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66

Keldysh Institute of Applied Mathematics

Russia Academy of Sciences

DVM Debugger

Detailed design

March 2000

Contents

1 The Functions of DVM debugger 3

1.1 The “Dynamic control of DVM-directives” method 4

1.2 Kinds of detected errors 4

1.3 The “Comparing execution results” method 4

2 The Content of DVM debugger 5

3 Prototypes of debugger functions 5

4 Base modules realization 8

4.1 Table 8

4.2 Hash-table 10

4.3 Table of variables 14

4.4 Diagnostics output module 18

4.4.1 Processing diagnostics context 18

4.4.2 Diagnostics output functions 21

5 Dynamic control of DVM-directives 25

5.1 Checking initialization of variables and elements of distributed arrays 25

5.2 Checking access to elements of distributed arrays 25

5.3 Checking private and read-only variables 26

5.4 Checking reduction variables 26

5.5 Checking usage of remote access buffer 27

5.6 Dynamic control of DVM directive implementation 27

5.6.1 Use the table of variables by dynamic control module 27

5.6.2 Base functions of dynamic control module 28

5.6.3 The parameters of dynamic control 35

6 The “Comparing execution results” method 36

6.1 Trace accumulation 36

6.2 Trace comparing 37

6.3 Checking reduction operations 37

6.4 Formats of input and output files 38

6.4.1 Trace configuration file 38

6.4.2 Trace file 40

6.5 Realization of comparing execution results 41

6.5.1 Base structures of comparing execution results module 41

6.5.2 Trace writing module 52

6.5.3 Trace accumulation module 54

6.5.4 Trace reading module 57

6.5.5 Trace comparing module 58

6.5.6 Reduction processing module 61

6.5.7 Parameters of comparing execution results sub-system 63

7 Diagnostic messages of the dynamic debugger 64

1The Functions of DVM debugger

DVM debugger is used for debugging DVM-program (written in Fortran-DVM or C-DVM languages). The following approach is used to debug DVM-programs. First, the program is debugged on a workstation as a sequential program using ordinary debugging methods and tools. Then the program is executed at the same workstation in the special mode of checking DVM-directives. At the next step the program may be executed on a parallel computer in the special mode, when intermediate results of its execution are compared with reference results (for example, the results of its sequential execution).

The DVM-program can contain the errors of different kinds. The DVM-debugger is used to detect errors not appeared during a sequential execution. In the common case the following classes of errors can be considered:

• Syntax errors in DVM-directives (incorrect usage of an operator, missing of brackets and so on). Violation of static semantics belongs to this class also.

• Incorrect order of DVM-directives or invalid parameters of DVM-directives.

• Wrong computation due to DVM-directive incorrectness and errors, not detected during program execution in sequential mode.

• Abnormal parallel program termination (GPFs, infinite looping, hanging) due to errors that not appear during program execution in sequential mode.

A compiler detects the errors of the first class.

Only simple errors of second class such as wrong parameter type may be detected by static analysis. To detect the errors of this class each Lib-DVM function checks the correctness of DVM-directives order and parameters. The checks are done dynamically, during program execution.

It is impossible to detect errors of third class without special compilation mode and considerable overhead in the parallel program. Unspecified data dependence in loop is example of the error of the third class. To detect this error it is necessary to fix all data modifications and usage on different processors and to determine the cases, when a variable is modified on one processor and used on the other. This detection can be more effectively performed on a single processor by emulation of parallel execution of the program.

The DVM-debugger is intended to detect errors of third class. It is based on the following two methods.

The first method, method of dynamic control of DVM-directives, allows verifying the correctness of the program parallelization specified by DVM-directives. It is based on the analysis of sequence of Lib-DVM function calls and references to the variables during parallel program execution simulated on a single processor.

The second method is based on the comparison of parallel and sequential program execution trace results. It allows to localize the program point and moment, when the results are beginning to differ. Unlike the first method, only part of program can be compiled in the special debug mode.

The facilities of parallel program tracing that are included into DVM-debugger may be useful for detection of error of fourth class. Moreover the system tracing facilities are intended to detect these errors (the tracing of DVM support library calls).

1.1The “Dynamic control of DVM-directives” method

The dynamic control is based on simulation of DVM-program parallel execution on a single processor. This method using can slow down the program execution significantly and requires large volumes of additional memory. Therefore it can be applicable to a program with specially chosen test data only.

1.2Kinds of detected errors

The dynamic control allows to detect the following kinds of errors:

1. Undeclared cross-iteration data dependencies in a parallel loop or task region.

2. Using of non-initialized variables.

3. Modification of non-distributed variables not specified as reduction or private.

4. Using reduction variables after the asynchronous reduction startup but before its completion.

5. Accessing to out of bound elements of distributed array.

6. Undeclared access to non-local elements of the distributed array.

7. Writing to shadow edges of the distributed array.

8. Reading shadow elements before completion of their updating.

1.3The “Comparing execution results” method

The dynamic control is intended first of all for checking DVM-directives correctness. The control area is limited by DVM-programs compiled in the special debug mode only. However, the program can contain calls of procedures written in ordinary sequential languages (including assembler). The procedure execution is not controlled and can cause incorrect program execution. Besides, correctness of reduction operation descriptions is not checked. And finally, the program can have errors (not related with its parallelization) which appear only during a parallel execution.

There is another control method aimed to detect such errors. This method is based on an accumulation of trace results for different conditions of execution and these results comparison. With this method the program may be executed in two modes. In the first mode, trace results (intermediate variable values) are accumulated and written to a file as the reference trace results. In the second mode, execution results are compared with the reference ones. It is taken into account that values of some variables may differ (for example, reduction variables inside parallel construction).

2The Content of DVM debugger

The DVM-debugger consists of two parts: the dynamic control system and the trace comparison system.

Both systems use the following base subsystems: tables allowing storing large volumes of uniformed information, hash-tables for fast data lookup, and diagnostic output module.

The dynamic control system consists of the following components:

• The table of variables. It provides means for saving information about usage of various variables and for fast retrieval of the information by the variable address.

• The dynamic control module. It performs DVM-directives verification. The functions of the module are called from certain Lib-DVM functions each time when a variable or array is accessed. This module uses the table of variables as auxiliary module to accumulate and retrieve information about variables.

The trace comparison system consists of the following components:

• Unit of accumulating the trace information in the memory. It accumulates and structures trace events during program execution.

• Unit of writing the trace to a file. Accumulation unit uses it when program is running with trace writing mode specified.

• Unit of reading the trace information from the files. It uses accumulation unit as auxiliary module.

• Unit of comparing traces. It compares trace events of executing program with the reference trace previously formed in the memory.

• Unit of checking reduction variables. It emulates execution of each iteration of parallel loop and each parallel task on the separate processor for checking of correctness of reduction operation descriptions. It works only in the special mode.

3Prototypes of debugger functions

The Lib-DVM system contains additional system functions relating to dynamic debugger that are responsible for dynamic control and comparing of execution results. The C-DVM and Fortran-DVM compilers insert an addressing to these functions in special debugging mode.

The prototypes of debugger functions is the following:

The function marks beginning of a variable or array element modification. The call should be inserted before a variable modification and up to an evaluation of expression to be assigned. The system call should be accompanied by dstv _ ().

The function marks the completion of a variable or array element modification. The call should be inserted after a variable modification and should be conjugate with dprstv_().

The function marks access to a variable or array element for reading.

The function marks parallel loop beginning. The function should be called before mapping the parallel loop on the abstract machine.

The function marks sequential loop beginning. The sequential loop rank is always equal to 1.

The function marks task region beginning. The task region rank is always equal to 1.

The function marks completion of a sequential or parallel loop or task region. The function should be called after completion of the last iteration of the loop or last parallel task in task region.

The function marks beginning of new iteration of a sequential or parallel loop or beginning of new parallel task. The function should be called after modification of all iteration variables of the loop (for multidimensional loops).

The function marks remote access buffer creation for dynamic control system. This call is necessary for correct monitoring of access to the elements of a DVM-array through the remote access buffer. The call should be used after creation of the buffer and initialization of its elements.

The function marks the completion of self-calculation block in sequential branch of the program. The function should be called after execution of the statement group and independently of the checking results.

4Base modules realization

4.1Table

The table is intended to keep large volume of data. A peculiarity of the implementation is that the memory is allocated not for each element but by large blocks of fixed size. The size of the allocated block is defined by parameters of table initialization. The table is expanded automatically, when the current allocated memory block is exhausted.

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