Work Scheduler Shutdown via Per-Thread Local Counters
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Solution Overview
Problem
Existing work schedulers face challenges in efficiently shutting down due to cache line contention and transaction failures, leading to high costs and inefficiencies in counting work across multiple threads.
Innovation Solution
A method for shutting down a work scheduler that involves determining when all virtual processors are idle, performing a sweep to ensure no work remains, and then messaging contexts to exit, with a multi-phase shutdown process that includes rollback mechanisms to handle changes in processor states and reference counting.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a work scheduler counts work across multiple threads using shared cache lines, then work completion tracking is achieved, but cache line contention and transaction failures increase significantly
Solution Approach 1:
The patent divides the work counting mechanism into per-thread local counters instead of a single shared counter. Each thread maintains its own work count locally, eliminating the need for frequent cache line locks and reducing contention. The global work completion status is determined by aggregating these local counts only when necessary, rather than continuously locking a shared resource.
Solution Approach 2:
The patent introduces an intermediary mechanism where each thread's work count is tracked through its context structure rather than directly incrementing a shared counter. This intermediary approach allows the system to track work completion without requiring all threads to simultaneously access and modify the same cache line, thereby reducing transaction failures and contention.
2Measurement precision
If the scheduler performs frequent work counting to ensure completion, then work tracking accuracy is maintained, but system performance deteriorates due to locking overhead
Solution Approach 1:
The patent performs preliminary actions by maintaining per-thread work counts continuously in local storage rather than counting globally only when shutdown is requested. This preliminary tracking ensures accuracy is already established before the counting operation is needed, eliminating the performance penalty of frequent global counting operations.
Solution Approach 2:
Each thread independently maintains and updates its own work count in its context structure without requiring service from other threads or centralized coordination. This self-service approach to work tracking eliminates the locking overhead and inter-thread dependency that plagues centralized counting mechanisms.
3Reliability
If the scheduler shuts down by messaging all contexts to exit, then clean termination is achieved, but shutdown time increases due to the multi-phase process
Solution Approach 1:
The patent performs preliminary actions during normal operation by maintaining accurate per-thread work counts and identifying which contexts are actively processing work. When shutdown is requested, this pre-established information allows the system to immediately identify and message only the relevant contexts for termination, rather than systematically checking all contexts.
Solution Approach 2:
The patent implements feedback mechanisms where the scheduler monitors context states and work completion in real-time. During shutdown, this feedback loop allows the system to detect when contexts have finished their current work and can be safely terminated, enabling more aggressive and efficient shutdown sequencing without sacrificing clean termination.
Data Source
AI summary
Shutting down a computer work scheduler. The work scheduler includes a number of virtual processors, each of which is either active or inactive. An active processor executes work, searches for work, or is idle. An inactive has no context running atop it. The method includes determining that all processors controlled by the scheduler are idle. As a result of determining that all controlled by the scheduler are idle, the method proceeds to a first phase of a shutdown operation, which when successful, includes: performing a sweep of all collections searching for any work in the scheduler and determining that no work is found in the scheduler. As a result of determining that no work is found in the scheduler, the method proceeds to a second phase of a shutdown operation, which when successful includes messaging all contexts in the scheduler and telling them to exit.