base/tracked_objects.h - chromium/src - Git at Google

 // Copyright (c) 2006-2008 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.

 #ifndef BASE_TRACKED_OBJECTS_H_
 #define BASE_TRACKED_OBJECTS_H_
 #pragma once

 #include <map>
 #include <string>
 #include <vector>

 #include "base/lock.h"
 #include "base/thread_local_storage.h"
 #include "base/tracked.h"

 // TrackedObjects provides a database of stats about objects (generally Tasks)
 // that are tracked.  Tracking means their birth, death, duration, birth thread,
 // death thread, and birth place are recorded.  This data is carefully spread
 // across a series of objects so that the counts and times can be rapidly
 // updated without (usually) having to lock the data, and hence there is usually
 // very little contention caused by the tracking.  The data can be viewed via
 // the about:tasks URL, with a variety of sorting and filtering choices.
 //
 // These classes serve as the basis of a profiler of sorts for the Tasks system.
 // As a result, design decisions were made to maximize speed, by minimizing
 // recurring allocation/deallocation, lock contention and data copying.  In the
 // "stable" state, which is reached relatively quickly, there is no separate
 // marginal allocation cost associated with construction or destruction of
 // tracked objects, no locks are generally employed, and probably the largest
 // computational cost is associated with obtaining start and stop times for
 // instances as they are created and destroyed.  The introduction of worker
 // threads had a slight impact on this approach, and required use of some locks
 // when accessing data from the worker threads.
 //
 // The following describes the lifecycle of tracking an instance.
 //
 // First off, when the instance is created, the FROM_HERE macro is expanded
 // to specify the birth place (file, line, function) where the instance was
 // created.  That data is used to create a transient Location instance
 // encapsulating the above triple of information.  The strings (like __FILE__)
 // are passed around by reference, with the assumption that they are static, and
 // will never go away.  This ensures that the strings can be dealt with as atoms
 // with great efficiency (i.e., copying of strings is never needed, and
 // comparisons for equality can be based on pointer comparisons).
 //
 // Next, a Births instance is created for use ONLY on the thread where this
 // instance was created.  That Births instance records (in a base class
 // BirthOnThread) references to the static data provided in a Location instance,
 // as well as a pointer specifying the thread on which the birth takes place.
 // Hence there is at most one Births instance for each Location on each thread.
 // The derived Births class contains slots for recording statistics about all
 // instances born at the same location.  Statistics currently include only the
 // count of instances constructed.
 // Since the base class BirthOnThread contains only constant data, it can be
 // freely accessed by any thread at any time (i.e., only the statistic needs to
 // be handled carefully, and it is ONLY read or written by the birth thread).
 //
 // Having now either constructed or found the Births instance described above, a
 // pointer to the Births instance is then embedded in a base class of the
 // instance we're tracking (usually a Task). This fact alone is very useful in
 // debugging, when there is a question of where an instance came from.  In
 // addition, the birth time is also embedded in the base class Tracked (see
 // tracked.h), and used to later evaluate the lifetime duration.
 // As a result of the above embedding, we can (for any tracked instance) find
 // out its location of birth, and thread of birth, without using any locks, as
 // all that data is constant across the life of the process.
 //
 // The amount of memory used in the above data structures depends on how many
 // threads there are, and how many Locations of construction there are.
 // Fortunately, we don't use memory that is the product of those two counts, but
 // rather we only need one Births instance for each thread that constructs an
 // instance at a Location. In many cases, instances (such as Tasks) are only
 // created on one thread, so the memory utilization is actually fairly
 // restrained.
 //
 // Lastly, when an instance is deleted, the final tallies of statistics are
 // carefully accumulated.  That tallying wrties into slots (members) in a
 // collection of DeathData instances.  For each birth place Location that is
 // destroyed on a thread, there is a DeathData instance to record the additional
 // death count, as well as accumulate the lifetime duration of the instance as
 // it is destroyed (dies).  By maintaining a single place to aggregate this
 // addition *only* for the given thread, we avoid the need to lock such
 // DeathData instances.
 //
 // With the above lifecycle description complete, the major remaining detail is
 // explaining how each thread maintains a list of DeathData instances, and of
 // Births instances, and is able to avoid additional (redundant/unnecessary)
 // allocations.
 //
 // Each thread maintains a list of data items specific to that thread in a
 // ThreadData instance (for that specific thread only).  The two critical items
 // are lists of DeathData and Births instances.  These lists are maintained in
 // STL maps, which are indexed by Location. As noted earlier, we can compare
 // locations very efficiently as we consider the underlying data (file,
 // function, line) to be atoms, and hence pointer comparison is used rather than
 // (slow) string comparisons.
 //
 // To provide a mechanism for iterating over all "known threads," which means
 // threads that have recorded a birth or a death, we create a singly linked list
 // of ThreadData instances. Each such instance maintains a pointer to the next
 // one.  A static member of ThreadData provides a pointer to the first_ item on
 // this global list, and access to that first_ item requires the use of a lock_.
 // When new ThreadData instances is added to the global list, it is pre-pended,
 // which ensures that any prior acquisition of the list is valid (i.e., the
 // holder can iterate over it without fear of it changing, or the necessity of
 // using an additional lock.  Iterations are actually pretty rare (used
 // primarilly for cleanup, or snapshotting data for display), so this lock has
 // very little global performance impact.
 //
 // The above description tries to define the high performance (run time)
 // portions of these classes.  After gathering statistics, calls instigated
 // by visiting about:tasks will assemble and aggregate data for display. The
 // following data structures are used for producing such displays.  They are
 // not performance critical, and their only major constraint is that they should
 // be able to run concurrently with ongoing augmentation of the birth and death
 // data.
 //
 // For a given birth location, information about births are spread across data
 // structures that are asynchronously changing on various threads.  For display
 // purposes, we need to construct Snapshot instances for each combination of
 // birth thread, death thread, and location, along with the count of such
 // lifetimes.  We gather such data into a Snapshot instances, so that such
 // instances can be sorted and aggregated (and remain frozen during our
 // processing).  Snapshot instances use pointers to constant portions of the
 // birth and death datastructures, but have local (frozen) copies of the actual
 // statistics (birth count, durations, etc. etc.).
 //
 // A DataCollector is a container object that holds a set of Snapshots.  A
 // DataCollector can be passed from thread to thread, and each thread
 // contributes to it by adding or updating Snapshot instances.  DataCollector
 // instances are thread safe containers which are passed to various threads to
 // accumulate all Snapshot instances.
 //
 // After an array of Snapshots instances are colleted into a DataCollector, they
 // need to be sorted, and possibly aggregated (example: how many threads are in
 // a specific consecutive set of Snapshots?  What was the total birth count for
 // that set? etc.).  Aggregation instances collect running sums of any set of
 // snapshot instances, and are used to print sub-totals in an about:tasks page.
 //
 // TODO(jar): I need to store DataCollections, and provide facilities for taking
 // the difference between two gathered DataCollections.  For now, I'm just
 // adding a hack that Reset()'s to zero all counts and stats.  This is also
 // done in a slighly thread-unsafe fashion, as the reseting is done
 // asynchronously relative to ongoing updates, and worse yet, some data fields
 // are 64bit quantities, and are not atomicly accessed (reset or incremented
 // etc.).  For basic profiling, this will work "most of the time," and should be
 // sufficient... but storing away DataCollections is the "right way" to do this.
 //
 class MessageLoop;


 namespace tracked_objects {

 //------------------------------------------------------------------------------
 // For a specific thread, and a specific birth place, the collection of all
 // death info (with tallies for each death thread, to prevent access conflicts).
 class ThreadData;
 class BirthOnThread {
  public:
   explicit BirthOnThread(const Location& location);

   const Location location() const { return location_; }
   const ThreadData* birth_thread() const { return birth_thread_; }

  private:
   // File/lineno of birth.  This defines the essence of the type, as the context
   // of the birth (construction) often tell what the item is for.  This field
   // is const, and hence safe to access from any thread.
   const Location location_;

   // The thread that records births into this object.  Only this thread is
   // allowed to access birth_count_ (which changes over time).
   const ThreadData* birth_thread_;  // The thread this birth took place on.

   DISALLOW_COPY_AND_ASSIGN(BirthOnThread);
 };

 //------------------------------------------------------------------------------
 // A class for accumulating counts of births (without bothering with a map<>).

 class Births: public BirthOnThread {
  public:
   explicit Births(const Location& location);

   int birth_count() const { return birth_count_; }

   // When we have a birth we update the count for this BirhPLace.
   void RecordBirth() { ++birth_count_; }

   // When a birthplace is changed (updated), we need to decrement the counter
   // for the old instance.
   void ForgetBirth() { --birth_count_; }  // We corrected a birth place.

   // Hack to quickly reset all counts to zero.
   void Clear() { birth_count_ = 0; }

  private:
   // The number of births on this thread for our location_.
   int birth_count_;

   DISALLOW_COPY_AND_ASSIGN(Births);
 };

 //------------------------------------------------------------------------------
 // Basic info summarizing multiple destructions of an object with a single
 // birthplace (fixed Location).  Used both on specific threads, and also used
 // in snapshots when integrating assembled data.

 class DeathData {
  public:
   // Default initializer.
   DeathData() : count_(0), square_duration_(0) {}

   // When deaths have not yet taken place, and we gather data from all the
   // threads, we create DeathData stats that tally the number of births without
   // a corrosponding death.
   explicit DeathData(int count) : count_(count), square_duration_(0) {}

   void RecordDeath(const base::TimeDelta& duration);

   // Metrics accessors.
   int count() const { return count_; }
   base::TimeDelta life_duration() const { return life_duration_; }
   int64 square_duration() const { return square_duration_; }
   int AverageMsDuration() const;
   double StandardDeviation() const;

   // Accumulate metrics from other into this.
   void AddDeathData(const DeathData& other);

   // Simple print of internal state.
   void Write(std::string* output) const;

   // Reset all tallies to zero.
   void Clear();

  private:
   int count_;                // Number of destructions.
   base::TimeDelta life_duration_;    // Sum of all lifetime durations.
   int64 square_duration_;  // Sum of squares in milliseconds.
 };

 //------------------------------------------------------------------------------
 // A temporary collection of data that can be sorted and summarized.  It is
 // gathered (carefully) from many threads.  Instances are held in arrays and
 // processed, filtered, and rendered.
 // The source of this data was collected on many threads, and is asynchronously
 // changing.  The data in this instance is not asynchronously changing.

 class Snapshot {
  public:
   // When snapshotting a full life cycle set (birth-to-death), use this:
   Snapshot(const BirthOnThread& birth_on_thread, const ThreadData& death_thread,
            const DeathData& death_data);

   // When snapshotting a birth, with no death yet, use this:
   Snapshot(const BirthOnThread& birth_on_thread, int count);


   const ThreadData* birth_thread() const { return birth_->birth_thread(); }
   const Location location() const { return birth_->location(); }
   const BirthOnThread& birth() const { return *birth_; }
   const ThreadData* death_thread() const {return death_thread_; }
   const DeathData& death_data() const { return death_data_; }
   const std::string DeathThreadName() const;

   int count() const { return death_data_.count(); }
   base::TimeDelta life_duration() const { return death_data_.life_duration(); }
   int64 square_duration() const { return death_data_.square_duration(); }
   int AverageMsDuration() const { return death_data_.AverageMsDuration(); }

   void Write(std::string* output) const;

   void Add(const Snapshot& other);

  private:
   const BirthOnThread* birth_;  // Includes Location and birth_thread.
   const ThreadData* death_thread_;
   DeathData death_data_;
 };
 //------------------------------------------------------------------------------
 // DataCollector is a container class for Snapshot and BirthOnThread count
 // items.  It protects the gathering under locks, so that it could be called via
 // Posttask on any threads, or passed to all the target threads in parallel.

 class DataCollector {
  public:
   typedef std::vector<Snapshot> Collection;

   // Construct with a list of how many threads should contribute.  This helps us
   // determine (in the async case) when we are done with all contributions.
   DataCollector();
   ~DataCollector();

   // Add all stats from the indicated thread into our arrays.  This function is
   // mutex protected, and *could* be called from any threads (although current
   // implementation serialized calls to Append).
   void Append(const ThreadData& thread_data);

   // After the accumulation phase, the following accessor is used to process the
   // data.
   Collection* collection();

   // After collection of death data is complete, we can add entries for all the
   // remaining living objects.
   void AddListOfLivingObjects();

  private:
   // This instance may be provided to several threads to contribute data.  The
   // following counter tracks how many more threads will contribute.  When it is
   // zero, then all asynchronous contributions are complete, and locked access
   // is no longer needed.
   int count_of_contributing_threads_;

   // The array that we collect data into.
   Collection collection_;

   // The total number of births recorded at each location for which we have not
   // seen a death count.
   typedef std::map<const BirthOnThread*, int> BirthCount;
   BirthCount global_birth_count_;

   Lock accumulation_lock_;  // Protects access during accumulation phase.

   DISALLOW_COPY_AND_ASSIGN(DataCollector);
 };

 //------------------------------------------------------------------------------
 // Aggregation contains summaries (totals and subtotals) of groups of Snapshot
 // instances to provide printing of these collections on a single line.

 class Aggregation: public DeathData {
  public:
   Aggregation();
   ~Aggregation();

   void AddDeathSnapshot(const Snapshot& snapshot);
   void AddBirths(const Births& births);
   void AddBirth(const BirthOnThread& birth);
   void AddBirthPlace(const Location& location);
   void Write(std::string* output) const;
   void Clear();

  private:
   int birth_count_;
   std::map<std::string, int> birth_files_;
   std::map<Location, int> locations_;
   std::map<const ThreadData*, int> birth_threads_;
   DeathData death_data_;
   std::map<const ThreadData*, int> death_threads_;

   DISALLOW_COPY_AND_ASSIGN(Aggregation);
 };

 //------------------------------------------------------------------------------
 // Comparator is a class that supports the comparison of Snapshot instances.
 // An instance is actually a list of chained Comparitors, that can provide for
 // arbitrary ordering.  The path portion of an about:objects URL is translated
 // into such a chain, which is then used to order Snapshot instances in a
 // vector.  It orders them into groups (for aggregation), and can also order
 // instances within the groups (for detailed rendering of the instances in an
 // aggregation).

 class Comparator {
  public:
   // Selector enum is the token identifier for each parsed keyword, most of
   // which specify a sort order.
   // Since it is not meaningful to sort more than once on a specific key, we
   // use bitfields to accumulate what we have sorted on so far.
   enum Selector {
     // Sort orders.
     NIL = 0,
     BIRTH_THREAD = 1,
     DEATH_THREAD = 2,
     BIRTH_FILE = 4,
     BIRTH_FUNCTION = 8,
     BIRTH_LINE = 16,
     COUNT = 32,
     AVERAGE_DURATION = 64,
     TOTAL_DURATION = 128,

     // Imediate action keywords.
     RESET_ALL_DATA = -1,
   };

   explicit Comparator();

   // Reset the comparator to a NIL selector.  Clear() and recursively delete any
   // tiebreaker_ entries.  NOTE: We can't use a standard destructor, because
   // the sort algorithm makes copies of this object, and then deletes them,
   // which would cause problems (either we'd make expensive deep copies, or we'd
   // do more thna one delete on a tiebreaker_.
   void Clear();

   // The less() operator for sorting the array via std::sort().
   bool operator()(const Snapshot& left, const Snapshot& right) const;

   void Sort(DataCollector::Collection* collection) const;

   // Check to see if the items are sort equivalents (should be aggregated).
   bool Equivalent(const Snapshot& left, const Snapshot& right) const;

   // Check to see if all required fields are present in the given sample.
   bool Acceptable(const Snapshot& sample) const;

   // A comparator can be refined by specifying what to do if the selected basis
   // for comparison is insufficient to establish an ordering.  This call adds
   // the indicated attribute as the new "least significant" basis of comparison.
   void SetTiebreaker(Selector selector, const std::string& required);

   // Indicate if this instance is set up to sort by the given Selector, thereby
   // putting that information in the SortGrouping, so it is not needed in each
   // printed line.
   bool IsGroupedBy(Selector selector) const;

   // Using the tiebreakers as set above, we mostly get an ordering, which
   // equivalent groups.  If those groups are displayed (rather than just being
   // aggregated, then the following is used to order them (within the group).
   void SetSubgroupTiebreaker(Selector selector);

   // Translate a keyword and restriction in URL path to a selector for sorting.
   void ParseKeyphrase(const std::string& key_phrase);

   // Parse a query in an about:objects URL to decide on sort ordering.
   bool ParseQuery(const std::string& query);

   // Output a header line that can be used to indicated what items will be
   // collected in the group.  It lists all (potentially) tested attributes and
   // their values (in the sample item).
   bool WriteSortGrouping(const Snapshot& sample, std::string* output) const;

   // Output a sample, with SortGroup details not displayed.
   void WriteSnapshot(const Snapshot& sample, std::string* output) const;

  private:
   // The selector directs this instance to compare based on the specified
   // members of the tested elements.
   enum Selector selector_;

   // For filtering into acceptable and unacceptable snapshot instance, the
   // following is required to be a substring of the selector_ field.
   std::string required_;

   // If this instance can't decide on an ordering, we can consult a tie-breaker
   // which may have a different basis of comparison.
   Comparator* tiebreaker_;

   // We or together all the selectors we sort on (not counting sub-group
   // selectors), so that we can tell if we've decided to group on any given
   // criteria.
   int combined_selectors_;

   // Some tiebreakrs are for subgroup ordering, and not for basic ordering (in
   // preparation for aggregation).  The subgroup tiebreakers are not consulted
   // when deciding if two items are in equivalent groups.  This flag tells us
   // to ignore the tiebreaker when doing Equivalent() testing.
   bool use_tiebreaker_for_sort_only_;
 };


 //------------------------------------------------------------------------------
 // For each thread, we have a ThreadData that stores all tracking info generated
 // on this thread.  This prevents the need for locking as data accumulates.

 class ThreadData {
  public:
   typedef std::map<Location, Births*> BirthMap;
   typedef std::map<const Births*, DeathData> DeathMap;

   ThreadData();
   ~ThreadData();

   // Using Thread Local Store, find the current instance for collecting data.
   // If an instance does not exist, construct one (and remember it for use on
   // this thread.
   // If shutdown has already started, and we don't yet have an instance, then
   // return null.
   static ThreadData* current();

   // For a given about:objects URL, develop resulting HTML, and append to
   // output.
   static void WriteHTML(const std::string& query, std::string* output);

   // For a given accumulated array of results, use the comparator to sort and
   // subtotal, writing the results to the output.
   static void WriteHTMLTotalAndSubtotals(
       const DataCollector::Collection& match_array,
       const Comparator& comparator, std::string* output);

   // In this thread's data, record a new birth.
   Births* TallyABirth(const Location& location);

   // Find a place to record a death on this thread.
   void TallyADeath(const Births& lifetimes, const base::TimeDelta& duration);

   // (Thread safe) Get start of list of instances.
   static ThreadData* first();
   // Iterate through the null terminated list of instances.
   ThreadData* next() const { return next_; }

   MessageLoop* message_loop() const { return message_loop_; }
   const std::string ThreadName() const;

   // Using our lock, make a copy of the specified maps.  These calls may arrive
   // from non-local threads, and are used to quickly scan data from all threads
   // in order to build an HTML page for about:objects.
   void SnapshotBirthMap(BirthMap *output) const;
   void SnapshotDeathMap(DeathMap *output) const;

   // Hack: asynchronously clear all birth counts and death tallies data values
   // in all ThreadData instances.  The numerical (zeroing) part is done without
   // use of a locks or atomics exchanges, and may (for int64 values) produce
   // bogus counts VERY rarely.
   static void ResetAllThreadData();

   // Using our lock to protect the iteration, Clear all birth and death data.
   void Reset();

   // Using the "known list of threads" gathered during births and deaths, the
   // following attempts to run the given function once all all such threads.
   // Note that the function can only be run on threads which have a message
   // loop!
   static void RunOnAllThreads(void (*Func)());

   // Set internal status_ to either become ACTIVE, or later, to be SHUTDOWN,
   // based on argument being true or false respectively.
   // IF tracking is not compiled in, this function will return false.
   static bool StartTracking(bool status);
   static bool IsActive();

 #ifdef OS_WIN
   // WARNING: ONLY call this function when all MessageLoops are still intact for
   // all registered threads.  IF you call it later, you will crash.
   // Note: You don't need to call it at all, and you can wait till you are
   // single threaded (again) to do the cleanup via
   // ShutdownSingleThreadedCleanup().
   // Start the teardown (shutdown) process in a multi-thread mode by disabling
   // further additions to thread database on all threads.  First it makes a
   // local (locked) change to prevent any more threads from registering.  Then
   // it Posts a Task to all registered threads to be sure they are aware that no
   // more accumulation can take place.
   static void ShutdownMultiThreadTracking();
 #endif

   // WARNING: ONLY call this function when you are running single threaded
   // (again) and all message loops and threads have terminated.  Until that
   // point some threads may still attempt to write into our data structures.
   // Delete recursively all data structures, starting with the list of
   // ThreadData instances.
   static void ShutdownSingleThreadedCleanup();

  private:
   // Current allowable states of the tracking system.  The states always
   // proceed towards SHUTDOWN, and never go backwards.
   enum Status {
     UNINITIALIZED,
     ACTIVE,
     SHUTDOWN,
   };

 #if defined(OS_WIN)
   class ThreadSafeDownCounter;
   class RunTheStatic;
 #endif

   // Each registered thread is called to set status_ to SHUTDOWN.
   // This is done redundantly on every registered thread because it is not
   // protected by a mutex.  Running on all threads guarantees we get the
   // notification into the memory cache of all possible threads.
   static void ShutdownDisablingFurtherTracking();

   // We use thread local store to identify which ThreadData to interact with.
   static TLSSlot tls_index_;

   // Link to the most recently created instance (starts a null terminated list).
   static ThreadData* first_;
   // Protection for access to first_.
   static Lock list_lock_;

   // We set status_ to SHUTDOWN when we shut down the tracking service. This
   // setting is redundantly established by all participating threads so that we
   // are *guaranteed* (without locking) that all threads can "see" the status
   // and avoid additional calls into the  service.
   static Status status_;

   // Link to next instance (null terminated list). Used to globally track all
   // registered instances (corresponds to all registered threads where we keep
   // data).
   ThreadData* next_;

   // The message loop where tasks needing to access this instance's private data
   // should be directed.  Since some threads have no message loop, some
   // instances have data that can't be (safely) modified externally.
   MessageLoop* message_loop_;

   // A map used on each thread to keep track of Births on this thread.
   // This map should only be accessed on the thread it was constructed on.
   // When a snapshot is needed, this structure can be locked in place for the
   // duration of the snapshotting activity.
   BirthMap birth_map_;

   // Similar to birth_map_, this records informations about death of tracked
   // instances (i.e., when a tracked instance was destroyed on this thread).
   // It is locked before changing, and hence other threads may access it by
   // locking before reading it.
   DeathMap death_map_;

   // Lock to protect *some* access to BirthMap and DeathMap.  The maps are
   // regularly read and written on this thread, but may only be read from other
   // threads.  To support this, we acquire this lock if we are writing from this
   // thread, or reading from another thread.  For reading from this thread we
   // don't need a lock, as there is no potential for a conflict since the
   // writing is only done from this thread.
   mutable Lock lock_;

   DISALLOW_COPY_AND_ASSIGN(ThreadData);
 };


 //------------------------------------------------------------------------------
 // Provide simple way to to start global tracking, and to tear down tracking
 // when done.  Note that construction and destruction of this object must be
 // done when running in  threaded mode (before spawning a lot of threads
 // for construction, and after shutting down all the threads for destruction).

 // To prevent grabbing thread local store resources time and again if someone
 // chooses to try to re-run the browser many times, we maintain global state and
 // only allow the tracking system to be started up at most once, and shutdown
 // at most once.  See bug 31344 for an example.

 class AutoTracking {
  public:
   AutoTracking() {
     if (state_ != kNeverBeenRun)
       return;
     ThreadData::StartTracking(true);
     state_ = kRunning;
   }

   ~AutoTracking() {
 #ifndef NDEBUG
     if (state_ != kRunning)
       return;
     // Don't call these in a Release build: they just waste time.
     // The following should ONLY be called when in single threaded mode. It is
     // unsafe to do this cleanup if other threads are still active.
     // It is also very unnecessary, so I'm only doing this in debug to satisfy
     // purify (if we need to!).
     ThreadData::ShutdownSingleThreadedCleanup();
     state_ = kTornDownAndStopped;
 #endif
   }

  private:
   enum State {
     kNeverBeenRun,
     kRunning,
     kTornDownAndStopped,
   };
   static State state_;

   DISALLOW_COPY_AND_ASSIGN(AutoTracking);
 };


 }  // namespace tracked_objects

 #endif  // BASE_TRACKED_OBJECTS_H_