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Memory Pool System Project

                      THE DESIGN OF THE MPS THREAD MANAGER
                           design.mps.thread-manager
                               incomplete design
                               richard 1995-11-20

    PURPOSE

    The Thread Manager handles various thread-related functions required by the
    MPS.  These are:

      - stack scanning

      - suspension and resumption of the mutator threads

    CONTEXT

    The barrier requires suspension and resumption of threads in order to ensure
    that the collector has exclusive access to part of memory.
    [design.mps.barrier.@@@]

    Stack scanning is provided as a service to the client.  [Link?@@@]

    OVERVIEW

    Each thread is represented by an object of type "Thread".  The Thread type is
    implemented as an ADT.  A deque of Thread objects is maintained in the Space.
    The Thread object contains OS dependent information about the thread --
    information necessary for manipulating the thread and for scanning the thread
    context.  Thread "registration" adds or removes the current thread to the
    Thread deque in the Space.

    DETAILED DESIGN

    Stack Scan
    This is a module providing a stack scanning function.  The scanning is arch/os
    dependent.  Typically the function will push the save registers (those
    preserved across function calls) which plausibly contain pointers (not FP/debug
    registers) and call TraceScanStack on the appropriate range.

    Thread Interface

    - Register/Deregister
      register returns "Thread"

    - Suspend/Resume
      suspends registered threads which are not the current thread

    - ThreadDequeScan
      ambiguously scans the stacks and root registers of all threads.  The exact
    definition is os/arch dependent

    Single-Threaded Generic Implementation

    - single thread
    - Suspend/Resume do nothing because there are no other threads.
    - registration records stack base only.
    - ThreadDequeScan calls StackScan

    Win32 Implementation
    - supports multiple threads
    - structured exception style faults are expected
    - suspend/resume loop over threads and call Win32 suspend/resume
    - registration records information for current thread
      - stack base for current thread
      - Win32 "handle" with SUSPEND/RESUME and GET CONTEXT access to the thread.
    This handle is needed as parameter to
        - Suspend/ResumeThread
        - ThreadGetContext
      - Win32 ThreadId this is so that the current thread may be identified.
    - stack scanning is Win32 specific
      - ThreadDequeScan uses GetThreadContext for other threads, giving root
    registers and the stack pointer
      - The thread's registers are dumped into the CONTEXT structure and fixed in
    memory.
      - scan the stack (getting sp from CONTEXT)
      - ThreadDequeScan calls StackScan to do the current thread.  The current
    thread is different because GetContext doesn't work on it.   (The context would
    not necessarily have the values which were in the saved registers on entry to
    the MM).

    ISSUES

    Scanning after Exceptions
    StackScan relies on the non-preserved registers having been pushed on the
    stack.  If we want to scan after a fault we must make sure that these registers
    are either already stored on the stack, or, have an extra function to do this
    explicitly.

    Multiple Registration
    It is not clear whether a thread should be allowed to be registered multiple
    times.  We do not provide a mechanism for knowing whether a thread is already
    registered with a space.

## A. References

## B. Document History

2002-06-21 | RB | Converted from MMInfo database design document.

## C. Copyright and License

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