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OpenVMS Programming Concepts Manual

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2.7 Debugging a Subprocess or a Detached Process

You have several ways to debug a subprocess or a detached process including the following:

  • Using DBG$ logical names
  • Using the workstation device (see Section 2.4.3)
  • Using a DECwindows DECterm display

DBG$ Logical Names

You can allow a program to be debugged within a subprocess or a detached process by using DBG$INPUT and DBG$OUTPUT. To allow debug operations with DBG$INPUT and DBG$OUTPUT, equate the subprocess logical names DBG$INPUT and DBG$OUTPUT to the terminal. When the subprocess executes the program, which has been compiled and linked with the debugger, the debugger reads input from DBG$INPUT and writes output to DBG$OUTPUT.

If you are executing the subprocess concurrently, you should restrict debugging to the program in the subprocess. The debugger prompt DBG> should enable you to differentiate between input required by the parent process and input required by the subprocess. However, each time the debugger displays information, you must press the Return key to display the DBG> prompt. (By pressing the Return key, you actually write to the parent process, which has regained control of the terminal following the subprocess's writing to the terminal. Writing to the parent process allows the subprocess to regain control of the terminal.)

DECwindows DECterm Display

If you have DECwindows installed, you can use display for debugging a subprocess or detached process. The following debugging example with DECterm shows how to create a DECterm display, and pass it into the SYS$CREPRC call for use with an application that is built using the OpenVMS Debugger:

#pragma module  CREATE_DECTERM

#include <descrip.h>
#include <lib$routines.h>
#include <pqldef.h>
#include <prcdef.h>
#include <ssdef.h>
#include <starlet.h>
#include <stsdef.h>

// Comment syntax used here assumes Compaq C compiler support (eg: V6.2)

// To build and run:
//    $ link CREATE_DECTERM,sys$input/option
//    sys$share:DECW$TERMINALSHR.EXE/share
//    $ run CREATE_DECTERM

// This routine is not declared in a currently-available library
extern int decw$term_port(void *,...);

main( void )
  int RetStat;
  int StsFlg;
  int DbgTermLen = 0;
  char DbgTermBuf[DBGTERMBUFLEN];
  $DESCRIPTOR( Customization,
"DECW$TERMINAL.iconName:\tDebugging Session\n\
DECW$TERMINAL.title:\tDebugging Session" );
  struct dsc$descriptor DbgTerm;

  DbgTerm.dsc$w_length  = DBGTERMBUFLEN;
  DbgTerm.dsc$b_dtype   = DSC$K_DTYPE_T;
  DbgTerm.dsc$b_class   = DSC$K_CLASS_S;
  DbgTerm.dsc$a_pointer = DbgTermBuf;

  // Request creation of a DECterm display
  RetStat = decw$term_port(
            0,              // display (use default)
            0,              // setup file (use default)
            &Customization, // customization
            &DbgTerm,       // resulting device name
            &DbgTermLen,    // resulting device name length
            0,              // controller (use default)
            0,              // char buffer (use default)
            0 );            // char change buffer (default)
  if ( !$VMS_STATUS_SUCCESS (RetStat ))
    lib$signal( RetStat );

  DbgTerm.dsc$w_length  = DbgTermLen;

  // Create the process as detached.
  StsFlg = PRC$M_DETACH;

  // Now create the process
  RetStat = sys$creprc(
            0,              // PID
            &Command,       // Image to invoke
            &DbgTerm,       // Input
            &DbgTerm,       // Output
            0, 0, 0, 0, 0, 0, 0,
            StsFlg );       // Process creation flags
  if ( !$VMS_STATUS_SUCCESS( RetStat ))
    lib$signal( RetStat );

  return SS$_NORMAL;

2.8 Kernel Threads and the Kernel Threads Process Structure (Alpha Only)

This section defines and describes some advantages of using kernel threads. It also describes some kernel threads features and type of model, as well as the design changes made to the OpenVMS operating system.


For information about the concepts and implementation of user threads with Compaq POSIX Threads Library, see the Guide to POSIX Threads Library.

2.8.1 Definition and Advantages of Kernel Threads

A thread is a single, sequential flow of execution within a process's address space. A single process contains an address space wherein either a single thread or multiple threads execute concurrently. Programs typically have a single flow of execution and therefore a single thread; whereas multithreaded programs have multiple points of execution at any one time.

By using threads as a programming model, you can gain the following advantages:

  • More modular code design
  • Simpler application design and maintenance
  • The potential to run independent flows of execution in parallel on multiple CPUs
  • The potential to make better use of available CPU resources through parallel execution

2.8.2 Kernel Threads Features

With kernel threads, the OpenVMS operating system implements the following two features:

  • Multiple execution contexts within a process
  • Efficient use of the OpenVMS and POSIX Threads Library schedulers Multiple Execution Contexts Within a Process

Before the implementation of kernel threads, the scheduling model for the OpenVMS operating system was per process. The only scheduling context was the process itself, that is, only one execution context per process. Since a threaded application could create thousands of threads, many of these threads could potentially be executing at the same time. But because OpenVMS processes had only a single execution context, in effect, only one of those application threads was running at any one time. If this multithreaded application was running on a multiprocessor system, the application could not make use of more than a single CPU.

After the implementation of kernel threads, the scheduling model allows for multiple execution contexts within a process; that is, more than one application thread can be executing concurrently. These execution contexts are called kernel threads. Kernel threads allow a multithreaded application to have a thread executing on every CPU in a multiprocessor system. Kernel threads, therefore, allow a threaded application to take advantage of multiple CPUs in a symmetric multiprocessing (SMP) system.

In the initial release of kernel threads, OpenVMS allowed a maximum of 16 kernel threads per process. This enabled an application to have threads executing on up to 16 CPUs at one time. With OpenVMS Alpha Version 7.2, the number of kernel threads that can be created per-process increased to 256. The maximum value for the MULTITHREAD system parameter has also increased to 256. Efficient Use of the OpenVMS and POSIX Threads Library Schedulers

The user mode thread manager schedules individual user mode application threads. On OpenVMS, POSIX Threads Library is the user mode threading package of choice. Before the implementation of kernel threads, POSIX Threads Library multiplexed user mode threads on the single OpenVMS execution context---the process. POSIX Threads Library implemented parts of its scheduling by using a periodic timer. When the AST executed and the thread manager gained control, the thread manager could then select a new application thread for execution. But because the thread manager could not detect that a thread had entered an OpenVMS wait state, the entire application blocked until that periodic AST was delivered. That resulted in a delay until the thread manager regained control and could schedule another thread. Once the thread manager gained control, it could schedule a previously preempted thread unaware that the thread was in a wait state. The lack of integration between the OpenVMS and POSIX Threads Library schedulers could result in wasted CPU resources.

After the implementation of kernel threads, the scheduling model provides for scheduler callbacks, which is not the default. A scheduler callback is an upcall from the OpenVMS scheduler to the thread manager whenever a thread changes state. This upcall allows the OpenVMS scheduler to inform the thread manager that the current thread is stalled and that another thread should be scheduled. Upcalls also inform the thread manager that an event a thread is waiting on has completed. With kernel threads, the two schedulers are better integrated, minimizing application thread scheduling delays. Terminating a POSIX Threads Image

To avoid hangs or a disorderly shutdown of a multithreaded process, Compaq recommends that you issue an upcall with an EXIT command at the DCL prompt ($). This procedure causes a normal termination of the image currently executing. If the image declared any exit-handling routines, for instance, they are then given control. The exit handlers are run in a separate thread, which allows them to be synchronized with activities in other threads. This allows them to block without danger of entering a self-deadlock due to the handler having been involved in a context which already held resources.

The effect of calling the EXIT command on the calling thread is the same as calling pthread_exit(): the caller's stack is unwound and the thread is terminated. This allows each frame on the stack to have an opportunity to be notified and to take action during the termination, so that it can then release any resource which it holds that might be required for an exit handler. By using upcalls, you have a way out of self-deadlock problems that can impede image shutdown.

You can optionally perform a shutdown by using the control y EXIT (Ctrl-- Y/EXIT) command. By doing this and with upcalls enabled, you release the exit handler thread. All other threads continue to execute untouched. This removes the possibility of the self-deadlock problem which is common when you invoke exit handlers asynchronously in an existing context. However, by invoking exit handlers, you do not automatically initiate any kind of implicit shutdown of the threads in the process. Because of this, it is up to the application to request explicitly the shutdown of its threads from its exit handler and to ensure that their shutdown is complete before returning from the exit handler. By having the application do this, you ensure that subsequent exit handlers do not encounter adverse operating conditions, such as threads which access files after they've been closed, or the inability to close files because they are being accessed by threads.

Along with using control y EXIT (Ctrl--Y/EXIT) to perform shutdowns, you can issue a control y (Ctrl--Y/STOP) command. If you use a control y STOP (Ctrl--Y/STOP) command, it is recommended that you do this with upcalls. To use a control y STOP (Ctrl--Y/STOP) command, can cause a disorderly or unexpected outcome.

2.8.3 Kernel Threads Model and Design Features

This section presents the type of kernel threads model that OpenVMS Alpha implements, and some features of the operating system design that changed to implement the kernel thread model. Kernel Threads Model

The OpenVMS kernel threads model is one that implements a few kernel threads to many user threads with integrated schedulers. With this model, there is a mapping of many user threads to only several execution contexts or kernel threads. The kernel threads have no knowledge of the individual threads within an application. The thread manager multiplexes those user threads on an execution context, though a single process can have multiple execution contexts. This model also integrates the user mode thread manager scheduler with the OpenVMS scheduler. Kernel Threads Design Features

Design additions and modifications made to various features of OpenVMS Alpha are as follows:

  • Process Structure
  • Access to Inner Modes
  • Scheduling
  • ASTs
  • Event Flags
  • Process Control Services Process Structure

With the implementation of OpenVMS kernel threads, all processes are a threaded process with at least one kernel thread. Every kernel thread gets a set of stacks, one for each access mode. Quotas and limits are maintained and enforced at the process level. The process virtual address space remains per process and is shared by all threads. The scheduling entity moves from the process to the kernel thread. In general, ASTs are delivered directly to the kernel threads. Event flags and locks remain per process. See Section 2.8.4 for more information. Access to Inner Modes

With the implementation of kernel threads, a single threaded process continues to function exactly as it has in the past. A multithreaded process may have multiple threads executing in user mode or in user mode ASTs, as is also possible for supervisor mode. Except in cases where an activity in inner mode is considered thread safe, a multithreaded process may have only a single thread executing in an inner mode at any one time. Multithreaded processes retain the normal preemption of inner mode by more inner mode ASTs. A special inner mode semaphore serializes access to inner mode. Scheduling

With the implementation of kernel threads, the OpenVMS scheduler concerns itself with kernel threads, and not processes. At certain points in the OpenVMS executive at which the scheduler could wait a kernel thread, it can instead transfer control to the thread manager. This transfer of control, known as a callback or upcall, allows the thread manager the chance to reschedule stalled application threads. ASTs

With the implementation of kernel threads, ASTs are not delivered to the process. They are delivered to the kernel thread on which the event was initiated. Inner mode ASTs are generally delivered to the kernel thread already in inner mode. If no thread is in inner mode, the AST is delivered to the kernel thread that initiated the event. Event Flags

With the implementation of kernel threads, event flags continue to function on a per-process basis, maintaining compatibility with existing application behavior. Process Control Services

With the implementation of kernel threads, many process control services continue to function at the process level. SYS$SUSPEND and SYS$RESUME system services, for example, continue to change the scheduling state of the entire process, including all of its threads. Other services such as SYS$HIBER and SYS$SCHDWK act on individual kernel threads instead of the entire process.

2.8.4 Kernel Threads Process Structure

This section describes the components that make up a kernel threads process. It describes the following components:

  • Process control block (PCB) and process header (PHD)
  • Kernel thread block (KTB)
  • Floating-point registers and execution data block (FRED)
  • Kernel threads region
  • Per-kernel thread stacks
  • Per-kernel thread data cells
  • Process status bits
  • Kernel thread priorities Process Control Block (PCB) and Process Header (PHD)

Two primary data structures exist in the OpenVMS executive that describe the context of a process:

  • Software process control block (PCB)
  • Process header (PHD)

The PCB contains fields that identify the process to the system. The PCB comprises contexts that pertain to quotas and limits, scheduling state, privileges, AST queues, and identifiers. In general, any information that is required to be resident at all times is in the PCB. Therefore, the PCB is allocated from nonpaged pool.

The PHD contains fields that pertain to a process's virtual address space. The PHD consists of the working set list and the process section table. The PHD also contains the hardware process control block (HWPCB) and a floating-point register save area. The HWPCB contains the hardware execution context of the process. The PHD is allocated as part of a balance set slot, and it can be outswapped. Effect of a Multithreaded Process on the PCB and PHD

With multiple execution contexts within the same process, the multiple threads of execution all share the same address space, but have some independent software and hardware context. This change to a multithreaded process results in an impact on the PCB and PHD structures, and on any code that references them.

Before the implementation of kernel threads, the PCB contained much context that was per-process. Now, with the introduction of multiple threads of execution, much context becomes per-thread. To accommodate per-thread context, a new data structure, the kernel thread block (KTB), is created, with the per-thread context removed from the PCB. However, the PCB continues to contain context common to all threads, such as quotas and limits. The new per-kernel thread structure contains the scheduling state, priority, and the AST queues.

The PHD contains the HWPCB that gives a process its single execution context. The HWPCB remains in the PHD; this HWPCB is used by a process when it is first created. This execution context is also called the initial thread. A single threaded process has only this one execution context. A new structure, the floating-point registers and execution data block (FRED), is created to contain the hardware context of the newly created kernel threads. Since all threads in a process share the same address space, the PHD continues to describe the entire virtual memory layout of the process. Kernel Thread Block (KTB)

The kernel thread block (KTB) is a new per-kernel-thread data structure. The KTB contains all per-thread software context moved from the PCB. The KTB is the basic unit of scheduling, a role previously performed by the PCB, and is the data structure placed in the scheduling state queues. Since the KTB is the logical extension of the PCB, the SCHED spinlock synchronizes access to the KTB and the PCB.

Typically, the number of KTBs a multithreaded process has is the same as the number of CPUs on the system. Actually, the number of KTBs is limited by the value of the system parameter MULTITHREAD. If MULTITHREAD is zero, the OpenVMS kernel support is disabled. With kernel threads disabled, user-level threading is still possible with POSIX Threads Library. The environment is identical to the OpenVMS environment prior to the OpenVMS Version 7.0 release. If MULTITHREAD is nonzero, it represents the maximum number of execution contexts or kernel threads that a process can own, including the initial one.

The KTB, in reality, is not an independent structure from the PCB. Both the PCB and KTB are defined as sparse structures. The fields of the PCB that move to the KTB retain their original PCB offsets in the KTB. In the PCB, these fields are unused. In effect, if the two structures are overlaid, the result is the PCB as it currently exists with new fields appended at the end. The PCB and KTB for the initial thread occupy the same block of nonpaged pool; therefore, the KTB address for the initial thread is the same as for the PCB. Floating-Point Registers and Execution Data Blocks (FREDs)

To allow for multiple execution contexts, not only are additional KTBs required to maintain the software context, but additional HWPCBs must be created to maintain the hardware context. Each HWPCB has allocated with it a block of 256 bytes for preserving the contents of the floating-point registers across context switches. Another 128 bytes is allocated for per-kernel thread data.

The combined structure that contains the HWPCB, floating-point register save area, and per-kernel thread data is called the floating-point registers and execution data (FRED) block. These structures reside in the process's balance set slot. This allows the FREDs to be outswapped with the process header. Kernel Threads Region

Much process context resides in P1 space, taking the form of data cells and the process stacks. Some of these data cells need to be per kernel thread, as do the stacks. During initialization of the multithread environment, a kernel thread region in P1 space is initialized to contain the per-kernel-thread data cells and stacks. The region begins at the boundary between P0 and P1 space at address 40000000x, and it grows toward higher addresses and the initial thread's user stack. The region is divided into per-kernel-thread areas. Each area contains pages for data cells and the four stacks. Per-Kernel Thread Stacks

A process is created with four stacks; each access mode has one stack. All four of these stacks are located in P1 space. Stack sizes are either fixed, determined by a SYSGEN parameter, or expandable. The parameter KSTACKPAGES controls the size of the kernel stack. This parameter continues to control all kernel stack sizes, including those created for new execution contexts of kernel threads. The executive stack is a fixed size of two pages; with kernel threads implementation, the executive stack for new execution contexts continues to be two pages in size. The supervisor stack is a fixed size of four pages; with kernel threads implementation, the supervisor stack for new execution contexts is reduced to two pages in size.

For the user stack, a more complex situation exists. OpenVMS allocates P1 space from high to lower addresses. The user stack is placed after the lowest P1 space address allocated. This allows the user stack to expand on demand toward P0 space. With the introduction of multiple sets of stacks, the locations of these stacks impose a limit on the size of each area in which they can reside. With the implementation of kernel threads, the user stack is no longer boundless. The initial user stack remains semi-boundless; it still grows toward P0 space, but the limit is the per-kernel thread region instead of P0 space.

The default user stack in a process can expand on demand to be quite large, so single threaded applications do not typically run out of user stack. When an application is written using POSIX Threads Library, each thread gets its own user stack, which is a fixed size. If the application developer underestimates the stack requirements, the application may fail due to a thread overflowing its stack. This failure is typically reported as an access violation and is very difficult to diagnose. To address this problem, yellow stack zones have been introduced in OpenVMS Version 7.2 and are available to applications using POSIX Threads Library.

Yellow stack zones are a mechanism by which the stack overflow can be signaled back to the application. The application can then choose either to provide a stack overflow handler or do nothing. If the application does nothing, this mechanism helps pinpoint the failure for the application developer. Instead of an access violation being signaled, a stack overflow error is signaled.

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