Transactional Memory Register Checkpointing via Rename Logic
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Solution Overview
Problem
Checkpointing the register state in processing devices for transactional memory consumes significant machine resources and processing overhead, particularly due to the latency of writing architectural registers to private memory and the need for additional physical registers when using a rename table.
Innovation Solution
Implementing rename logic to map a predetermined number of architectural registers to physical registers and using microcode to write these values to memory when the number of modified registers exceeds the threshold, thereby optimizing resource allocation and reducing overhead.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If architectural registers are written to private memory for checkpointing, then the checkpoint operation is straightforward and does not require allocating physical registers, but it requires significant upfront latency (50-100 cycles) which reduces performance for short transactions
Solution Approach 1:
The checkpointing process is segmented into two parts: a fast path using physical register allocation for transactions with few modified registers, and a fallback path using private memory for transactions with many modified registers. This segmentation allows the system to optimize for the common case (short transactions) while still handling the general case correctly.
Solution Approach 2:
The system dynamically selects the checkpointing method based on the number of modified registers. The rename table size is configured as a threshold parameter that determines whether to use the fast physical register path or the slower private memory path, allowing adaptive optimization based on transaction characteristics.
2Loss of time
If an additional rename table is implemented to map checkpointed values to physical registers, then checkpointing is faster than writing to private memory, but it requires allocating additional physical registers which may adversely affect performance by leaving fewer physical registers available for in-flight instructions
Solution Approach 1:
The system changes the parameter of rename table size to optimize the balance between checkpointing speed and register availability. By configuring the rename table size as a threshold, the system can adjust this parameter to match the specific performance requirements and hardware resource constraints of different implementations.
Solution Approach 2:
Physical registers allocated for checkpointing in the rename table are discarded (made unavailable for other purposes) during the transaction, but are recovered and made available for in-flight instructions after the transaction completes or aborts. This temporary allocation allows fast checkpointing without permanently reducing the register file size.
3Productivity
If a larger physical register file is built to achieve acceptable performance on transactions that modify a large number of architectural registers, then performance on such transactions improves, but it increases the overall device complexity and resource consumption
Solution Approach 1:
The system segments the transaction handling into two paths based on the number of modified registers: a fast path for transactions modifying fewer registers than the rename table size, and a fallback path for transactions modifying more registers. This avoids the need to provision a large register file for the rare case of transactions modifying many registers.
Solution Approach 2:
The system uses a relatively small, fixed-size rename table as a disposable resource for checkpointing. Rather than provisioning a large register file that would be partially utilized, the system uses a smaller rename table that is allocated temporarily when needed and then freed, accepting the occasional fallback to private memory for extreme cases.
Data Source
AI summary
The present invention provides a method and apparatus for checkpointing registers for transactional memory. Some embodiments of the apparatus include first rename logic configured to map up to a predetermined number of architectural registers to corresponding first physical registers that hold first values associated with the architectural registers. The mapping is responsive to a transaction modifying one or more of the first values associated with the architectural registers. Some embodiments of the apparatus also include microcode configured to write contents of the first physical registers to a memory in response to the transaction modifying first values associated with a number of the architectural registers that is larger than the predetermined number.