Lazy Translation Lookaside Buffer Coherence via Periodic Revalidation
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Solution Overview
Problem
Current systems face inefficiencies in maintaining TLB coherence, particularly due to the high cost of TLB shootdowns, which are necessary for propagating page table entry changes in a synchronous manner, and this is expected to worsen as PTE changes increase in frequency.
Innovation Solution
Implementing a hardware-supported mechanism for periodic TLB revalidation, ensuring that TLBs are refreshed after a certain number of context switches, allowing for deterministic software schemes to manage PTE changes without requiring all cores to complete a context switch, and using micro-batched PTE updates to accelerate the process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If TLB shootdowns are used to propagate PTE changes synchronously, then TLB coherence is maintained, but system performance deteriorates due to prohibitive cost
Solution Approach 1:
The patent implements periodic revalidation of TLB entries at predetermined intervals instead of immediate shootdowns. The revalidation unit checks whether cached PTE information is still valid by comparing against updated PTE data, allowing the system to tolerate stale entries temporarily while maintaining coherence when necessary. This periodic approach dramatically reduces the frequency of full TLB invalidation events.
Solution Approach 2:
The system performs preliminary validation checks before actually invalidating TLB entries. The revalidation unit proactively verifies PTE validity using validation data from memory, allowing the system to detect and handle coherence issues before they cause incorrect address translations. This preliminary action prevents the need for frequent emergency shootdowns.
2Adaptability or versatility
If frequent PTE changes are supported, then system adaptability improves, but TLB coherence maintenance becomes more expensive
Solution Approach 1:
The revalidation unit autonomously manages TLB entry validation without requiring external intervention or full system-wide shootdowns. It independently compares cached PTE information against validation data stored in memory, and only initiates invalidation when actually necessary. This self-service mechanism allows frequent PTE changes to be tolerated while minimizing the energy cost of coherence maintenance.
Solution Approach 2:
The system changes the temporal parameter of coherence enforcement from immediate (synchronous) to delayed (periodic). By allowing TLB entries to remain valid for predetermined intervals even after PTE changes occur in memory, the system accommodates frequent PTE updates without triggering expensive coherence protocols each time. The validation interval parameter can be adjusted based on system requirements.
3Reliability
If synchronous PTE propagation is implemented, then address translation consistency is improved, but time consumption increases
Solution Approach 1:
Instead of propagating PTE changes synchronously across all processors immediately, the system uses periodic revalidation at predetermined intervals. Each processor independently validates its cached PTE information against validation data in memory at these intervals, ensuring consistency without requiring coordinated system-wide actions. This asynchronous periodic approach eliminates propagation delay while maintaining reliability.
Solution Approach 2:
The patent introduces validation data as an intermediary between the actual PTE changes in memory and the cached translations in TLBs. Rather than directly invalidating TLB entries when PTEs change, the system updates validation data in memory and allows TLBs to continue using cached translations until their next periodic validation check. This intermediary layer decouples the timing of PTE updates from translation invalidation.
Data Source
AI summary
An apparatus and method are described for managing TLB coherence. For example, one embodiment of a processor comprises: one or more cores to execute instructions and process data; one or more translation lookaside buffers (TLBs) each comprising a plurality of entries to cache virtual-to-physical address translations usable by the set of one or more cores when executing the instructions; one or more epoch counters each programmed with a specified epoch value; and TLB validation logic to validate a specified set of TLB entries at intervals specified by the epoch value.