Data Cache Block Zero Speculative Probe Transaction
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Solution Overview
Problem
In a sequentially consistent multi-processor system, the requirement for non-speculative invalidate transactions when executing a series of data cache block zero (DCBZ) instructions leads to increased latency and slowed execution due to the need for explicit invalidate and allocate operations, especially when DCBZ instructions occur in close proximity, impacting data bandwidth and processor efficiency.
Innovation Solution
A processor is configured to issue either a probe transaction for speculative DCBZ instructions to determine the state of affected cache blocks, allowing the transaction latency to be hidden, and an invalidate transaction for non-speculative instructions, enabling rapid completion once the state is determined to be invalid, thus optimizing the execution of DCBZ instructions by reducing the need for explicit invalidate transactions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If invalidate transactions are used for DCBZ instructions to ensure sequential consistency, then memory access correctness is guaranteed, but execution latency increases due to the need for explicit invalidate and allocate operations
Solution Approach 1:
The system dynamically adjusts the transaction type based on the speculative status of the DCBZ instruction. When an instruction is speculative, a probe transaction is issued instead of an invalidate transaction. This dynamic adaptation allows the system to maintain sequential consistency for non-speculative instructions while enabling speculative execution for instructions that can tolerate delayed validation, thereby resolving the contradiction between reliability and execution latency.
Solution Approach 2:
The invention changes the parameter of transaction type from fixed (always invalidate) to variable (probe or invalidate based on speculative status). By introducing the speculative status parameter and adjusting the transaction type accordingly, the system achieves both sequential consistency where needed and reduced latency for speculative operations, effectively resolving the contradiction between memory access correctness and execution latency.
2Stability of the object's composition
If DCBZ instructions are executed non-speculatively to maintain sequential consistency, then memory ordering is preserved, but processor efficiency decreases due to the latency of each invalidate transaction
Solution Approach 1:
The system dynamically determines whether to execute DCBZ instructions speculatively or non-speculatively based on the speculative status flag. This dynamic approach allows the processor to maintain memory ordering when required while improving efficiency by allowing speculative execution for instructions where ordering constraints are less critical, thus resolving the contradiction between memory ordering stability and processor efficiency.
Solution Approach 2:
The system performs preliminary determination of the speculative status for each DCBZ instruction before executing the invalidate or probe transaction. By anticipating whether an instruction is speculative in advance, the processor can optimize the transaction type selection, maintaining memory ordering where necessary while improving overall processor efficiency through speculative execution paths, thereby resolving the contradiction between memory ordering and processor efficiency.
3Reliability
If a series of DCBZ instructions are executed with full invalidate transactions, then each instruction completes correctly, but the cumulative latency of M*L clock cycles slows down the routine execution
Solution Approach 1:
The system dynamically adjusts the transaction execution strategy for each DCBZ instruction in the series based on its speculative status. By issuing probe transactions for speculative instructions and invalidate transactions only when necessary, the system maintains instruction completion correctness while significantly reducing the cumulative execution time of the routine, effectively resolving the contradiction between reliability and execution duration.
Solution Approach 2:
The invention changes the transaction parameter from always using invalidate to conditionally using probe or invalidate based on speculative status. This parameter change allows the system to maintain correctness for all DCBZ instructions while reducing the cumulative latency from M*L clock cycles to a much lower value by eliminating unnecessary invalidate transactions, thereby resolving the contradiction between instruction correctness and routine execution time.
Data Source
AI summary
In one embodiment, a processor comprises a core configured to execute a data cache block write instruction and an interface unit coupled to the core and to an interconnect on which the processor is configured to communicate. The core is configured to transmit a request to the interface unit in response to the data cache block write instruction. If the request is speculative, the interface unit is configured to issue a first transaction on the interconnect. On the other hand, if the request is non-speculative, the interface unit is configured to issue a second transaction on the interconnect. The second transaction is different from the first transaction. For example, the second transaction may be an invalidate transaction and the first transaction may be a probe transaction. In some embodiments, the processor may be in a system including the interconnect and one or more caching agents.