ProcessMetrics::GetCPUUsage() is Slow

base/process/process_metrics_linux.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/process/process_metrics.h"
 
 #include <dirent.h>
 #include <fcntl.h>
 #include <sys/stat.h>
 #include <sys/time.h>
@@ skipping 126 matching lines @@
     if (!tid)
       continue;
 
     // Synchronously reading files in /proc does not hit the disk.
     ThreadRestrictions::ScopedAllowIO allow_io;
 
     std::string stat;
     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;
     }
   }
   closedir(dir);
 
   return total_cpu;
 }
 
 }  // namespace
@@ skipping 247 matching lines @@
   if (!internal::ParseProcStats(input, &proc_stats))
     return -1;
 
   if (proc_stats.size() <= internal::VM_STIME)
     return -1;
   int utime = GetProcStatsFieldAsInt64(proc_stats, internal::VM_UTIME);
   int stime = GetProcStatsFieldAsInt64(proc_stats, internal::VM_STIME);
   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) {
   return internal::ReadProcStatsAndGetFieldAsInt64(process,
                                                    internal::VM_NUMTHREADS);
 }
 
 namespace {
 
 // The format of /proc/diskstats is:
@@ skipping 463 matching lines @@
 #if defined(OS_LINUX)
 int ProcessMetrics::GetIdleWakeupsPerSecond() {
   uint64 wake_ups;
   const char kWakeupStat[] = "se.statistics.nr_wakeups";
   return ReadProcSchedAndGetFieldAsUint64(process_, kWakeupStat, &wake_ups) ?
       CalculateIdleWakeupsPerSecond(wake_ups) : 0;
 }
 #endif  // defined(OS_LINUX)
 
 }  // namespace base