Introduce ThreadPool and use it for quicksort

.github/workflows/c-cpp.yml

@@ -235,7 +235,7 @@ jobs:
         CXX: g++-10
       run: |
         make clean
-        meson setup -Dbuild_tests=true -Duse_openmp=true --warnlevel 2 --werror --buildtype release builddir
+        meson setup -Dbuild_tests=true -Duse_openmp=true --Duse_stdthreads=true --warnlevel 2 --werror --buildtype release builddir
         cd builddir
         ninja
 

Makefile

@@ -6,6 +6,10 @@ test_openmp:
 	meson setup -Dbuild_tests=true -Duse_openmp=true --warnlevel 2 --werror --buildtype release builddir
 	cd builddir && ninja
 
+test_stdthreads:
+	meson setup -Dbuild_tests=true -Duse_stdthreads=true --warnlevel 2 --werror --buildtype release builddir
+	cd builddir && ninja
+
 test_asan:
 	meson setup -Dbuild_tests=true -Duse_openmp=true -Db_sanitize=address,undefined -Dfatal_sanitizers=true -Db_lundef=false -Dasan_ci_dont_validate=true --warnlevel 0 --buildtype debugoptimized builddir
 	cd builddir && ninja

lib/meson.build

@@ -1,18 +1,13 @@
 libtargets = []
 
-# Add compile flags for OpenMP if enabled
-openmpflags = []
-if get_option('use_openmp')
-  openmpflags = ['-DXSS_USE_OPENMP=true', '-fopenmp']
-endif
-
 if cpp.has_argument('-march=haswell')
   libtargets += static_library('libavx',
     files(
       'x86simdsort-avx2.cpp',
       ),
     include_directories : [src],
-    cpp_args : ['-march=haswell', openmpflags],
+    cpp_args : ['-march=haswell', stdthreadsflag],
+    dependencies: [omp_dep],
     gnu_symbol_visibility : 'inlineshidden',
     )
 endif
@@ -23,7 +18,8 @@ if cpp.has_argument('-march=skylake-avx512')
       'x86simdsort-skx.cpp',
       ),
     include_directories : [src],
-    cpp_args : ['-march=skylake-avx512', openmpflags],
+    cpp_args : ['-march=skylake-avx512', stdthreadsflag],
+    dependencies: [omp_dep],
     gnu_symbol_visibility : 'inlineshidden',
     )
 endif
@@ -34,7 +30,8 @@ if cpp.has_argument('-march=icelake-client')
       'x86simdsort-icl.cpp',
       ),
     include_directories : [src],
-    cpp_args : ['-march=icelake-client', openmpflags],
+    cpp_args : ['-march=icelake-client', stdthreadsflag],
+    dependencies: [omp_dep],
     gnu_symbol_visibility : 'inlineshidden',
     )
 endif
@@ -45,7 +42,8 @@ if cancompilefp16
       'x86simdsort-spr.cpp',
       ),
     include_directories : [src],
-    cpp_args : ['-march=sapphirerapids', openmpflags],
+    cpp_args : ['-march=sapphirerapids', stdthreadsflag],
+    dependencies: [omp_dep],
     gnu_symbol_visibility : 'inlineshidden',
     )
 endif

meson.build

@@ -29,6 +29,20 @@ if get_option('build_vqsortbench')
   benchvq = true
 endif
 
+# build with openmp
+omp = []
+omp_dep = []
+if get_option('use_openmp')
+  omp = dependency('openmp', required : true)
+  omp_dep = declare_dependency(dependencies: omp, compile_args: ['-DXSS_USE_OPENMP'])
+endif
+
+# build with std::threads
+stdthreadsflag = []
+if get_option('use_stdthreads')
+  stdthreadsflag += ['-DXSS_BUILD_WITH_STD_THREADS']
+endif
+
 fp16code = '''#include<immintrin.h>
 int main() {
   __m512h temp = _mm512_set1_ph(1.0f);
@@ -43,8 +57,8 @@ if get_option('lib_type') == 'shared'
   libsimdsort = shared_library('x86simdsortcpp',
                              'lib/x86simdsort.cpp',
                              include_directories : [src, utils, lib],
-                             link_args : [openmpflags],
                              link_with : [libtargets],
+                             dependencies: [omp],
                              gnu_symbol_visibility : 'inlineshidden',
                              install : true,
                              soversion : 1,
@@ -53,7 +67,7 @@ else
   libsimdsort = static_library('x86simdsortcpp',
                              'lib/x86simdsort.cpp',
                              include_directories : [src, utils, lib],
-                             link_args : [openmpflags],
+                             dependencies: [omp],
                              link_with : [libtargets],
                              gnu_symbol_visibility : 'inlineshidden',
                              install : true,

meson_options.txt

@@ -8,6 +8,8 @@ option('build_vqsortbench', type : 'boolean', value : true,
   description : 'Add google vqsort to benchmarks (default: "true").')
 option('use_openmp', type : 'boolean', value : false,
   description : 'Use OpenMP to accelerate key-value sort (default: "false").')
+option('use_stdthreads', type : 'boolean', value : false,
+  description : 'Use std::threads to accelerate qsort (default: "false").')
 option('lib_type', type : 'string', value : 'shared',
   description : 'Library type: shared or static (default: "shared").')
 option('fatal_sanitizers', type : 'boolean', value : 'false',

scripts/bench-compare.sh

@@ -11,7 +11,7 @@ if [ ! -d .bench/google-benchmark ]; then
 fi
 compare=$(realpath .bench/google-benchmark/tools/compare.py)
 
-meson setup -Dbuild_benchmarks=true -Dbuild_ippbench=true --warnlevel 0 --buildtype release builddir-${branch}
+meson setup -Dbuild_benchmarks=true -Duse_stdthreads=true -Duse_openmp=true --warnlevel 0 --buildtype release builddir-${branch}
 cd builddir-${branch}
 ninja
 $compare filters ./benchexe $1 $2 --benchmark_repetitions=$3

scripts/branch-compare.sh

@@ -27,7 +27,7 @@ build_branch() {
         fi
     fi
     cd $dir_name
-    meson setup -Dbuild_benchmarks=true -Duse_openmp=true --warnlevel 0 --buildtype release builddir
+    meson setup -Dbuild_benchmarks=true -Duse_openmp=true -Duse_stdthreads=true --warnlevel 0 --buildtype release builddir
     cd builddir
     ninja
     cd ../../

src/avx512-16bit-qsort.hpp

@@ -548,25 +548,35 @@ avx512_qsort_fp16_helper(uint16_t *arr, arrsize_t arrsize)
     using T = uint16_t;
     using vtype = zmm_vector<float16>;
 
-#ifdef XSS_COMPILE_OPENMP
+#ifdef XSS_BUILD_WITH_STD_THREADS
     bool use_parallel = arrsize > 100000;
+#else
+    bool use_parallel = false;
+#endif
 
     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
+#ifdef XSS_BUILD_WITH_STD_THREADS
+
+        // This thread limit was determined experimentally
         constexpr int thread_limit = 8;
-        int thread_count = std::min(thread_limit, omp_get_max_threads());
+        int thread_count = std::min(thread_limit,
+                                    (int)std::thread::hardware_concurrency());
         arrsize_t task_threshold = std::max((arrsize_t)100000, 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);
+        // Create a thread pool
+        ThreadPool pool(thread_count);
+
+        // Initial sort task
+        qsort_threads<vtype, comparator, T>(arr,
+                                            0,
+                                            arrsize - 1,
+                                            2 * (arrsize_t)log2(arrsize),
+                                            task_threshold,
+                                            pool);
+
+        // Wait for all tasks to complete
+        pool.wait_all();
+#endif
     }
     else {
         qsort_<vtype, comparator, T>(arr,
@@ -575,11 +585,6 @@ avx512_qsort_fp16_helper(uint16_t *arr, arrsize_t arrsize)
                                      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), 0);
-#endif
 }
 
 [[maybe_unused]] X86_SIMD_SORT_INLINE void