Add OpenMP acceleration for normal quicksort

Author: sterrettm2

src/avx512-16bit-qsort.hpp

@@ -556,6 +556,7 @@ avx512_qsort_fp16(uint16_t *arr,
 {
     using vtype = zmm_vector<float16>;
 
+    // TODO multithreading support here
     if (arrsize > 1) {
         arrsize_t nan_count = 0;
         if (UNLIKELY(hasnan)) {
@@ -564,11 +565,11 @@ avx512_qsort_fp16(uint16_t *arr,
         }
         if (descending) {
             qsort_<vtype, Comparator<vtype, true>, uint16_t>(
-                    arr, 0, arrsize - 1, 2 * (arrsize_t)log2(arrsize));
+                    arr, 0, arrsize - 1, 2 * (arrsize_t)log2(arrsize), 0);
         }
         else {
             qsort_<vtype, Comparator<vtype, false>, uint16_t>(
-                    arr, 0, arrsize - 1, 2 * (arrsize_t)log2(arrsize));
+                    arr, 0, arrsize - 1, 2 * (arrsize_t)log2(arrsize), 0);
         }
         replace_inf_with_nan(arr, arrsize, nan_count, descending);
     }

src/avx512-64bit-common.h

@@ -968,7 +968,8 @@ struct zmm_vector<uint64_t> {
 static_assert(sizeof(size_t) == sizeof(uint64_t),
               "Size of size_t and uint64_t are not the same");
 template <>
-struct zmm_vector<size_t> : public zmm_vector<uint64_t> {};
+struct zmm_vector<size_t> : public zmm_vector<uint64_t> {
+};
 #endif
 
 template <>

src/xss-common-includes.h

@@ -82,6 +82,11 @@
     0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, \
             21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31
 
+#if defined(XSS_USE_OPENMP) && defined(_OPENMP)
+#define XSS_COMPILE_OPENMP
+#include <omp.h>
+#endif
+
 template <class... T>
 constexpr bool always_false = false;
 

src/xss-common-keyvaluesort.hpp

@@ -11,11 +11,6 @@
 #include "xss-common-qsort.h"
 #include "xss-network-keyvaluesort.hpp"
 
-#if defined(XSS_USE_OPENMP) && defined(_OPENMP)
-#define XSS_COMPILE_OPENMP
-#include <omp.h>
-#endif
-
 /*
  * Parition one ZMM register based on the pivot and returns the index of the
  * last element that is less than equal to the pivot.

src/xss-common-qsort.h

@@ -521,8 +521,11 @@ template <typename vtype, int maxN>
 void sort_n(typename vtype::type_t *arr, int N);
 
 template <typename vtype, typename comparator, typename type_t>
-static void
-qsort_(type_t *arr, arrsize_t left, arrsize_t right, arrsize_t max_iters)
+static void qsort_(type_t *arr,
+                   arrsize_t left,
+                   arrsize_t right,
+                   arrsize_t max_iters,
+                   arrsize_t task_threshold)
 {
     /*
      * Resort to std::sort if quicksort isnt making any progress
@@ -559,10 +562,40 @@ qsort_(type_t *arr, arrsize_t left, arrsize_t right, arrsize_t max_iters)
     type_t leftmostValue = comparator::leftmost(smallest, biggest);
     type_t rightmostValue = comparator::rightmost(smallest, biggest);
 
+#ifdef XSS_COMPILE_OPENMP
+    if (pivot != leftmostValue) {
+        bool parallel_left = (pivot_index - left) > task_threshold;
+        if (parallel_left) {
+#pragma omp task
+            qsort_<vtype, comparator>(
+                    arr, left, pivot_index - 1, max_iters - 1, task_threshold);
+        }
+        else {
+            qsort_<vtype, comparator>(
+                    arr, left, pivot_index - 1, max_iters - 1, task_threshold);
+        }
+    }
+    if (pivot != rightmostValue) {
+        bool parallel_right = (right - pivot_index) > task_threshold;
+
+        if (parallel_right) {
+#pragma omp task
+            qsort_<vtype, comparator>(
+                    arr, pivot_index, right, max_iters - 1, task_threshold);
+        }
+        else {
+            qsort_<vtype, comparator>(
+                    arr, pivot_index, right, max_iters - 1, task_threshold);
+        }
+    }
+#else
+    UNUSED(task_threshold);
+
     if (pivot != leftmostValue)
-        qsort_<vtype, comparator>(arr, left, pivot_index - 1, max_iters - 1);
+        qsort_<vtype, comparator>(arr, left, pivot_index - 1, max_iters - 1, 0);
     if (pivot != rightmostValue)
-        qsort_<vtype, comparator>(arr, pivot_index, right, max_iters - 1);
+        qsort_<vtype, comparator>(arr, pivot_index, right, max_iters - 1, 0);
+#endif
 }
 
 template <typename vtype, typename comparator, typename type_t>
@@ -627,8 +660,41 @@ X86_SIMD_SORT_INLINE void xss_qsort(T *arr, arrsize_t arrsize, bool hasnan)
         }
 
         UNUSED(hasnan);
+
+#ifdef XSS_COMPILE_OPENMP
+
+        bool use_parallel = arrsize > 10000;
+
+        if (use_parallel) {
+            // This thread limit was determined experimentally; it may be better for it to be the number of physical cores on the system
+            constexpr int thread_limit = 8;
+            int thread_count = std::min(thread_limit, omp_get_max_threads());
+            arrsize_t task_threshold
+                    = std::max((arrsize_t)10000, arrsize / 100);
+
+            // We use omp parallel and then omp single to setup the threads that will run the omp task calls in qsort_
+            // The omp single prevents multiple threads from running the initial qsort_ simultaneously and causing problems
+            // Note that we do not use the if(...) clause built into OpenMP, because it causes a performance regression for small arrays
+#pragma omp parallel num_threads(thread_count)
+#pragma omp single
+            qsort_<vtype, comparator, T>(arr,
+                                         0,
+                                         arrsize - 1,
+                                         2 * (arrsize_t)log2(arrsize),
+                                         task_threshold);
+        }
+        else {
+            qsort_<vtype, comparator, T>(arr,
+                                         0,
+                                         arrsize - 1,
+                                         2 * (arrsize_t)log2(arrsize),
+                                         std::numeric_limits<arrsize_t>::max());
+        }
+#pragma omp taskwait
+#else
         qsort_<vtype, comparator, T>(
-                arr, 0, arrsize - 1, 2 * (arrsize_t)log2(arrsize));
+                arr, 0, arrsize - 1, 2 * (arrsize_t)log2(arrsize), 0);
+#endif
 
         replace_inf_with_nan(arr, arrsize, nan_count, descending);
     }

tests/test-qsort.cpp

@@ -11,6 +11,10 @@ class simdsort : public ::testing::Test {
     simdsort()
     {
         std::iota(arrsize.begin(), arrsize.end(), 1);
+        arrsize.push_back(10'000);
+        arrsize.push_back(100'000);
+        arrsize.push_back(1'000'000);
+
         arrtype = {"random",
                    "constant",
                    "sorted",