Memory Command Selection Policy for Low-Latency Reads
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Solution Overview
Problem
Existing command selection policies in memory devices, such as FCFS and FRFCFS, fail to effectively prioritize read commands, leading to increased latency and suboptimal bandwidth utilization due to timing constraints and lack of consideration for different types of access commands.
Innovation Solution
A modified FRFCFS policy that prioritizes the selection of banks associated with the oldest read commands, tracks ready banks, and dynamically adjusts a threshold value based on the number of read commands to preempt write commands when a certain threshold is reached, thereby reducing latency with minimal impact on bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If traditional FCFS or FRFCFS command selection policies are used, then command execution order is determined, but read commands are not prioritized leading to increased latency
Solution Approach 1:
The command selection policy dynamically adapts between FRFCFS and read-priority modes based on the number of ready banks. When ready banks exceed the threshold, the system switches to prioritizing read commands; otherwise, it maintains FRFCFS ordering. This dynamic adjustment resolves the contradiction by optimizing for latency when possible while preserving bandwidth utilization when constraints exist.
Solution Approach 2:
The system changes the command selection parameter (priority criterion) based on system state. The threshold parameter compares the number of ready banks against a configurable value to determine whether to apply read-command prioritization. This parameter-based control enables the system to reduce latency for read commands while maintaining overall bandwidth efficiency.
2Loss of time
If read commands are prioritized over write commands, then read latency is reduced, but bandwidth utilization may be impacted
Solution Approach 1:
The system dynamically switches between FRFCFS and read-priority selection based on whether the number of ready banks exceeds the threshold. This dynamic behavior ensures read latency is reduced only when bank availability permits, thereby maintaining optimal bandwidth utilization without unnecessary prioritization that would impact write commands and overall data bus efficiency.
Solution Approach 2:
The command selection logic continuously monitors the number of ready banks and uses this feedback to adjust command prioritization. When ready banks exceed the threshold, the system feedback indicates it is safe to prioritize reads; when below threshold, feedback triggers return to FRFCFS mode. This feedback mechanism resolves the contradiction by enabling read prioritization only under conditions that preserve bandwidth utilization.
3Productivity
If FRFCFS policy is used without read prioritization, then bandwidth is maintained, but read command latency increases
Solution Approach 1:
The system dynamically adjusts command selection based on ready bank availability. When the number of ready banks exceeds the threshold, it switches to read-priority mode to reduce read latency; otherwise, it maintains FRFCFS mode to preserve bandwidth efficiency. This dynamic adaptation resolves the contradiction by optimizing for the right metric at the right time.
Solution Approach 2:
The command selection parameter changes based on the threshold comparison. The system uses the ready bank count parameter to determine whether to apply read-command prioritization or maintain FRFCFS ordering. This parameter-driven approach enables the system to reduce read access time when banks are available while maintaining bandwidth efficiency when they are not.
4Loss of time
If the threshold for preempting write commands is lowered, then read latency is reduced more aggressively, but bandwidth utilization decreases
Solution Approach 1:
The threshold parameter controls the aggressiveness of read command prioritization. By adjusting this parameter, the system can optimize for different operating conditions: a lower threshold enables more aggressive read prioritization to reduce latency, while a higher threshold preserves bandwidth utilization. This parameter tuning resolves the contradiction by allowing optimization based on specific performance requirements.
Solution Approach 2:
The system dynamically responds to the threshold setting by preempting write commands only when ready banks exceed it. This dynamic behavior ensures that read latency is reduced to the extent permitted by the threshold configuration without unnecessarily impacting write commands and overall throughput. The threshold acts as a control point that balances latency reduction against bandwidth preservation.
Data Source
AI summary
Memory controller commands to be sent to a memory device may be prioritized based one or more factors. For instance, a memory controller may employ a first-ready, first-come, first-served (FRFCFS) policy in which certain types of commands (e.g., read) are prioritized over other types of commands (e.g., write). The policy may be modified so when deciding which bank to open, the bank with the oldest read command is opened. While continuing to issue commands per this modified FRFCFS policy, the controller may keep track of banks whose pages are ready to receive their respective read commands. Once a number of ready banks meets or exceeds a threshold, issuance of write commands are paused and read commands for the ready banks are issued.