Event level parallelism in Geant4 Version 10.0

The threading model of Geant4 and several aspects and the new classes are explained in detail here: Geant4MTAdvandedTopicsForApplicationDevelopers
In this page we will concentrate on aspects that are important for kernel developers. In particular we will discuss the most important aspect for Geant4 Version 10.0 multi-threading model, memory handling, split-classes and thread-local storage.
A beginner guide to multi-threading targeted to Geant4 developers can be found in the 18th Collaboration Meeting: https://indico.cern.ch/getFile.py/access?contribId=3&sessionId=7&resId=0&materialId=slides&confId=250021

Memory handling in Geant4 Version 10.0

Introduction

In Geant4 we distinguish two broad types of classes overall: ones whose instances are separate for each thread (such as a physics process, which has a state), and ones whose instances are shared between threads (e.g. an element G4Element which holds constant data ).

In a few cases classes exist which are split - part of their state is constant, and part is per-worker. A simple example of this is a particle definitions, such as G4Electron, which holds both data (which is constant) and a pointer to the G4ProcessManager object for electrons - which must be different for each worker (thread).

We handle these 'split' classes specially, to enable data members and methods which correspond to the per-thread state to give a different result on each worker thread. The implementation of this requires an array for each worker (thread) and an additional indirection - which imposes a cost for each time the method is called.

Thread safety and sharing of objects

To better understand how memory is handled in multi-threaded Geant4 it is better to proceed with a simplified example:
Let us consider a class G4Class that, for simplicity, contains a single data member:
class G4Class {
    [static] G4double fValue; //static keyword is optional
};
Our goal is to transfer the code of G4Class to make it thread-safe. We define as thread-safe if more than one thread can simultaneously operate on the class data member of methods without interfering with each other in an unpredictable way. A classical way to proceed is to protect concurrent access to a shared memory location by thread using a lock on a mutex. However this technique substantially reduce performances because only one thread at a time is allowed to be executed. In Geant4 Version 10.0 we have achieved thread safety via the use of thread local storage. For an explanation of what is thread local storage several sources exists, for a basic introduction adequate for our discussion, web resources give enough details (e.g. wikipedia).
We define an instance of a variable thread-local (or thread-private) if each thread owns a copy of the variable. On the other side a thread-shared variable is an instance of a variable that is shared among the threads (i.e. all thread have access to the same memory location that holds the value of the variable). In addition, if we need to share fValue between several instances of G4Class we call the data field instance-shared otherwise it is instance-local.
It is easy understandable that in the case of thread-shared variables thread needs synchronization to avoid race condition (it is worth to remind that there are no race conditions if the variable is accessed only to be read, for example in the case the variable is marked as const).
One or more instances of G4Class can exist in our application. These instances can be thread-local (e.g. a G4VProcess) or thread-shared (e.g. a G4LogicalVolume). In addition the class data field fValue can be by itself thread-local or thread-shared. The action to be taken to transform the code to be thread-safe depends on three key aspects:
  • Do we need to make the instance(s) of G4Class thread-local or thread-shared?
  • Do we need to make the data field fValue thread-local or thread-shared?
  • In case more than one instance of G4Class exits, do we need fValue to be instance-local or instance-shared?
This gives rise to 8 different possible combinations, summarized in the following figures:


It is worth to remind that in Geant4 only few well identified class instances are thread-shared: geometry, particle-definition, materials, EM data-tables.

Case A: thread-local class instance(s), thread-shared and instance-shared data field

In this case each thread has its own instance(s) of G4Class. We need to share fValue both among threads and among instances. As for a sequential application we can simply add the static keyword to the declaration of fValue. This technique is very common in Geant4 but has the disadvantage that the result code is not thread-safe (unless locks are used) unless the shared variable is also declared as const. Trying to add const (or modify its value, with the use of a lock) outside of the event loop is the best solution:
class G4Class {
    static const G4double fValue;
};

Case B: thread-local class instance(s), thread-local and instance-shared data field.

This scenario is common in Geant4: we need to share a variable (e.g. a data table) between instances of the same class. However it is impractical to share among threads this data field. To make the code thread-safe we can make the data field thread-local declaring the variable thread-local-storage:
#include "G4Types.hh"
class G4Class {
    static G4ThreadLocal G4double fValue;
};
It should be noted that only simple data types can be declared G4ThreadLocal. More information and the procedure to make an object instance G4ThreadLocal is explained in Geant4MTTipsAndTricks#4_Why_I_cannot_simply_add_G4Thre

Case C: thread-local class instance(s), thread-shared and instance-local data field

This case is probably the less frequent. A possible use-case is the reduction of application memory footprint, providing a thread-shared component to the thread-private instances of G4Class (e.g. a cross-section data table). Since this scenario strongly depends on the implementation details it is not possible to define a common strategy to guarantee thread-safety. The best one being to try to make this shared component const.

Case D: thread-local class instance(s), thread-local and instance-local data field

This case is the simplest, nothing has to be changed in the original code.

Case E: thread-shared class instance(s), thread-shared and instance-shared data field

This case is equivalent to Case A, and the same recommendations and comments are valid.

Case F: thread-shared class instance(s), thread-local and instance-shared data field

This case is equivalent to Case B, and the same recommendations and comments are valid.

Case G: thread-shared class instance(s), thread-shared and instance-shared data field

Since the class instances are shared among threads the data field are automatically thread-shared. No action is needed, however access to data field is not, in general thread safe, and the same comments as for case A holds.

Case H: thread-sahred class instance(s), thread-local and instance-local data field

This is the most complex case and is quite common in Geant4 Version 10.0. For example G4ParticleDefinition instances are shared among the threads, but the G4ProcessManager needs to be thread and instance local. To obtain this there are two possible solutions:
  • Use the split-class mechanism. This requires some deep understanding of Geant4 MT and coordination with kernel developers. This code is recognized as critical and while provides a thread-safe code with good CPU performances it also requires modification in other aspects of kernel category (in particular the introduction of a new split-class requires changes in run category). The idea behind the split-class mechanism is that each thread-shared instance instance of G4Class initializes the thread-local data fields copying the initial status from the master thread that is guaranteed to be fully configured. Additional details on split classes are available in this chapter.
  • If performances are not a concern a simple solution is available. It is a simplified version of the split-class mechanism that does not copy the initial status of the thread-local data field from the master thread. A typical example is a cache variable that reduces CPU usage keeping in memory a value of a CPU intensive calculation that depends on the current event. In such a case the G4Cache utility class can be employed.

Details on the split classes mechanism

Copy from Geant4MTTipsAndTricks#10_Splitting_of_classes. Add Makoto's diagrams from CM.

List of split-classes

In Geant4 Version 10.0 the following are split-classes:
  • For geometry related split classes the class G4GeomSplitter implements the split-class mechanism. These are the geometry related split-classes:
  1. G4LogicalVolume
  2. G4PhysicalVolume
  3. G4PVReplica
  4. G4Region
  5. G4PloyconeSide
  6. G4PolyhedraSide
  • For Physics related split-classes the classes G4PDefSplitter and G!4VUPLSplitter implement the split-class mechanism. These are the physics related split-classes:
  1. G4ParticleDefinition
  2. G4VUserPhysicsList
  3. G4VModularPhysicsList
  4. G4VPhysicsConstructor

Explicit memory handling in Geant4 Version 10.0

In the following some utility classes and functions to help the memory handling for multi-threading development are discussed.
Before going in the detail it should be noted that all of these utilities has a (small) CPU and emory performance penalty, they should be used with caution only if other simpler methods are not available. In addition limitation on their usage are present.

G4Cache template class

As discussed in previous paragraph the split-class mechanism allows for an efficient implementation of many thread-shared instances with thread- and instance- local data field. This technique is efficient because it is based on the assumption that when worker threads start the thread-local part of the instances can be initialized, for each worker, copying from the fully initialized thread-local memory from master thread.
In many cases this is not needed and what we really need is a simple thread-local and instance-local data field in thread-shared instances of G4Class. For example a class representing a cross-section is made shared because of its memory footprint. However it requires a data field to act as a cache to store a value of a CPU intensive calculation. Since different thread share this instance we need to transform the code in a manner similar to what we do for split-classes mechanism. The helper class G4Cache can be used for this purpose.
This is a template class that implements a light-weight split-classes mechanism. Being a template it allows for storing any user-defined type. The public API of this class is very simple and provides two methods
T& G4Cache<T>::Get() const;
void G4Cache<T>::Put(const T& val) const;
to get/set a thread-local instance of the cached object.
#include "G4Cache.hh"
class G4Class {
    G4Cache<G4double> fValue;
    void foo() {
          // Store a thread-local value
          G4double val = someHeavyCalc();
          fValue.Put( val );
    }
    void bar() {
         //Get a thread-local value:
         G4double local = fValue.Get();
    }
};
Since Get returns a reference to the cached object is possible to avoid to use Put to update the cache if we modify bar() to:
void G4Class::bar() {
    //Get a reference to the thread-local value:
   G4double& local = fValue.Get();
   // Use local as in the original sequential code, cache is updated, without the need to use Put
  local++;
}
In case the cache holds a instance of an object it is possible to implement a lazy initialization, as in the following example:
#include "G4Cache.hh"
class G4Class {
    G4Cache<G4Something*> fValue;
    void bar() {
         //Get a thread-local value:
         G4Something* local = fValue.Get();
         if ( local == 0 ) {
                   local = new G4Something( . );
                   //warning this may cause a memory leak. Use of G4AutoDelete can help, see later
         }
    }
};
The use of G4Cache implies some CPU penalty, it is a good habit to try to minimize its use. For example, do not make G4Cache several data field independently, but use a helper structure and use this structure as template parameter to G4Cache:
class G4Class {
   struct {
       G4double fValue1;
       G4Something* fValue2;
   } ToBeCached_t;
  G4Cache&ltToBeCached_t&gt fCache;
};
Finally two specialized versions of G4Cache exists that implement the semantics of std::vector and std::map:
  • G4VectorCache<T> implements thread-local std::vector<T> = with methods: =Push_back() , operator[], Begin(), End(), Clear(), Size(), Pop_back()
  • G4MapCache<K,V> implements thread-local std::map<K,V> with methods: Insert(), Begin(), End(), Find(), Size(), Get(), Erase(), operator[] and introduces the method Has()

G4AutoDelete namespace

In the previous discussion about G4Cache we have shown the example of using a pointer to a dynamically created object as the template parameter of G4Cache. A common problem is to correctly delete this object at the end of its lifecicle. Since the G4Class instance is shared among threads it is not possible to delete the cached object in the destructor of the class, because the destructors is called by master thread and thread-local instances of G4Something will not be deleted causing a memory leak. To partially solve this problem it is possible to use a helper introduced in G4AutoDelete namespace. This introduces a simplified garbage collection without reference counting. With reference to the previous example:
#include "G4AutoDelete.hh"
void G4Class::bar() {
    //Get a thread-local value:
    G4Something* local = fValue.Get();
    if ( local == 0 ) {
            local = new G4Something( . );
            G4AutoDelete::Register( local ); //All thread instances will be delete automatically
    }
}
This technique will delete all instances of the registered objects at the end of the program after the main function has returned (as they would be static).
This method requires some attention and has several limitations: 1 Registered objects will be deleted only at the end of the program as they would be "static" 1 The order in which objects of different type will be deleted is not specified 1 Once an object is registered it cannot be deleted anymore explicitly by user 1 The objects that are registered with this method cannot contain G4ThreadLocal data and cannot be split-classes 1 Registered object cannot make use of G4Allocator In particular since the objects will be deleted after the main program exit in a non-specified order their destructors should be simple and should not depend on other objects.

Thread Private singleton

In Geant4 the singleton pattern is used in several areas. The majority of managers are implemented via the singleton pattern, that in the most simple implementation is:
class G4Singleton {
public:
   G4Singleton* GetInstance() {
        static G4Singleton anInstance;
        return &anInstance;
   }   
};
With multi-threading many managers and singletons are required to become thread-local. For this reason they have been transformed to:
class G4Singleton {
private:
   static G4ThreadLocal* instance;
public:
   G4Singleton* GetInstance() {
        if ( instance == 0 ) instance = new G4Singleton;
        return instance;
   }   
};
This causes a memory leak since it is not possible to delete thread-local instances of the singletons. The class G4ThreadLocalSingleton can be used to solve this problem. This template class has a single public method T* G4ThreadLocalSingleton<T>::Instance() that returns a pointer to a thread-local instance of T.
The code in the example can be transformed to:
#include "G4ThreadLocalSingleton.hh"
class G4Singleton {
   friend class G4ThreadLocalSingleton<G4Singleton>;
public:
   G4Singleton* GetInstance() {
       static G4ThreadLocalSingleton<G4Singleton> theInstance; 
       return theInstance.Instance();
   }   
};

Additional material

For additional information consult this page Geant4MTAdvandedTopicsForApplicationDevelopers
and this page Geant4MTTipsAndTricks
Several contribution at the 18th Collaboration Meeting discussed multi-threading: Plenary Session 3 - Geant4 version 10 (part 1)
Parallel session 7B - Hadronics issues related to MT
Developments for multi-threading: work-spaces
Status of the planned developments: coding guidelines, MT migration, g4tools migration, code review
G4MT CP on MIC Architecture
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