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Note: Iterations of a parallel loop can depend from each other due to usage of reduction variables inside loop. But in this case such dependence doesn’t prevent parallel loop execution and should be declared by programmer when specifying the parallel loop. If a program hasn’t such declaration the variable will be considered as a private variable and private variable usage error will be detected.

When the program accesses to a distributed array inside a parallel construction the local part of the array is calculated.

If the local part of the array doesn’t contain the accessed element an error of access to non-local part is reported. If a shadow edge contains the accessed element the shadow edge renewal checking is performed.

The following three states are assigned to shadow edges of a distributed array: the renewal isn’t performed, renewal being started and renewal has been finished. The program can access to the shadow edges only for reading and only after the renewal has been completed.

The state “renewal isn’t performed” is set when exported elements are modified. It provides a detection of incorrect access to shadow elements before the next edge renewal will be performed. The error is reported when exported elements are modified before renewal of edges has been completed.

5.3Checking private and read-only variables

The type of a variable is determined when the first use of the variable inside a parallel construction is detected. If the variable is not reduction variable or distributed array then its type is determined by the following way. If the variable is used for writing it will be registered as private. Otherwise it will be registered as read-only.

Each private variable has its initialization flag. The flag is set if the variable is initialized inside given loop iteration or task and otherwise flag is cleared. When variable is modified the flag is set. When the variable is read the flag setting is checked. The private variable flags are cleared after parallel loop exit.

Access type checking is performed for read-only variables. If the program attempts to write to read-only variable the error will be detected.

5.4Checking reduction variables

Reduction variables are registered when included into a reduction group. The reduction variable has a flag that can assume the following states:

• The variable is used inside a parallel loop. This is initial state of the reduction variable.

• The parallel construction has been finished, but asynchronous reduction for the variable is not started yet.

• The asynchronous reduction for the variable being started.

• The asynchronous reduction for the variable has been completed. This is final state of reduction variable. The reduction variable will be deleted from the table of variables and can be used as a variable of other class.

The state of the reduction variable is checked when the variable is accessed. If reduction variable is used after a parallel construction leaving but before asynchronous reduction completion the error will be reported.

Note: Incorrect specification of a reduction function (for example, specifying MIN instead of MAX) is not detected by this method. Use the trace comparison method to detect such errors.

5.5Checking usage of remote access buffer

The remote data buffer is used for pre-loading of non-local elements required for the each processor. The same verifications are performed for a remote data buffer and for a distributed array. But there are little exclusions:

• Verifying the data dependencies in a parallel construction takes into account that the remote data buffer is used instead of distributed array.

• Verifying the array initialization is performed both for remote data buffer elements and elements of related distributed array. The initialization of remote data buffer is copying of distributed array elements to remote data buffer.

• Only read operations are allowed for elements of the remote data buffer.

5.6Dynamic control of DVM directive implementation

Dynamic control subsystem includes the following components:

• Table of variables – provides methods to accumulate the information about variables of various types and quick find this information by address.

• Module of dynamic control – performs DVM directive checking. Functions of this module are called form some functions of Lib-DVM and for each variable or array accessing. The table of variable is used as auxiliary module, which accumulates and provides information about variables during dynamic control.

5.6.1Use the table of variables by dynamic control module

The dynamic control subsystem uses such concept as execution level.

The root level is a level of the initial sequential branch of the program . This level has zero number. When entering a parallel construction the number of current level is incremented by one unit. When leaving parallel construction the number of current level is decrement by on unit. Thus, the number of current level defines a depth of parallel constructions.

The number of the current level is important for work with variables, since the ideology of using variables depends on current level. This level is taken into account with registering a new variable in the dynamic control and is kept together with a basic information about a variable.

There are three variants when dynamic control finds a variable in table of variables:

1. There is no variable information in the table. In this case a new record for the variable is created. The type of usage this variable is detected dependently on context of variable accessing. The current executed level is set as level for this variable. The formed record is returned as result of searching.

2. There is variable information in the table but its level does not correspond to current. In this case also a new record will be created. The reference to a variable information on the previous level is stored in the formed record. The context of variable accessing and class of using of variable on previous level defines a new class of variable usage. The formed record is returned as result of searching. After removing variable information from the table the necessary information will be updated for a variable on the previous level.

3. There is variable information in the table and its level corresponds to current. In this case the found record is returned as result of searching.

Also some functions of Lib-DVM create records in the table of variables. For example the definition of a distributed DVM array or including variable into reduction groups automatically creates a records in table of variables with respective class of usage.

After completion of each parallel structure, all record, which level is equal to level of completed structure, are removed from that table of variables. Thus the lifetime of executed parallel structure defines the lifetime of record in the table of variables.

5.6.2Base functions of dynamic control module

The following functions performs dynamic control:

The function initializes all required structures and variables for dynamic control subsystem.

The function un-initializes all dynamic control structures and prints all detected errors.

The function is called when a parallel structure is completed. It removes all registered variables for the current execution level.

The function removes all data that is used by dynamic control module.

The function finds variable information by its address. The function returns NULL if variable is not registered.

This is general function of dynamic control module. The function checks all variable accessing for read and write.

The function validates the variable information for current executed level. If the Current is not valid for current level then it creates a new variable information structure, which is returned as result of the function. Otherwise, the Current pointer is returned.

The function checks access to read-only variables.

The function checks access to private variables.

The function checks access to reduction variables.

The function checks access to distributed DVM-arrays.

This is general function of variable registration.

The function registers a reduction variable.

The function registers a distributed DVM array.

The function registers a variable with private type of access.

The function registers a variable with read-only type of access.

The function registers the remote access buffer for several elements of the source distributed array. The remote access buffer is created as a distributed array.

The function registers the remote access buffer for a single element of the source distributed array. The remote access buffer is created as a scalar variable.

Destructor of a reduction variable. The function of reduction variable registration assigns this destructor for variable information structure.

Destructor of a private variable. The function of private variable registration assigns this destructor for variable information structure.

Destructor of a read-only variable. The function of read-only variable registration assigns this destructor for variable information structure.

Destructor of a distributed DVM array. The function of distributed DVM array registration assigns this destructor for variable information structure.

The function removes variable information for the current execution level.

The function sets initialization flag for the variable or element of distributed array.

The function clears initialization flag for the variable or all elements of distributed array.

The function checks initialization of variable or element of distributed array . It returns 1 if variable or element has been initialized.

The function sets the flag of using the specified array in the parallel loop with ACROSS type of data dependency. The dynamic control will not check data dependency for this array within the limits of executed loop.

The function calculates the current local part of the distributed array allocated on the current abstract machine and checks whether the element belongs to this local part or its edges or not. In addition, the correctness of edge usage is checked.

The function validates the access to an element of distributed DVM array in the sequential part of a program.

The function checks the undeclared loop data dependence when the specified distributed array element is used.

The function checks out-of-range access for the specified distributed array. It returns 1 if the element with the index LI belongs to the local array part.

The function calculates the block of a distributed array, elements of which can be accessed inside the current iteration of a parallel loop. It returns 0 in case of a successful evaluation of the block.

The function registers the shadow definition of the distributed array in the dynamic control module and links they to the specified bound exchange structure.

The function sets the flag of completion of bound exchange for edges that are included into the specified bound group.

The function removes all shadow definitions for the specified distributed array. It is called after modification of an exportable element of the array.

The function finds index of a shadow description that contains the specified array’s element.

5.6.3The parameters of dynamic control

There is the special set of parameters that manages the dynamic control module. By using these parameters user can turn on/off some validations, tune the effectiveness of algorithms, manage diagnostic output, and so on.

Характеристики

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