base/metrics/stats_table.h

   1 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4 //
   5 // A StatsTable is a table of statistics.  It can be used across multiple
   6 // processes and threads, maintaining cheap statistics counters without
   7 // locking.
   8 //
   9 // The goal is to make it very cheap and easy for developers to add
  10 // counters to code, without having to build one-off utilities or mechanisms
  11 // to track the counters, and also to allow a single "view" to display
  12 // the contents of all counters.
  13 //
  14 // To achieve this, StatsTable creates a shared memory segment to store
  15 // the data for the counters.  Upon creation, it has a specific size
  16 // which governs the maximum number of counters and concurrent
  17 // threads/processes which can use it.
  18 //
  19
  20 #ifndef BASE_METRICS_STATS_TABLE_H_
  21 #define BASE_METRICS_STATS_TABLE_H_
  22
  23 #include <string>
  24
  25 #include "base/base_export.h"
  26 #include "base/basictypes.h"
  27 #include "base/containers/hash_tables.h"
  28 #include "base/memory/shared_memory.h"
  29 #include "base/synchronization/lock.h"
  30 #include "base/threading/thread_local_storage.h"
  31
  32 namespace base {
  33
  34 class BASE_EXPORT StatsTable {
  35  public:
  36   // Create a new StatsTable.
  37   // If a StatsTable already exists with the specified name, this StatsTable
  38   // will use the same shared memory segment as the original.  Otherwise,
  39   // a new StatsTable is created and all counters are zeroed.
  40   //
  41   // name is the name of the StatsTable to use.
  42   //
  43   // max_threads is the maximum number of threads the table will support.
  44   // If the StatsTable already exists, this number is ignored.
  45   //
  46   // max_counters is the maximum number of counters the table will support.
  47   // If the StatsTable already exists, this number is ignored.
  48   StatsTable(const std::string& name, int max_threads, int max_counters);
  49
  50   // Destroys the StatsTable.  When the last StatsTable is destroyed
  51   // (across all processes), the StatsTable is removed from disk.
  52   ~StatsTable();
  53
  54   // For convenience, we create a static table.  This is generally
  55   // used automatically by the counters.
  56   static StatsTable* current();
  57
  58   // Set the global table for use in this process.
  59   static void set_current(StatsTable* value);
  60
  61   // Get the slot id for the calling thread. Returns 0 if no
  62   // slot is assigned.
  63   int GetSlot() const;
  64
  65   // All threads that contribute data to the table must register with the
  66   // table first.  This function will set thread local storage for the
  67   // thread containing the location in the table where this thread will
  68   // write its counter data.
  69   //
  70   // name is just a debugging tag to label the thread, and it does not
  71   // need to be unique.  It will be truncated to kMaxThreadNameLength-1
  72   // characters.
  73   //
  74   // On success, returns the slot id for this thread.  On failure,
  75   // returns 0.
  76   int RegisterThread(const std::string& name);
  77
  78   // Returns the number of threads currently registered.  This is really not
  79   // useful except for diagnostics and debugging.
  80   int CountThreadsRegistered() const;
  81
  82   // Find a counter in the StatsTable.
  83   //
  84   // Returns an id for the counter which can be used to call GetLocation().
  85   // If the counter does not exist, attempts to create a row for the new
  86   // counter.  If there is no space in the table for the new counter,
  87   // returns 0.
  88   int FindCounter(const std::string& name);
  89
  90   // TODO(mbelshe): implement RemoveCounter.
  91
  92   // Gets the location of a particular value in the table based on
  93   // the counter id and slot id.
  94   int* GetLocation(int counter_id, int slot_id) const;
  95
  96   // Gets the counter name at a particular row.  If the row is empty,
  97   // returns NULL.
  98   const char* GetRowName(int index) const;
  99
 100   // Gets the sum of the values for a particular row.
 101   int GetRowValue(int index) const;
 102
 103   // Gets the sum of the values for a particular row for a given pid.
 104   int GetRowValue(int index, int pid) const;
 105
 106   // Gets the sum of the values for a particular counter.  If the counter
 107   // does not exist, creates the counter.
 108   int GetCounterValue(const std::string& name);
 109
 110   // Gets the sum of the values for a particular counter for a given pid.
 111   // If the counter does not exist, creates the counter.
 112   int GetCounterValue(const std::string& name, int pid);
 113
 114   // The maxinum number of counters/rows in the table.
 115   int GetMaxCounters() const;
 116
 117   // The maxinum number of threads/columns in the table.
 118   int GetMaxThreads() const;
 119
 120 #if defined(OS_POSIX)
 121   // Get the underlying shared memory handle for the table.
 122   base::SharedMemoryHandle GetSharedMemoryHandle() const;
 123 #endif
 124
 125   // The maximum length (in characters) of a Thread's name including
 126   // null terminator, as stored in the shared memory.
 127   static const int kMaxThreadNameLength = 32;
 128
 129   // The maximum length (in characters) of a Counter's name including
 130   // null terminator, as stored in the shared memory.
 131   static const int kMaxCounterNameLength = 64;
 132
 133   // Convenience function to lookup a counter location for a
 134   // counter by name for the calling thread.  Will register
 135   // the thread if it is not already registered.
 136   static int* FindLocation(const char *name);
 137
 138  private:
 139   class Internal;
 140   struct TLSData;
 141   typedef hash_map<std::string, int> CountersMap;
 142
 143   // Returns the space occupied by a thread in the table.  Generally used
 144   // if a thread terminates but the process continues.  This function
 145   // does not zero out the thread's counters.
 146   // Cannot be used inside a posix tls destructor.
 147   void UnregisterThread();
 148
 149   // This variant expects the tls data to be passed in, so it is safe to
 150   // call from inside a posix tls destructor (see doc for pthread_key_create).
 151   void UnregisterThread(TLSData* tls_data);
 152
 153   // The SlotReturnFunction is called at thread exit for each thread
 154   // which used the StatsTable.
 155   static void SlotReturnFunction(void* data);
 156
 157   // Locates a free slot in the table.  Returns a number > 0 on success,
 158   // or 0 on failure.  The caller must hold the shared_memory lock when
 159   // calling this function.
 160   int FindEmptyThread() const;
 161
 162   // Locates a counter in the table or finds an empty row.  Returns a
 163   // number > 0 on success, or 0 on failure.  The caller must hold the
 164   // shared_memory_lock when calling this function.
 165   int FindCounterOrEmptyRow(const std::string& name) const;
 166
 167   // Internal function to add a counter to the StatsTable.  Assumes that
 168   // the counter does not already exist in the table.
 169   //
 170   // name is a unique identifier for this counter, and will be truncated
 171   // to kMaxCounterNameLength-1 characters.
 172   //
 173   // On success, returns the counter_id for the newly added counter.
 174   // On failure, returns 0.
 175   int AddCounter(const std::string& name);
 176
 177   // Get the TLS data for the calling thread.  Returns NULL if none is
 178   // initialized.
 179   TLSData* GetTLSData() const;
 180
 181   Internal* internal_;
 182
 183   // The counters_lock_ protects the counters_ hash table.
 184   base::Lock counters_lock_;
 185
 186   // The counters_ hash map is an in-memory hash of the counters.
 187   // It is used for quick lookup of counters, but is cannot be used
 188   // as a substitute for what is in the shared memory.  Even though
 189   // we don't have a counter in our hash table, another process may
 190   // have created it.
 191   CountersMap counters_;
 192   ThreadLocalStorage::Slot tls_index_;
 193
 194   DISALLOW_COPY_AND_ASSIGN(StatsTable);
 195 };
 196
 197 }  // namespace base
 198
 199 #endif  // BASE_METRICS_STATS_TABLE_H_