PCIe Atomic Operations for Cache Coherence and Bandwidth
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Solution Overview
Problem
Current PCIe links face inefficiencies due to lack of cache coherence, expensive I/O operations, data serialization, and inefficient memory access, particularly in complex systems with heavy workloads, which impact processing bandwidth and power management.
Innovation Solution
The enhancement of serial point-to-point interconnect architecture through the implementation of cache/prefetch hints, atomic operations, and transaction reordering, along with power management sub-states, to improve cache coherence, reduce serialization delays, and optimize memory access and power usage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If I/O devices use conventional multi-drop parallel bus architecture, then device connectivity is achieved, but processing bandwidth is limited and access efficiency is low
Solution Approach 1:
The patent segments the traditional multi-drop parallel bus into multiple independent serial point-to-point lanes. Each lane operates independently with dedicated transmit and receive pairs, allowing parallel data transmission across multiple channels. This segmentation enables higher processing bandwidth by eliminating the serial access bottleneck of conventional buses while maintaining manageable complexity through standardized lane configurations (1, 2, 4, 8, 16 lanes).
Solution Approach 2:
The patent replaces the mechanical/electrical parallel bus architecture with a serial communication mechanism. Instead of using multiple wires in parallel for data transmission, the system uses time-division multiplexing over serial channels, where data is transmitted bit-by-bit sequentially across multiple lanes. This substitution increases bandwidth efficiency while reducing signal interference and routing complexity.
2Productivity
If I/O accesses use direct I/O mode without cache coherence, then device access is simplified, but access cost increases and processing bandwidth decreases
Solution Approach 1:
The patent implements preliminary cache coherence actions by establishing coherent cache states for I/O memory regions before data access occurs. The system pre-establishes coherence protocols and cache mapping information, allowing subsequent I/O accesses to utilize cached data without requiring expensive coherence checks or re-transmissions. This preliminary setup reduces access time and improves processing bandwidth by eliminating redundant coherence verification steps.
3Reliability
If devices acquire system-wide locks for shared memory access, then data consistency is maintained, but data serialization occurs and processing bandwidth is reduced
Solution Approach 1:
The patent segments the system-wide lock into per-lane or per-transaction atomic operation mechanisms. Instead of requiring a single system-wide lock that serializes all access, the system implements atomic operations that can proceed independently across different lanes or memory regions. This segmentation maintains data consistency through atomic read-modify-write operations while eliminating the serialization bottleneck, allowing multiple devices to access shared memory simultaneously without waiting for a global lock.
Solution Approach 2:
The patent implements feedback mechanisms through atomic operations that provide immediate response about memory access status. The atomic operation protocol includes feedback signals that indicate whether a memory location is readable, writable, or locked, allowing devices to make real-time decisions about access timing. This feedback system maintains data consistency by coordinating access based on current memory state while avoiding the need for system-wide locks that cause serialization.
4Productivity
If devices issue memory transactions in any order, then device autonomy is maintained, but memory access efficiency decreases due to page thrashing
Solution Approach 1:
The patent implements preliminary transaction reordering by the root controller or memory subsystem that analyzes outgoing transactions and rearranges them to optimize memory access patterns. Before transactions are issued to memory, the system pre-sorts and groups them by memory page and access type, consolidating requests for the same page together. This preliminary action prevents page thrashing by ensuring sequential or grouped page accesses rather than random scattering, significantly improving memory access efficiency.
Solution Approach 2:
The patent implements feedback mechanisms where the memory subsystem monitors transaction patterns and provides feedback to the root controller about memory access efficiency. Based on this feedback, the system dynamically adjusts transaction ordering strategies, learning from access patterns to optimize future transaction sequencing. This feedback loop improves memory access efficiency by continuously adapting to workload characteristics while maintaining manageable complexity through automated optimization.
Data Source
AI summary
A method and apparatus for enhancing/extending a serial point-to-point interconnect architecture, such as Peripheral Component Interconnect Express (PCIe) is herein described. Temporal and locality caching hints and prefetching hints are provided to improve system wide caching and prefetching. Message codes for atomic operations to arbitrate ownership between system devices/resources are included to allow efficient access/ownership of shared data. Loose transaction ordering provided for while maintaining corresponding transaction priority to memory locations to ensure data integrity and efficient memory access. Active power sub-states and setting thereof is included to allow for more efficient power management. And, caching of device local memory in a host address space, as well as caching of system memory in a device local memory address space is provided for to improve bandwidth and latency for memory accesses.