ProcessMetrics::GetCPUUsage() is Slow

base/process/process_metrics_linux.cc

Index: base/process/process_metrics_linux.cc
diff --git a/base/process/process_metrics_linux.cc b/base/process/process_metrics_linux.cc
index e8db571a317b380b17e7efc5c0595bce35faffb8..e72b8b4e424e5bcc13ecd83fc675f977a5247f54 100644
--- a/base/process/process_metrics_linux.cc
+++ b/base/process/process_metrics_linux.cc
@@ -144,7 +144,7 @@ int GetProcessCPU(pid_t pid) {
     FilePath stat_path =
         task_path.Append(ent->d_name).Append(internal::kStatFile);
     if (ReadFileToString(stat_path, &stat)) {
-      int cpu = ParseProcStatCPU(stat);
+      int cpu = GetCpuFromProcStats(stat);
       if (cpu > 0)
         total_cpu += cpu;
     }
@@ -412,6 +412,46 @@ int ParseProcStatCPU(const std::string& input) {
   return utime + stime;
 }
 
+// Exposed for testing.

[danakj, 2015/02/03 18:51:09]

this comment is in a weird place, itd expect it in a header where exposed or not
is decided

[afakhry, 2015/02/03 20:23:54]

I was following the same style of the original ParseProcStatCPU(). I removed it
anyway in this patch.

+int GetCpuFromProcStats(const std::string& input) {
+  if (input.empty())
+    return -1;
+
+  int j = 0;
+  int last_space_index = 0;
+  int utime_start_index = 0;
+  bool prevWasSpace = false;

[danakj, 2015/02/03 18:51:09]

fix style

[afakhry, 2015/02/03 20:23:54]

Done.

+  for (auto& ch : input) {
+    if (ch == ' ') {
+      if (!prevWasSpace)
+        ++last_space_index;
+
+      prevWasSpace = true;
+    } else {
+      if (ch == ')') {
+        // Restart the counter, we're looking to count from the last ')' we
+        // encounter.
+        last_space_index = 0;
+      } else if (prevWasSpace && last_space_index == internal::VM_UTIME - 2) {
+        utime_start_index = j + 1;
+        break;
+      }
+      prevWasSpace = false;
+    }
+    ++j;
+  }
+
+  if (last_space_index < internal::VM_UTIME - 2)
+    return -1;
+
+  int utime = 0;
+  int stime = 0;
+  if (sscanf(&input.data()[utime_start_index], "%d %d", &utime, &stime) != 2)
+    return -1;
+
+  return utime + stime;
+}
+
 const char kProcSelfExe[] = "/proc/self/exe";
 
 int GetNumberOfThreads(ProcessHandle process) {