Better exercice !

5-CollectiveDay/4-Optimization/en.2-operations.ipynb

@@ -1006,12 +1006,15 @@
    "source": [
     "# Exercice\n",
     "\n",
-    "Rather than looking at each isolated code line, it is often profitable to take a look at the big picture : how to simply move one part of the computation, so to get exactly the same output, a lot faster ?"
+    "Let's assume that we always run the program below with a `degree`which is even.\n",
+    "1. Replace `val *= data` with `val *= data*data`, and `j<degree` with `j<(degree/2)`. What is the effect on the execution time ?\n",
+    "2. Ask for `std::cout.precision(18)`, and compare the original and the previous implementation. Why the result is slightly differing ?\n",
+    "3. Try to compile with `-Ofast`."
    ]
   },
   {
    "cell_type": "code",
-   "execution_count": 521,
+   "execution_count": 536,
    "metadata": {
     "slideshow": {
      "slide_type": "skip"
@@ -1029,50 +1032,50 @@
    "source": [
     "%%file tmp.operations.cpp\n",
     "\n",
+    "#include <valarray>\n",
     "#include <cstdlib>\n",
-    "#include <array>\n",
-    "\n",
-    "int const SIZE = 1024 ;\n",
-    "int const REPEAT = 100000 ;\n",
-    "\n",
-    "using Array = std::array<double,SIZE> ;\n",
+    "#include <cassert>\n",
+    "#include <iostream>\n",
     "\n",
-    "void randomize( Array & x ) {\n",
-    "  srand(1) ;\n",
-    "  for ( int i=0 ; i<SIZE ; ++i )\n",
-    "    x[i] = (std::rand()/(RAND_MAX+1.)-0.5) ;\n",
-    "}\n",
+    "std::valarray<double> generate( int size )\n",
+    " {\n",
+    "  std::valarray<double> datas(size) ;\n",
+    "  for ( double & data : datas )\n",
+    "   { data = std::rand()/(RAND_MAX+1.) ; }\n",
+    "  return datas ;\n",
+    " }\n",
     "\n",
-    "void multiply( Array const & x, Array & y ) {\n",
-    "  for ( int r=0 ; r<REPEAT ; ++r )\n",
-    "    for ( int i=0 ; i<SIZE ; ++i )\n",
-    "      y[i] += x[i]/10. ;\n",
-    "}\n",
-    "      \n",
-    "void reduce( Array const & y ) {\n",
-    "  double res {0.} ;\n",
-    "  for ( int i=0 ; i<SIZE ; ++i ) {\n",
-    "    res += y[i] ;\n",
-    "  }\n",
-    "  std::cout<<(res/SIZE)<<std::endl ;\n",
-    "}\n",
+    "double pow_reduce( std::valarray<double> const & datas, int degree )\n",
+    " {\n",
+    "  double res = 0 ;\n",
+    "  for ( double data : datas )\n",
+    "   {\n",
+    "    double val = 1 ; \n",
+    "    for ( int j=0 ; j<degree ; ++j )\n",
+    "      val *= data ;\n",
+    "    res += val ;\n",
+    "   }\n",
+    "  return res ;\n",
+    " }\n",
     "\n",
-    "int main()\n",
+    "int main( int argc, char * argv[] )\n",
     " {\n",
-    "  Array x, y ;\n",
-    "  randomize(x) ;\n",
-    "  y.fill(0.) ;\n",
-    "  multiply(x,y) ;\n",
-    "  reduce(y) ;\n",
+    "  assert(argc==3) ;\n",
+    "  int size {atoi(argv[1])} ;\n",
+    "  int degree {atoi(argv[2])} ;\n",
+    "\n",
+    "  auto datas = generate(size) ;\n",
+    "  auto res = pow_reduce(datas,degree) ;\n",
+    "  std::cout << res << std::endl ;\n",
     " }"
    ]
   },
   {
    "cell_type": "code",
-   "execution_count": 522,
+   "execution_count": 545,
    "metadata": {
     "slideshow": {
-     "slide_type": "subslide"
+     "slide_type": "skip"
     }
    },
    "outputs": [
@@ -1088,24 +1091,16 @@
     "%%file tmp.operations.sh\n",
     "\n",
     "rm -f tmp.operations.exe\n",
-    "g++ -std=c++17 -O1 tmp.operations.cpp -o tmp.operations.exe\n",
-    "./tmp.operations.exe $*\n",
-    "\n",
-    "rm -f tmp.operations.py\n",
-    "echo \"t = 0\" >> tmp.operations.py\n",
-    "for i in 0 1 2 3 4 5 6 7 8 9\n",
-    "do \\time -f \"t += %U\" -a -o ./tmp.operations.py ./tmp.operations.exe $* >> /dev/null\n",
-    "done\n",
-    "echo \"print('(~ {:.3f} s)'.format(t/10.))\" >> tmp.operations.py\n",
-    "python3 tmp.operations.py"
+    "g++ -std=c++17 -O3 tmp.operations.cpp -o tmp.operations.exe\n",
+    "time ./tmp.operations.exe $*"
    ]
   },
   {
    "cell_type": "code",
-   "execution_count": 523,
+   "execution_count": 546,
    "metadata": {
     "slideshow": {
-     "slide_type": "subslide"
+     "slide_type": "skip"
     }
    },
    "outputs": [
@@ -1113,13 +1108,16 @@
      "name": "stdout",
      "output_type": "stream",
      "text": [
-      "67.5054\n",
-      "(~ 0.142 s)\n"
+      "0.656155\r\n",
+      "\r\n",
+      "real\t0m1.767s\r\n",
+      "user\t0m1.767s\r\n",
+      "sys\t0m0.000s\r\n"
      ]
     }
    ],
    "source": [
-    "!bash -l ./tmp.operations.sh"
+    "!bash -l ./tmp.operations.sh 1024 65536"
    ]
   },
   {