CCoW: Optimizing Copy-on-Write Considering the Spatial Locality in Workloads

Minjong Ha,Sang-Hoon Kim

doi:10.3390/electronics11030461

Abstract

Copy-on-Write (CoW) is one of the most essential memory management techniques enabling efficient page sharing between processes. Specifically, combined CoW with the fork system call, applications, even with a huge memory footprint, can take a snapshot of the current in-memory data at low overhead. However, since the CoW takes place per page in the page fault handler, each time the page fault occurs, the operating system should get involved. This leads to significant performance degradation for write-intensive workloads. This paper proposes coverage-based copy-on-write (CCoW), an optimized CoW scheme considering the locality in memory accesses to mitigate the problem of CoW. CCoW measures the spatial locality in process address spaces with the concept of coverage. While processing CoW, CCoW copies multiple pages in advance for high-locality memory regions, thereby minimizing the involvement of OS for write-intensive workloads. We explain the challenges for measuring the locality and provide the optimization to implement the concept. Evaluation with a prototype demonstrates that this approach can improve the overall performance of applications by up to 10% with a small amount of memory overhead.

Highlights

The primary role of an operating system (OS) is to manage the precious system resources, and copy-on-write (CoW) is one of the most fundamental memory management techniques adopted by most of contemporary operating systems
As the snapshot is isolated from the address space of the main process but managed by the copy-on-write, the main and child processes do not require any complicated mechanism to maintain the consistency between the current data and the snapshot
Each logical page in the process address space is mapped to its physical location, and this mapping is stored in the form of a page table

Summary

Introduction

The primary role of an operating system (OS) is to manage the precious system resources, and copy-on-write (CoW) is one of the most fundamental memory management techniques adopted by most of contemporary operating systems. Virtual memory systems can effectively provide data sharing between processes, and various virtual memory features, such as duplicating the address space during process forks, deduplicating same pages, zero page sharing, are implemented based on the copy-on-write [1,2,3]. Owing to these features, applications can make a copy of themselves with low space overhead, and use the clone appropriately. Each logical page in the process address space is mapped to its physical location, and this mapping is stored in the form of a page table. Each PTE contains the mapping information and may have additional fields for describing the status of the corresponding page and mapping

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Conclusion