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// Copyright (c) 2010 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.
//
// A StatsTable is a table of statistics. It can be used across multiple
// processes and threads, maintaining cheap statistics counters without
// locking.
//
// The goal is to make it very cheap and easy for developers to add
// counters to code, without having to build one-off utilities or mechanisms
// to track the counters, and also to allow a single "view" to display
// the contents of all counters.
//
// To achieve this, StatsTable creates a shared memory segment to store
// the data for the counters. Upon creation, it has a specific size
// which governs the maximum number of counters and concurrent
// threads/processes which can use it.
//
#ifndef BASE_METRICS_STATS_TABLE_H_
#define BASE_METRICS_STATS_TABLE_H_
#pragma once
#include <string>
#include "base/basictypes.h"
#include "base/hash_tables.h"
#include "base/lock.h"
#include "base/thread_local_storage.h"
namespace base {
class StatsTable {
public:
// Create a new StatsTable.
// If a StatsTable already exists with the specified name, this StatsTable
// will use the same shared memory segment as the original. Otherwise,
// a new StatsTable is created and all counters are zeroed.
//
// name is the name of the StatsTable to use.
//
// max_threads is the maximum number of threads the table will support.
// If the StatsTable already exists, this number is ignored.
//
// max_counters is the maximum number of counters the table will support.
// If the StatsTable already exists, this number is ignored.
StatsTable(const std::string& name, int max_threads, int max_counters);
// Destroys the StatsTable. When the last StatsTable is destroyed
// (across all processes), the StatsTable is removed from disk.
~StatsTable();
// For convenience, we create a static table. This is generally
// used automatically by the counters.
static StatsTable* current() { return global_table_; }
// Set the global table for use in this process.
static void set_current(StatsTable* value) { global_table_ = value; }
// Get the slot id for the calling thread. Returns 0 if no
// slot is assigned.
int GetSlot() const;
// All threads that contribute data to the table must register with the
// table first. This function will set thread local storage for the
// thread containing the location in the table where this thread will
// write its counter data.
//
// name is just a debugging tag to label the thread, and it does not
// need to be unique. It will be truncated to kMaxThreadNameLength-1
// characters.
//
// On success, returns the slot id for this thread. On failure,
// returns 0.
int RegisterThread(const std::string& name);
// Returns the number of threads currently registered. This is really not
// useful except for diagnostics and debugging.
int CountThreadsRegistered() const;
// Find a counter in the StatsTable.
//
// Returns an id for the counter which can be used to call GetLocation().
// If the counter does not exist, attempts to create a row for the new
// counter. If there is no space in the table for the new counter,
// returns 0.
int FindCounter(const std::string& name);
// TODO(mbelshe): implement RemoveCounter.
// Gets the location of a particular value in the table based on
// the counter id and slot id.
int* GetLocation(int counter_id, int slot_id) const;
// Gets the counter name at a particular row. If the row is empty,
// returns NULL.
const char* GetRowName(int index) const;
// Gets the sum of the values for a particular row.
int GetRowValue(int index) const;
// Gets the sum of the values for a particular row for a given pid.
int GetRowValue(int index, int pid) const;
// Gets the sum of the values for a particular counter. If the counter
// does not exist, creates the counter.
int GetCounterValue(const std::string& name);
// Gets the sum of the values for a particular counter for a given pid.
// If the counter does not exist, creates the counter.
int GetCounterValue(const std::string& name, int pid);
// The maxinum number of counters/rows in the table.
int GetMaxCounters() const;
// The maxinum number of threads/columns in the table.
int GetMaxThreads() const;
// The maximum length (in characters) of a Thread's name including
// null terminator, as stored in the shared memory.
static const int kMaxThreadNameLength = 32;
// The maximum length (in characters) of a Counter's name including
// null terminator, as stored in the shared memory.
static const int kMaxCounterNameLength = 32;
// Convenience function to lookup a counter location for a
// counter by name for the calling thread. Will register
// the thread if it is not already registered.
static int* FindLocation(const char *name);
private:
class Private;
struct TLSData;
// Returns the space occupied by a thread in the table. Generally used
// if a thread terminates but the process continues. This function
// does not zero out the thread's counters.
// Cannot be used inside a posix tls destructor.
void UnregisterThread();
// This variant expects the tls data to be passed in, so it is safe to
// call from inside a posix tls destructor (see doc for pthread_key_create).
void UnregisterThread(TLSData* tls_data);
// The SlotReturnFunction is called at thread exit for each thread
// which used the StatsTable.
static void SlotReturnFunction(void* data);
// Locates a free slot in the table. Returns a number > 0 on success,
// or 0 on failure. The caller must hold the shared_memory lock when
// calling this function.
int FindEmptyThread() const;
// Locates a counter in the table or finds an empty row. Returns a
// number > 0 on success, or 0 on failure. The caller must hold the
// shared_memory_lock when calling this function.
int FindCounterOrEmptyRow(const std::string& name) const;
// Internal function to add a counter to the StatsTable. Assumes that
// the counter does not already exist in the table.
//
// name is a unique identifier for this counter, and will be truncated
// to kMaxCounterNameLength-1 characters.
//
// On success, returns the counter_id for the newly added counter.
// On failure, returns 0.
int AddCounter(const std::string& name);
// Get the TLS data for the calling thread. Returns NULL if none is
// initialized.
TLSData* GetTLSData() const;
typedef hash_map<std::string, int> CountersMap;
Private* impl_;
// The counters_lock_ protects the counters_ hash table.
Lock counters_lock_;
// The counters_ hash map is an in-memory hash of the counters.
// It is used for quick lookup of counters, but is cannot be used
// as a substitute for what is in the shared memory. Even though
// we don't have a counter in our hash table, another process may
// have created it.
CountersMap counters_;
TLSSlot tls_index_;
static StatsTable* global_table_;
DISALLOW_COPY_AND_ASSIGN(StatsTable);
};
} // namespace base
#endif // BASE_METRICS_STATS_TABLE_H_
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