C++ Distributed Counters

Committee: ISO/IEC JTC1 SC22 WG21 SG1 Concurrency
Document Number: P0261R2
Title: C++ Distributed Counters
Date: 2017-02-05
Authors: Lawrence Crowl
Reply To: Lawrence Crowl, Lawrence@Crowl.org
Audience: SG1 Concurrency

Abstract

We propose four class templates implementing distributed atomic counters.

Contents

Introduction
Solution
    General Counter
    Counter Broker
    Counter Array
Implementation
Wording
    ?.? Distributed counters [distctr]
    ?.?.1 General [distctr.general]
    ?.?.2 Synopsis [distctr.syn]
    ?.?.3 Synchronization [distctr.sync]
    ?.?.4 Bumper concept [distctr.bumper]
    ?.?.5 Class template general_counter [distctr.genctr]
    ?.?.6 Class template counter_broker [distctr.broker]
    ?.?.7 Class general_counter_array [distctr.generalarray]
    ?.?.8 Class counter_broker_array [distctr.brokerarray]
Revision History
References

Introduction

Counters are ubiquitous in computing. For multi-threaded programs, atomic integers can provide safe concurrent counting.

In many cases, counters are much more commonly incremented than read. For example, long-running multithreaded server programs may maintain many counters to aid in program diagnosis. Counters are often read only in response to a network query or program termination.

Unfortunately, the cost incrementing an atomic integer is significantly greater than reading one, even when reads are rare relative to increments. As the program scales, the cost of incrementing rarely-read atomic integers can limit program performance.

We need a mechanism that minimizes the cost of incrementing a counter, even if it has increased costs to read the counter.

Cilk adder-reducers [Cilk-Reducer] are one such mechanism. Unfortunately, the reduction is control-dependent and hence not suitable for external inspection of the count.

We propose a counter that has a complexity similar to atomic integers in the simple case, but enables programmers to easily distribute that counter across multiple threads when scalability becomes a concern.

This proposal restricts itself to precise counters. Statistical counters, where the read count may be slightly different from the count operations, can produce better performance [Dice-Lev-Moir-2013].

Solution

We propose two coordinated counter types that collectively provide for distributed counting. The essential component of the design is a trade-off between the cost of incrementing the counter, the cost of loading (reading) the counter value, and the "lag" between an increment and its appearance in a "read" value. That is, the read of a counter value may not reflect the most recent increments. However, no count will be lost.

These counters are parameterized by the base integer type that maintains the count. Avoid situations that overflow the integer, as that may have uninterpretable or undefined behavior. This constraint implies that counters must be sized to their use. (For dynamic mitigation, see the exchange operation below.)

The base of the design is the concept of a counter bumper. This concept only provides for increasing or decreasing a counter. None of its operations provide any means to read the count.

General Counter

The first type programmers will encounter is the general_counter. This counter implements the bumper concept and provides two additional operations, load and exchange. The load operation returns the current count. Note that 'current' is a fuzzy notion when other threads are concurrently incrementing the counter. The exchange operation replaces the current count with the given value and returns the old value. This operation has particular utility in long-running programs in occasionally draining the counter to prevent integer overflow.

general_counter<int> red_count;

void count_red( Bag bag ) {
    for ( auto i: bag )
        if ( i.is_red() )
            ++red_count;
}

int how_many_red() {
    return red_count.load();
}

Counter Broker

When contention becomes a problem, programmers distribute the counting with a counter_broker objects. These will typically be thread-local objects, but could be CPU-local objects or even stack-based objects.

general_counter<int> red_count;
thread_local counter_broker<int> thread_red( red_count );

void count_red( Bag bag )
    for ( auto i: bag )
        if ( i.is_red() )
            ++thread_red;
}

The association of counter brokers to general counter variables is an essential part of the distribution of counters, and not easily moved or copied. Therefore, we provide no copy or move operations for these types.

Counter Array

Managing the association between general counters and their attached brokers can require a non-trivial amount of space and time. Therefore, we provide a means to handle many counters with one name with the general_counter_array. The size of the counter arrays is fixed at construction time. Counter arrays have the same structure as single-value counters, with the following exceptions.

• Increment a counter by first indexing into its array and then applying one of the bumper operations. E.g. ++ctr_ary[i].

• The load and exchange operations take an additional index parameter.

Implementation

Olivier Giroux suggested an implementation in which the set of attached brokers was tracked by an intrusive list through the brokers themselves, rather than through a separate set data structure. This approach has some advantages.

• No dynamic allocation is required.

• The constructor of the general counter can be constexpr, which avoids initialization order problems for global objects.

• The head of the list is null until such time as a broker attaches itself, which substantially reduces the cost of using a general counter in the case where there are no brokers.

The benefit of a constexpr initializer is high enough so that we choose to adopt the constexpr initializer. Unfortunately, counter arrays require allocation for their array of bumpers, and so cannot have constexpr initializers.

An implementation of an earlier version of the design (with the separate broker set data structure) is available from the Google Concurrency Library https://github.com/alasdairmackintosh/google-concurrency-library at .../blob/master/include/counter.h. This paper's general_counter maps to counter::strong_duplex and this paper's counter_broker maps to counter::strong_broker.

Wording

The distributed counter definition is as follows.

?.? Distributed counters [distctr]

Add a new section.

?.?.1 General [distctr.general]

Add a new section.

This section provides mechanisms for distributed counters. These mechanisms ease the production of scalable concurrent programs.

?.?.2 Synopsis [distctr.syn]

Add a new section.

template< typename Integral > class general_counter;
template< typename Integral > class counter_broker;
template< typename Integral > class general_counter_array;
template< typename Integral > class counter_broker_array;

?.?.3 Synchronization [distctr.sync]

Add a new section.

Operations on a single general counter (and its brokers and referenceable subobjects), appear to execute in a single total order consistent with the happens before order, such that load and exchange operations observe the changes to the count of prior operations in this order, and no others. [Note: If the load() is concurrent with a modification to the counter, i.e. the modification and the load are not ordered by the happens before relation, the result of the load() should be treated as approximate. If the exchange() operation is concurrent with another modification to the counter, i.e. the other modification and the exchange are not ordered by the happens before relation, the returned value exactly reflects the adjustment to the counter, but it should otherwise be treated as approximate. Detecting a count that indicates that an event has occured does not mean that any other memory updates associated with that event are visible to the current thread. Acting specially on any particular thread count requires additional synchronization. —end note]

?.?.4 Bumper concept [distctr.bumper]

Add a new section.

The bumper concept provides a common interface for increasing or decreasing the value of a counter. It consists of the following operations. [Note: These operations specifically return void to avoid leaking information that would defeat the purpose of optimizing for increments over loads. —end note]

void operator +=( integer )
void operator -=( integer )

Effects:

Atomically add/subtract a value to/from the counter.

void operator ++()
void operator ++(int)
void operator --()
void operator --(int)

Effects:

As if counter+=1 or counter-=1, respectively.

?.?.5 Class template general_counter [distctr.genctr]

Add a new section.

namespace std {
template< typename Integral >
class general_counter
{
  general_counter( const general_counter& ) = delete;
  general_counter& operator=( const general_counter& ) = delete;
public:
  constexpr general_counter( Integral );
  constexpr general_counter();
  ~general_counter();
  void operator +=( Integral );
  void operator -=( Integral );
  void operator ++();
  void operator ++(int);
  void operator --();
  void operator --(int);
  Integral load();
  Integral exchange( Integral to );
}

The Integral template type parameter shall be an integral type for which std::atomic has a specialization (29.5 [atomics.types.generic]).

The general_counter class template implements the bumper concept ([distctr.bumper]).

constexpr general_counter( Integral )
constexpr general_counter()

Effects:

Initialize the general_counter with the given value, or zero if no value is given.

~general_counter()

Requirements:

All attached counter_broker objects shall have been previously destroyed.

Effects:

Destroys the general_counter.

Integer load()

Returns:

Atomically returns the value of the general_counter. [Note: The value will be an approximation if any threads may concurrently update the general_counter. —end note]

Integer exchange( Integer )

Effects:

Atomically replaces the value of the general_counter with the value of the parameter.

Returns:

The replaced value.

?.?.6 Class template counter_broker [distctr.broker]

Add a new section.

template< typename Integral >
class counter_broker
{
  counter_broker( const counter_broker& ) = delete;
  counter_broker& operator=( const counter_broker& ) = delete;
public:
  counter_broker( general_counter<Integral>& );
  ~counter_broker();
  void operator +=( Integral );
  void operator -=( Integral );
  void operator ++();
  void operator ++(int);
  void operator --();
  void operator --(int);
};

The Integral template type parameter shall be an integral type for which std::atomic has a specialization (29.5 [atomics.types.generic]).

The counter_broker class template implements the bumper concept ([distctr.bumper]).

counter_broker( general_counter<Integral>& )

Effects:

Initialize the broker, atomically associating it with the given general_counter.

~counter_broker()

Effects:

Atomically moves any part of the count value retained within the counter_broker to its associated general_counter. Atomically disassociates the counter_broker from the general_counter. Destroys the counter_broker.

?.?.7 Class general_counter_array [distctr.generalarray]

Add a new section.

template< typename Integral >
class general_counter_array
{
  general_counter_array( const general_counter_array& ) = delete;
  general_counter_array& operator=( const general_counter_array& ) = delete;
public:
  typedef size_t size_type;
  general_counter_array( size_type size );
  some_bumper_type& operator[]( size_type idx );
  size_type size();
  Integral load( size_type idx );
  Integral exchange( size_type idx, Integral value );
};

The Integral template type parameter shall be an integral type for which std::atomic has a specialization (29.5 [atomics.types.generic]).

constexpr general_counter_array( size_type );

Effects:

Initialize the general_counter_array with the given number of counters, each initialized to zero.

~general_counter_array()

Requirements:

All attached counter_broker_array objects shall have been previously destroyed.

Effects:

Destroys the general_counter_array.

some_bumper_type& operator[]( size_type index )

Returns:

A reference to an object, identified by the index, implementing the bumper concept [distctr.bumper].

Integer load( size_type index )

Returns:

the value of the bumper with the given index. [Note: The value will be an approximation if any threads may concurrently update the counter. —end note]

Integer exchange( size_type index, Integer value )

Effects:

Atomically replaces the value of the bumper at the given index with the given value.

Returns:

The replaced value.

?.?.8 Class counter_broker_array [distctr.brokerarray]

Add a new section.

template< typename Integral >
class counter_broker_array
{
  counter_broker_array( const counter_broker_array& ) = delete;
  counter_broker_array& operator=( const counter_broker_array& ) = delete;
public:
  typedef size_t size_type;
  counter_broker_array( general_counter_array<Integral>& );
  some_bumper_type& operator[]( size_type idx );
  size_type size();
};

The Integral template type parameter shall be an integral type for which std::atomic has a specialization (29.5 [atomics.types.generic]).

constexpr counter_broker_array( general_counter_array< Integral >& );

Effects:

Initialize the broker array. Atomically associate the counter_broker_array with the general_counter_array.

~counter_broker_array()

Effects:

Atomically moves each count value retained within the broker to its associated position in the counter array. Atomically disassociates the broker array from the counter array. Destroys the broker array.

some_bumper_type& operator[]( size_type index )

Returns:

A reference to an object, identified by the index, implementing the bumper concept [distctr.bumper].

Revision History

This paper revises P0261r1 - 2016-10-13. Its changes are as follows.

• Merge all synchronization clauses into a single new section. Require cache coherence on counts.

• Clarify that moving the count from the broker does so an atomic element at at time, not atomically in bulk.

• Remove "There is no default value" as it follows from the declaration and is potentially confusing.

• Move the note about increments returning void to the top of the section so that it applies to all operations.

P0261R1 revised P0261r0 - 2016-02-14. Its changes are as follows.

• Clarify that general_counter::load is atomic.

• Use the type-specific phrase "counter_broker object" rather than the general phrase "counter broker" in the proposed wording. Do the same with the array forms.

• Correct parameter and description of effective broker constructors.

• Add a size_type to the counter array classes.

• Clarify initialization of counter array values.

• Clarify the adoption of constexpr general_counter constructors.

• Expand on the problem of counter overflow.

• Update the document header.

• Add an abstract.

• Add subheadings to Solution.

• Do general editorial cleanup.

P0261r0 revised N3706 - 2013-09-01. Its changes are as follows:

• Add more context to the introduction and rework subsequent discussion.

• Reduce the number of types to four, a general_counter equivalent to the earlier strong_duplex, a counter_broker equivalent to the earlier strong_broker, a general_counter_array equivalent to the earlier strong_duplex_array, and a counter_broker_array equivalent to the earlier strong_broker_array.

• Remove the atomicity specification. Only full and relaxed atomicity is now supported.

• As a consequence of the above two points, drastically reduce the solution presentation.

• Update the implementation discussion with a new technique and new pointers to the code base.

• Add proposed wording.

N3706 revised N3355 = 12-0045 - 2012-01-14. Its changes are as follows:

• Add more context to the introduction.

• Change named inc and dec functions to operators.

• Place all declarations within namespace counter. The namespace serves to avoid redundancy in names.

• Rename types for increased clarity.

• Change separate serial and atomic counter types with a single type templatized on the atomicity.

• Add issues of memory order and contention to the atomicity parameter.

• Add counter arrays.

• Add open issues section.

• Add implementation section.

• Add revision history section.

• Add references section.

References

[Cilk-Reducer]

"Reducers", http://software.intel.com/sites/products/documentation/hpc/composerxe/en-us/2011Update/cpp/lin/cref_cls/common/cilk_bk_reducer_intro.htm; "Intel Cilk Plus", http://software.intel.com/sites/products/documentation/hpc/composerxe/en-us/2011Update/cpp/lin/cref_cls/common/cilk_bk_using_cilk.htm; "Intel C++ Compiler XE 12.1 User and Reference Guides", http://software.intel.com/sites/products/documentation/hpc/composerxe/en-us/2011Update/cpp/lin/main/main_cover_title.htm

[Dice-Lev-Moir-2013]

"Scalable Statstics Counters"; Dave Dice, Yossi Lev, Mark Moir; SPAA'13, June 23-25, 2013, Montrèal Quèbec, Canada.

[Lev-Moir-2011]

"Lightweight Parallel Accumulators Using C++ Templates"; Yossi Lev, Mark Moir; ICSE'11, May 21-28, 2011, Waikiki, Honolulu, HI, USA