[v1.05] Adapt frequency measurement iterations depending on CPU speed

This is achieved by running a first measurement which gets a taste of
CPU speed, then estimate a reasonable value for the iterations of the
real measurement and then running the actual measurement. This is very
helpful to reduce the runtime of the measurement, especially for slow
CPUs

src/common/freq.c

@@ -69,28 +69,15 @@ void nop_function(uint64_t iters) {
   }
 }
 
-// Differences between x86 measure_frequency this measure_max_frequency:
+// Run the nop_function with the number of iterations specified and
-// - measure_frequency employs all cores simultaneously wherease 
+// measure both the time and number of cycles
-//   measure_max_frequency only employs 1.
+int measure_freq_iters(uint64_t iters, uint32_t core, double* freq) {
-// - measure_frequency runs the computation and checks /proc/cpuinfo whereas
-//   measure_max_frequency does not rely on /proc/cpuinfo and simply
-//   counts cpu cycles to measure frequency.
-// - measure_frequency uses actual computation while measuring the frequency
-//   whereas measure_max_frequency uses nop instructions. This makes the former
-//   x86 dependant whereas the latter is architecture independant.
-int64_t measure_max_frequency(uint32_t core) {
-  if (!bind_to_cpu(core)) {
-    printErr("Failed binding the process to CPU %d", core);
-    return -1;
-  }
-
   clockid_t clock = CLOCK_PROCESS_CPUTIME_ID;
-
+  struct timespec start, end;
   struct perf_event_attr pe;
-  uint64_t instructions;
+  uint64_t cycles;
   int fd;
   int pid = 0;
-
   memset(&pe, 0, sizeof(struct perf_event_attr));
   pe.type = PERF_TYPE_HARDWARE;
   pe.size = sizeof(struct perf_event_attr);
@@ -109,12 +96,6 @@ int64_t measure_max_frequency(uint32_t core) {
     return -1;
   }
 
-  const char* frequency_banner = "cpufetch is measuring the max frequency...";
-  printf("%s", frequency_banner);
-  fflush(stdout);
-
-  uint64_t iters = 10000000;
-  struct timespec start, end;
   if (clock_gettime(clock, &start) == -1) {
     perror("clock_gettime");
     return -1;
@@ -130,10 +111,7 @@ int64_t measure_max_frequency(uint32_t core) {
 
   nop_function(iters);
 
-  // Clean screen once measurement is finished
+  ssize_t ret = read(fd, &cycles, sizeof(uint64_t));
-  printf("\r%*c\r", (int) strlen(frequency_banner), ' ');
-
-  ssize_t ret = read(fd, &instructions, sizeof(uint64_t));
   if (ret == -1) {
     perror("read");
     return -1;
@@ -153,7 +131,60 @@ int64_t measure_max_frequency(uint32_t core) {
 
   uint64_t nsecs = (end.tv_sec*1e9 + end.tv_nsec) - (start.tv_sec*1e9 + start.tv_nsec);
   uint64_t usecs = nsecs/1000;  
-  double frequency = instructions/((double)usecs);
+  *freq = cycles/((double)usecs);
+  return 0;
+}
+
+// Return a good number of iterations to run the nop_function in
+// order to get a precise measurement of the frequency without taking
+// too much time.
+uint64_t get_num_iters_from_freq(double frequency) {
+  // Truncate to reduce variability
+  uint64_t freq_trunc = ((uint64_t) frequency / 100) * 100;
+  uint64_t osp_per_iter = 4 * 1000;
+
+  return freq_trunc * 1e7 * 1/osp_per_iter;
+}
+
+// Differences between x86 measure_frequency and this measure_max_frequency:
+// - measure_frequency employs all cores simultaneously whereas
+//   measure_max_frequency only employs 1.
+// - measure_frequency runs the computation and checks /proc/cpuinfo whereas
+//   measure_max_frequency does not rely on /proc/cpuinfo and simply
+//   counts cpu cycles to measure frequency.
+// - measure_frequency uses actual computation while measuring the frequency
+//   whereas measure_max_frequency uses nop instructions. This makes the former
+//   x86 dependant whereas the latter is architecture independant.
+int64_t measure_max_frequency(uint32_t core) {
+  if (!bind_to_cpu(core)) {
+    printErr("Failed binding the process to CPU %d", core);
+    return -1;
+  }
+
+  // First, get very rough estimation of clock cycle to
+  // compute a reasonable value for the iterations
+  double estimation_freq, frequency;
+  uint64_t iters = 100000;
+  if (measure_freq_iters(iters, core, &estimation_freq) == -1)
+    return -1;
+
+  if (estimation_freq <= 0.0) {
+    printErr("First frequency measurement yielded an invalid value: %f", estimation_freq);
+    return -1;
+  }
+  iters = get_num_iters_from_freq(estimation_freq);
+  printWarn("Running frequency measurement with %ld iterations on core %d...", iters, core);
+
+  // Now perform actual measurement
+  const char* frequency_banner = "cpufetch is measuring the max frequency...";
+  printf("%s", frequency_banner);
+  fflush(stdout);
+
+  if (measure_freq_iters(iters, core, &frequency) == -1)
+    return -1; 
+  
+  // Clean screen once measurement is finished
+  printf("\r%*c\r", (int) strlen(frequency_banner), ' ');
   
   // Discard last digit in the frequency, which should help providing
   // more reliable and predictable values.