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..0d923af50e3d1112680f585f7832a3492b6dc560 100644
--- a/base/process/process_metrics_linux.cc
+++ b/base/process/process_metrics_linux.cc
@@ -399,16 +399,37 @@ size_t GetSystemCommitCharge() {
   return meminfo.total - meminfo.free - meminfo.buffers - meminfo.cached;
 }
 
-// Exposed for testing.
 int ParseProcStatCPU(const std::string& input) {
-  std::vector<std::string> proc_stats;
-  if (!internal::ParseProcStats(input, &proc_stats))
+  // |input| may be empty if the process disappeared somehow.
+  // e.g. http://crbug.com/145811.
+  if (input.empty())
     return -1;
 
-  if (proc_stats.size() <= internal::VM_STIME)
+  size_t start = input.find_last_of(')');
+  if (start == input.npos)
     return -1;
-  int utime = GetProcStatsFieldAsInt64(proc_stats, internal::VM_UTIME);
-  int stime = GetProcStatsFieldAsInt64(proc_stats, internal::VM_STIME);
+
+  // Number of spaces remaining until reaching utime's index starting after the
+  // last ')'.
+  int num_spaces_remaining = internal::VM_UTIME - 1;
+  int utime_start_index = 0;
+
+  size_t i = start;
+  while ((i = input.find(' ', i + 1)) != input.npos) {
+    if (input[i - 1] != ' ' && --num_spaces_remaining == 0) {

[danakj, 2015/02/04 18:33:06]

can you leave a comment like // Skips past contiguous spaces without counting
them?

btw none of the tests seem to have more than one space between values. can that
actually happen?

[afakhry, 2015/02/04 19:05:20]

It's unlikey but it's better be safe. The original code used SplitString which
took care of that.  

+      utime_start_index = i;
+      break;
+    }
+  }
+
+  if (num_spaces_remaining != 0)

[danakj, 2015/02/04 18:33:06]

nit: maybe slightly more obvious to check utime_start_index == 0?

[afakhry, 2015/02/04 19:05:20]

Done.

+    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;
 }