Dynamic Tag Allocation for Multithreaded Out-of-Order Processor
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Solution Overview
Problem
Computer processors face challenges in maintaining program order during out-of-order execution, particularly in multithreaded environments where ensuring that instructions are committed in the correct order without causing incorrect processor states is crucial.
Innovation Solution
A computer processor dynamically allocates tag values from a predetermined set to groups of instructions across multiple threads, using a reorder buffer to manage the commitment of instructions in program order, ensuring that subsequent instructions are only committed after earlier ones have completed execution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If instructions are executed out-of-order to improve processor throughput, then execution efficiency is improved, but maintaining program order for commitment becomes more complex
Solution Approach 1:
The patent assigns sequence numbers to instructions before execution begins. This preliminary assignment of ordering information allows the processor to execute instructions out-of-order while maintaining the ability to commit them in the correct program order, as the sequence numbers are already established and can be used to determine commitment order.
Solution Approach 2:
The patent introduces sequence numbers as an intermediary mechanism between out-of-order execution and in-order commitment. These sequence numbers act as a mediator that decouples the execution order from the commitment order, allowing instructions to be executed in any order while ensuring they are committed in program order based on their sequence number tags.
2Device complexity
If a fixed set of tag values is used to track instruction order, then resource usage is simplified, but dynamic allocation between multiple threads reduces efficiency
Solution Approach 1:
The patent implements dynamic tag allocation where tag values are not permanently assigned to specific threads but are allocated on-demand from a shared pool. When a thread needs a tag, it borrows one from the available pool, and when the thread's instructions are committed, the tag is returned to the pool for reuse. This dynamic approach allows efficient multithreaded execution while keeping the tag management system relatively simple.
Solution Approach 2:
The patent employs a tag recycling mechanism where tag values are discarded (returned to the pool) after their instructions are committed and can be recovered (reallocated) for new instructions. This allows the same fixed set of tag values to serve multiple threads over time, improving multithreaded efficiency without requiring an expanded tag pool for each thread.
Data Source
AI summary
Various techniques for dynamically allocating instruction tags and using those tags are disclosed. These techniques may apply to processors supporting out-of-order execution and to architectures that supports multiple threads. A group of instructions may be assigned a tag value from a pool of available tag values. A tag value may be usable to determine the program order of a group of instructions relative to other instructions in a thread. After the group of instructions has been (or is about to be) committed, the tag value may be freed so that it can be re-used on a second group of instructions. Tag values are dynamically allocated between threads; accordingly, a particular tag value or range of tag values is not dedicated to a particular thread.