TLB Shootdown State Consolidation for CPU Cache Contention
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Solution Overview
Problem
Conventional TLB shootdowns in symmetric multiprocessor systems are expensive and time-consuming due to complex protocols that burden multiple processing cores, leading to performance impacts and potential safety violations, especially since most processor architectures lack hardware TLB coherence.
Innovation Solution
The implementation of 'early acknowledgement' and 'shared state consolidation' optimizations in the TLB shootdown mechanism, where early acknowledgement allows remote cores to return acknowledgments immediately upon entering the shootdown interrupt handler, and shared state consolidation reduces CPU cache contention by consolidating shared variables and data structures, thereby reducing synchronous waiting and cache contention.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional TLB shootdown protocol is used to ensure TLB coherence across multiple cores, then correctness and security are maintained, but system performance deteriorates due to expensive synchronization operations taking several thousand compute cycles
Solution Approach 1:
The patent applies preliminary action by having the initiating core complete its TLB shootdown operation and update its local shared state before notifying remote cores. This allows the initiator to make progress without waiting for remote cores to acknowledge, thereby maintaining correctness while improving performance. The remote cores independently update their own shared state when they receive the notification, eliminating synchronous waiting.
2Reliability
If synchronous waiting is implemented for remote core acknowledgments during TLB shootdown, then correctness is ensured, but time consumption increases significantly
Solution Approach 1:
The patent eliminates synchronous waiting by having the initiating core perform the shootdown operation and update its shared state before waiting for remote acknowledgments. The initiating core does not block other operations while waiting, and remote cores independently update their shared state when notified, converting a synchronous operation into an asynchronous one that maintains correctness without time loss.
3Ease of operation
If shared state is distributed across multiple memory locations for TLB shootdown operations, then accessibility for different cores is improved, but CPU cache contention increases due to scattered access patterns
Solution Approach 1:
The patent merges the scattered shared state variables into a single consolidated memory location. Instead of having separate shared state structures for different TLB shootdown operations, the patent introduces a unified shared state that all cores access through a single location, thereby reducing cache contention while maintaining accessibility for all cores involved in TLB synchronization.
Data Source
AI summary
Techniques for consolidating shared state for translation lookaside buffer (TLB) shootdowns are provided. In one set of embodiments, an operating system (OS) kernel of a computer system can co-locate, in a system memory of the computer system, a plurality of shared data accessed by first and second processing cores of the computer system for performing a translation lookaside buffer (TLB) shootdown of the first processing core by the second processing core, where the co-locating allows the plurality of shared data to occupy a single cache line when brought from the system memory into a CPU (central processing unit) cache of the first or second processing core. This can include, e.g., (1) co-locating a lazy mode indicator and a call function queue (CFQ) head element of the first processing core, such that these two data components occupy the same cache line, and (2) co-locating a TLB flush info entry and a call function data (CFD) entry created by the second processing core at the time of initiating the TLB shootdown, such that these two data components occupy the same cache line.