Cache Footprint Tracking for Fast Context Switching

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Solution Overview

Problem

Context switching in modern processors is costly due to the time-consuming process of reloading cache footprints, particularly TLB, SLB, and cache states, which significantly impacts processing performance as processor speed outpaces memory speed.

Innovation Solution

Allowing an execution entity to continue executing for a predetermined period to track cache access patterns, saving this information, and selectively restoring the cache footprint based on past and expected performance when context switching in, thereby reducing the overhead of cache reloading.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If the CPU state including cache footprint is saved and restored during context switch, then the process can resume execution correctly, but the context switch time increases significantly due to cache reloading

Engineering Contradiction:

Improvecorrect process resumptionVSAvoidcontext switch time

Core Design Contradiction:

ReliabilityVSLoss of time

Solution Approach 1:

The patent applies preliminary action by allowing the execution entity to continue executing for a predetermined period after the context switch decision is made. During this period, the cache footprint is tracked and saved in advance, so that when the context switch actually occurs, the cache state is already prepared and can be restored quickly without stalling the processor.

Inventive Principle:

Principle #10Preliminary action

2Power

If the cache footprint is re-created after context switch, then the processor can access required data, but the processor performance deteriorates due to cache misses and stalls

Engineering Contradiction:

Improveprocessor access capabilityVSAvoidprocessor performance

Core Design Contradiction:

PowerVSProductivity

Solution Approach 1:

The cache footprint information is tracked and saved in advance during the predetermined period before the context switch takes effect. This preliminary tracking ensures that when the process is resumed, the cache can be quickly restored from the saved information without causing processor stalls or performance degradation.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The execution entity continues to execute during the predetermined period after the context switch decision, maintaining continuous useful action. This allows the cache access patterns to be captured in real-time while the processor remains productive, rather than stalling to save cache state immediately.

Inventive Principle:

Principle #20Continuity of useful action

3Adaptability or versatility

If registers and cache states are saved and restored, then multiple processes can share the CPU, but the system overhead increases due to the cache footprint reloading cost

Engineering Contradiction:

Improveprocess sharing capabilityVSAvoidsystem overhead

Core Design Contradiction:

Adaptability or versatilityVSLoss of energy

Solution Approach 1:

The cache footprint is tracked and saved in advance during the predetermined period, preparing the cache state before the context switch actually occurs. This preliminary action reduces the overhead of cache reloading when processes share the CPU, as the cache state is already prepared and can be restored quickly without significant energy consumption.

Inventive Principle:

Principle #10Preliminary action

Data Source

PatentUS7634642B2Mechanism to save and restore cache and translation trace for fast context switch

Publication Date: 2009.12.15 INTERNATIONAL BUSINESS MACHINE CORPORATION

AI summary

A method and system for efficient context switching are provided. An execution entity that is to be context switched out is allowed to continue executing for a predetermined period of time before being context switched out. During the predetermined period of time in which the execution entity continues to execute, the hardware or an operating system tracks and records its footprint such as the addresses and page and segment table entries and the like accessed by the continued execution. When the execution entity is being context switched back in, its page and segment table and cache states are reloaded for use in its immediate execution.