Microsoft Technology Licensing, LLC

Erez Petrank

Bjarne Steensgaard

Filip Pizlo

Gabriel Kliot

Object relocation often involves a multi-word copy of the object from a source address to a destination address, followed by updating the references (e.g., pointers) to the object. However, during the relocation, other threads may write to portions of the object that have already been relocated, and the updates may be lost when the references are updated to point to the destination address. A non-blocking relocation technique may be implemented to permit threads to write to the object during the relocation, wherein the memory accesses are monitored for a write to the object. If a write is detected during the relocation, the relocation fails and the memory at the destination address is deallocated; but if no write is detected, the relocation succeeds and the references are updated to point to the destination address. Refinements of this technique may be implemented to reduce other synchronization issues.

Optimistic object relocation

Concurrent, incremental, and lock-free stack scanning for garbage collectors is disclosed. This method uses a summary table and return barriers to allow high responsiveness. The method also supports programs that employ fine-synchronization to avoid locks, imposes negligible overhead on program execution, can be used with existing concurrent collectors, and supports the special in-stack references existing in languages such as C#.

Incremental lock-free stack scanning for garbage collection

Mechanism that employs code cloning and specialized code execution for barriers to minimize runtime overhead. This is facilitated by duplicating code and inserting specializations of the barriers in the code copies. The mechanism is effective for garbage collection when the garbage collection executes through different phases, and the barrier behavior and overheads depend on these phases. The duplicated and specialized code enables the program to run efficiently by reducing the dynamic count of a phase check when the phase is well-known and phase checks can be avoided.

Path specializations for runtime code with phase behavior

Object relocation often involves a multi-word copy of the object from a source memory to a destination memory, followed by updating the references (e.g., pointers) to the object. However, during the relocation, other threads may write to portions of the object that have already been relocated, and the updates may be lost when the references are updated to point to the destination memory. The object relocation may therefore mark the words of the object during relocation with a relocation value to indicate transfer to the destination memory without locking the threads. The threads may be configured to check the value the source memory during object access, and to access the corresponding word of the destination memory if the source memory word comprises the relocation value. While the probability of a large (e.g., 64-bit) relocation value appearing in the object is small, safety measures are provided to detect and mitigate conflicts.

Probabilistic object relocation

Described is a technology by which a real-time data relocating mechanism is provided for multiprocessing environments, including supporting lock-free programs that run in parallel. The relocating mechanism moves an object by using a status field related to the data field, possibly in an interim (wide) object space, which is then copied to a to-space object. The status information for each data field of the original object contains information indicating where a current version of the data for each field is present, that is, in the original, wide or to-space object. In one example, a handshake mechanism of a garbage collector establishes preparation and copy phases between the mechanism and other threads that determine where memory accesses occur. Also described is support for program thread compare-and-swap (CAS) operations and/or multi-word atomic operations.

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Redmond, WA, United States of America

Erez Petrank