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/hash_tables.h"
  28 #include "base/synchronization/lock.h"
  29 #include "base/threading/thread_local_storage.h"
  30
  31 namespace base {
  32
  33 class BASE_EXPORT StatsTable {
  34  public:
  35   // Create a new StatsTable.
  36   // If a StatsTable already exists with the specified name, this StatsTable
  37   // will use the same shared memory segment as the original.  Otherwise,
  38   // a new StatsTable is created and all counters are zeroed.
  39   //
  40   // name is the name of the StatsTable to use.
  41   //
  42   // max_threads is the maximum number of threads the table will support.
  43   // If the StatsTable already exists, this number is ignored.
  44   //
  45   // max_counters is the maximum number of counters the table will support.
  46   // If the StatsTable already exists, this number is ignored.
  47   StatsTable(const std::string& name, int max_threads, int max_counters);
  48
  49   // Destroys the StatsTable.  When the last StatsTable is destroyed
  50   // (across all processes), the StatsTable is removed from disk.
  51   ~StatsTable();
  52
  53   // For convenience, we create a static table.  This is generally
  54   // used automatically by the counters.
  55   static StatsTable* current() { return global_table_; }
  56
  57   // Set the global table for use in this process.
  58   static void set_current(StatsTable* value) { global_table_ = value; }
  59
  60   // Get the slot id for the calling thread. Returns 0 if no
  61   // slot is assigned.
  62   int GetSlot() const;
  63
  64   // All threads that contribute data to the table must register with the
  65   // table first.  This function will set thread local storage for the
  66   // thread containing the location in the table where this thread will
  67   // write its counter data.
  68   //
  69   // name is just a debugging tag to label the thread, and it does not
  70   // need to be unique.  It will be truncated to kMaxThreadNameLength-1
  71   // characters.
  72   //
  73   // On success, returns the slot id for this thread.  On failure,
  74   // returns 0.
  75   int RegisterThread(const std::string& name);
  76
  77   // Returns the number of threads currently registered.  This is really not
  78   // useful except for diagnostics and debugging.
  79   int CountThreadsRegistered() const;
  80
  81   // Find a counter in the StatsTable.
  82   //
  83   // Returns an id for the counter which can be used to call GetLocation().
  84   // If the counter does not exist, attempts to create a row for the new
  85   // counter.  If there is no space in the table for the new counter,
  86   // returns 0.
  87   int FindCounter(const std::string& name);
  88
  89   // TODO(mbelshe): implement RemoveCounter.
  90
  91   // Gets the location of a particular value in the table based on
  92   // the counter id and slot id.
  93   int* GetLocation(int counter_id, int slot_id) const;
  94
  95   // Gets the counter name at a particular row.  If the row is empty,
  96   // returns NULL.
  97   const char* GetRowName(int index) const;
  98
  99   // Gets the sum of the values for a particular row.
 100   int GetRowValue(int index) const;
 101
 102   // Gets the sum of the values for a particular row for a given pid.
 103   int GetRowValue(int index, int pid) const;
 104
 105   // Gets the sum of the values for a particular counter.  If the counter
 106   // does not exist, creates the counter.
 107   int GetCounterValue(const std::string& name);
 108
 109   // Gets the sum of the values for a particular counter for a given pid.
 110   // If the counter does not exist, creates the counter.
 111   int GetCounterValue(const std::string& name, int pid);
 112
 113   // The maxinum number of counters/rows in the table.
 114   int GetMaxCounters() const;
 115
 116   // The maxinum number of threads/columns in the table.
 117   int GetMaxThreads() const;
 118
 119   // The maximum length (in characters) of a Thread's name including
 120   // null terminator, as stored in the shared memory.
 121   static const int kMaxThreadNameLength = 32;
 122
 123   // The maximum length (in characters) of a Counter's name including
 124   // null terminator, as stored in the shared memory.
 125   static const int kMaxCounterNameLength = 64;
 126
 127   // Convenience function to lookup a counter location for a
 128   // counter by name for the calling thread.  Will register
 129   // the thread if it is not already registered.
 130   static int* FindLocation(const char *name);
 131
 132  private:
 133   class Private;
 134   struct TLSData;
 135   typedef hash_map<std::string, int> CountersMap;
 136
 137   // Returns the space occupied by a thread in the table.  Generally used
 138   // if a thread terminates but the process continues.  This function
 139   // does not zero out the thread's counters.
 140   // Cannot be used inside a posix tls destructor.
 141   void UnregisterThread();
 142
 143   // This variant expects the tls data to be passed in, so it is safe to
 144   // call from inside a posix tls destructor (see doc for pthread_key_create).
 145   void UnregisterThread(TLSData* tls_data);
 146
 147   // The SlotReturnFunction is called at thread exit for each thread
 148   // which used the StatsTable.
 149   static void SlotReturnFunction(void* data);
 150
 151   // Locates a free slot in the table.  Returns a number > 0 on success,
 152   // or 0 on failure.  The caller must hold the shared_memory lock when
 153   // calling this function.
 154   int FindEmptyThread() const;
 155
 156   // Locates a counter in the table or finds an empty row.  Returns a
 157   // number > 0 on success, or 0 on failure.  The caller must hold the
 158   // shared_memory_lock when calling this function.
 159   int FindCounterOrEmptyRow(const std::string& name) const;
 160
 161   // Internal function to add a counter to the StatsTable.  Assumes that
 162   // the counter does not already exist in the table.
 163   //
 164   // name is a unique identifier for this counter, and will be truncated
 165   // to kMaxCounterNameLength-1 characters.
 166   //
 167   // On success, returns the counter_id for the newly added counter.
 168   // On failure, returns 0.
 169   int AddCounter(const std::string& name);
 170
 171   // Get the TLS data for the calling thread.  Returns NULL if none is
 172   // initialized.
 173   TLSData* GetTLSData() const;
 174
 175   Private* impl_;
 176
 177   // The counters_lock_ protects the counters_ hash table.
 178   base::Lock counters_lock_;
 179
 180   // The counters_ hash map is an in-memory hash of the counters.
 181   // It is used for quick lookup of counters, but is cannot be used
 182   // as a substitute for what is in the shared memory.  Even though
 183   // we don't have a counter in our hash table, another process may
 184   // have created it.
 185   CountersMap counters_;
 186   ThreadLocalStorage::Slot tls_index_;
 187
 188   static StatsTable* global_table_;
 189
 190   DISALLOW_COPY_AND_ASSIGN(StatsTable);
 191 };
 192
 193 }  // namespace base
 194
 195 #endif  // BASE_METRICS_STATS_TABLE_H_