Speed up CTF process name parsing

Cache result of process name parsing to not constantly rebuild the
string.

src/TraceEvent/Ctf/CtfReader.cs

@@ -37,6 +37,7 @@ namespace Microsoft.Diagnostics.Tracing.Ctf
         private CtfMetadata _metadata;
         private CtfStream _streamDefinition;
         CtfEventHeader _header = new CtfEventHeader();
+        public Dictionary<int, string> _processes = new Dictionary<int, string>();
 
         private bool _eof;
         private GCHandle _handle;
@@ -149,6 +150,8 @@ namespace Microsoft.Diagnostics.Tracing.Ctf
                     _header.Pid = eventContext.GetFieldValue<int>(result, "_vpid");
                     _header.Tid = eventContext.GetFieldValue<int>(result, "_vtid");
                     
+                    if (IsProcessStartEvent(evt) || !_processes.TryGetValue(_header.Pid, out _header.ProcessName))
+                    {
                         int procnameIndex = eventContext.GetFieldIndex("_procname");
                         object[] procname = (object[])(result[procnameIndex]);
                         processName.Clear();
@@ -161,7 +164,10 @@ namespace Microsoft.Diagnostics.Tracing.Ctf
                             processName.Append((char)b);
                         }
 
-                    _header.ProcessName = processName.ToString();
+                        string name = processName.ToString();
+                        _header.ProcessName = name;
+                        _processes[_header.Pid] = name;
+                    }
                 }
 
                 _readHeader = true;
@@ -169,6 +175,11 @@ namespace Microsoft.Diagnostics.Tracing.Ctf
             }
         }
 
+        private static bool IsProcessStartEvent(CtfEvent evt)
+        {
+            return evt.Name == "sched_process_exec";
+        }
+
         public void ResetBuffer()
         {
             _bitOffset = 0;

src/TraceEvent/CtfTraceEventSource.cs

@@ -36,6 +36,7 @@ namespace Microsoft.Diagnostics.Tracing
         private List<Tuple<ZipArchiveEntry, CtfMetadata>> _channels;
         private TraceEventNativeMethods.EVENT_RECORD* _header;
         private Dictionary<string, ETWMapping> _eventMapping;
+        private Dictionary<int, string> _processes = new Dictionary<int, string>();
 
 #if DEBUG
         private StreamWriter _debugOut;
@@ -482,6 +483,7 @@ namespace Microsoft.Diagnostics.Tracing
                 CtfEventHeader header = entry.Current;
                 CtfEvent evt = header.Event;
                 lastTimestamp = header.Timestamp;
+                _processes[header.Pid] = header.ProcessName;
 
 #if DEBUG
                 if (_debugOut != null)
@@ -549,6 +551,15 @@ namespace Microsoft.Diagnostics.Tracing
             return _header;
         }
 
+        internal override string ProcessName(int processID, long timeQPC)
+        {
+            string name;
+            if (!_processes.TryGetValue(processID, out name))
+                name = $"Process({processID.ToString("x")})";
+
+            return name;
+        }
+
         private ETWMapping GetTraceEvent(CtfEvent evt)
         {
             ETWMapping result;

src/TraceEvent/Parsers/LinuxKernelTraceEventParser.cs

@@ -72,6 +72,9 @@ namespace Microsoft.Diagnostics.Tracing.Parsers
         private Action<SchedulerProcessTraceData> _action;
         private static string[] _payloadNames;
         
+
+        public int Tid { get { throw new NotImplementedException(); } }
+
         public SchedulerProcessTraceData(Action<SchedulerProcessTraceData> action, int eventID, int task, string taskName, Guid taskGuid, int opcode, string opcodeName, Guid providerGuid, string providerName)
             : base(eventID, task, taskName, taskGuid, opcode, opcodeName, providerGuid, providerName)
         {
@@ -124,7 +127,7 @@ namespace Microsoft.Diagnostics.Tracing.Parsers
         {
             Prefix(sb);
             XmlAttrib(sb, "ProcessID", ProcessID);
-            XmlAttrib(sb, "ImageFileName", ImageFileName);
+            XmlAttrib(sb, "ProcessName", ProcessName);
             sb.Append("/>");
             return sb;
         }