TLB Lease Values for Shootdown Latency Reduction
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Solution Overview
Problem
The latency associated with TLB shootdowns in computing devices is high, especially as the number of processors increases, leading to significant delays and inefficiencies in virtual address to physical address translations.
Innovation Solution
Implementing a lease value system for TLB entries, where each entry has a valid time, allowing processors to avoid unnecessary TLB shootdowns by checking the latest lease value for page table entries, thereby determining if cached copies are still valid before modifying them.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If TLB shootdown is performed to modify page table entries, then consistency between page table and TLB copies is maintained, but latency increases significantly especially as number of processors increases
Solution Approach 1:
The patent applies preliminary action by assigning lease values to TLB entries before modification is needed. When a page table entry is to be modified, the system checks the lease value in advance to determine if a shootdown is necessary, rather than performing shootdown unconditionally. This advance preparation allows the system to avoid unnecessary shootdown operations and their associated latency.
Solution Approach 2:
The patent introduces lease values as a new parameter associated with TLB entries. This parameter change enables the system to track the validity period of cached translations and make informed decisions about shootdown necessity. By adding this temporal parameter, the system can distinguish between entries that require immediate invalidation and those that can remain cached, thereby reducing overall shootdown latency.
2Reliability
If TLB shootdown is performed frequently to maintain consistency, then data consistency is ensured, but communication bandwidth and power consumption increase
Solution Approach 1:
By introducing lease values as a new parameter, the system can track when TLB entries were last updated and make informed decisions about shootdown necessity. This parameter enables the system to avoid unnecessary shootdown operations, thereby reducing communication bandwidth consumption and power usage while maintaining data consistency.
Solution Approach 2:
The lease value mechanism enables TLB entries to essentially self-manage their validity state. Rather than requiring active coordination through shootdown operations, the lease value automatically indicates whether an entry is still valid or needs invalidation, reducing the need for inter-processor communication and associated energy costs.
Data Source
AI summary
The described embodiments include a computing device with two or more translation lookaside buffers (TLB) that performs operations for handling entries in the TLBs. During operation, the computing device maintains lease values for entries in the TLBs, the lease values representing times until leases for the entries expire, wherein a given entry in the TLB is invalid when the associated lease has expired. The computing device uses the lease value to control operations that are allowed to be performed using information from the entries in the TLBs. In addition, the computing device maintains, in a page table, longest lease values for page table entries indicating when corresponding longest leases for entries in TLBs expire. The longest lease values are used to determine when and if a TLB shootdown is to be performed.