to be used for the subsequent tasks
+ *    submitted.
+ */
+size_t pthreadpool_get_num_threads_to_use(void);
+
 /**
  * Process items on a 1D grid.
  *

src/fastpath.c

@@ -39,7 +39,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_thread_parallelize_1d_fastpath(
       (pthreadpool_task_1d_t)pthreadpool_load_relaxed_void_p(&threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -77,7 +78,8 @@ pthreadpool_thread_parallelize_1d_with_thread_fastpath(
           &threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -126,7 +128,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_thread_parallelize_1d_with_uarch_fastpath(
   }
 #endif
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -163,7 +166,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_thread_parallelize_1d_tile_1d_fastpath(
           &threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -206,7 +210,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_thread_parallelize_2d_fastpath(
       (pthreadpool_task_2d_t)pthreadpool_load_relaxed_void_p(&threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -258,7 +263,8 @@ pthreadpool_thread_parallelize_2d_with_thread_fastpath(
           &threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -309,7 +315,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_thread_parallelize_2d_tile_1d_fastpath(
           &threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -378,7 +385,8 @@ pthreadpool_thread_parallelize_2d_tile_1d_with_uarch_fastpath(
   }
 #endif
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -449,7 +457,8 @@ pthreadpool_thread_parallelize_2d_tile_1d_with_uarch_with_thread_fastpath(
   }
 #endif
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -508,7 +517,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_thread_parallelize_2d_tile_2d_fastpath(
           &threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -581,7 +591,8 @@ pthreadpool_thread_parallelize_2d_tile_2d_with_uarch_fastpath(
   }
 #endif
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -644,7 +655,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_thread_parallelize_3d_fastpath(
       (pthreadpool_task_3d_t)pthreadpool_load_relaxed_void_p(&threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -706,7 +718,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_thread_parallelize_3d_tile_1d_fastpath(
           &threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -773,7 +786,8 @@ pthreadpool_thread_parallelize_3d_tile_1d_with_thread_fastpath(
           pthreadpool_load_relaxed_void_p(&threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -853,7 +867,8 @@ pthreadpool_thread_parallelize_3d_tile_1d_with_uarch_fastpath(
   }
 #endif
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -934,7 +949,8 @@ pthreadpool_thread_parallelize_3d_tile_1d_with_uarch_with_thread_fastpath(
   }
 #endif
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -1003,7 +1019,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_thread_parallelize_3d_tile_2d_fastpath(
           &threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -1087,7 +1104,8 @@ pthreadpool_thread_parallelize_3d_tile_2d_with_uarch_fastpath(
   }
 #endif
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -1161,7 +1179,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_thread_parallelize_4d_fastpath(
       (pthreadpool_task_4d_t)pthreadpool_load_relaxed_void_p(&threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -1234,7 +1253,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_thread_parallelize_4d_tile_1d_fastpath(
           &threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -1311,7 +1331,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_thread_parallelize_4d_tile_2d_fastpath(
           &threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -1405,7 +1426,8 @@ pthreadpool_thread_parallelize_4d_tile_2d_with_uarch_fastpath(
   }
 #endif
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -1490,7 +1512,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_thread_parallelize_5d_fastpath(
       (pthreadpool_task_5d_t)pthreadpool_load_relaxed_void_p(&threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -1573,7 +1596,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_thread_parallelize_5d_tile_1d_fastpath(
           &threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -1661,7 +1685,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_thread_parallelize_5d_tile_2d_fastpath(
           &threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -1752,7 +1777,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_thread_parallelize_6d_fastpath(
       (pthreadpool_task_6d_t)pthreadpool_load_relaxed_void_p(&threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -1846,7 +1872,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_thread_parallelize_6d_tile_1d_fastpath(
           &threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */
@@ -1944,7 +1971,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_thread_parallelize_6d_tile_2d_fastpath(
           &threadpool->task);
   void* const argument = pthreadpool_load_relaxed_void_p(&threadpool->argument);
 
-  const size_t threads_count = threadpool->threads_count.value;
+  const size_t threads_count =
+      pthreadpool_load_relaxed_size_t(&threadpool->num_threads_to_use);
   const size_t range_threshold = -threads_count;
 
   /* Process thread's own range of items */

src/gcd.c

@@ -30,6 +30,8 @@
 #include "threadpool-object.h"
 #include "threadpool-utils.h"
 
+thread_local size_t max_num_threads = UINT_MAX;
+
 static void thread_main(void* arg, size_t thread_index) {
   struct pthreadpool* threadpool = (struct pthreadpool*)arg;
   struct thread_info* thread = &threadpool->threads[thread_index];
@@ -73,6 +75,8 @@ struct pthreadpool* pthreadpool_create(size_t threads_count) {
     return NULL;
   }
   threadpool->threads_count = fxdiv_init_size_t(threads_count);
+  pthreadpool_store_relaxed_size_t(&threadpool->num_threads_to_use,
+                                   threads_count);
   for (size_t tid = 0; tid < threads_count; tid++) {
     threadpool->threads[tid].thread_number = tid;
   }
@@ -85,6 +89,12 @@ struct pthreadpool* pthreadpool_create(size_t threads_count) {
   return threadpool;
 }
 
+void pthreadpool_set_num_threads_to_use(size_t num_threads) {
+  max_num_threads = num_threads;
+}
+
+size_t pthreadpool_get_num_threads_to_use() { return max_num_threads; }
+
 PTHREADPOOL_INTERNAL void pthreadpool_parallelize(
     struct pthreadpool* threadpool, thread_function_t thread_function,
     const void* params, size_t params_size, void* task, void* context,
@@ -107,7 +117,8 @@ PTHREADPOOL_INTERNAL void pthreadpool_parallelize(
 
   /* Locking of completion_mutex not needed: readers are sleeping on
    * command_condvar */
-  const struct fxdiv_divisor_size_t threads_count = threadpool->threads_count;
+  const struct fxdiv_divisor_size_t threads_count = fxdiv_init_size_t(min(
+      threadpool->threads_count.value, pthreadpool_get_num_threads_to_use()));
 
   if (params_size != 0) {
     memcpy(&threadpool->params, params, params_size);