PSU: A Framework for Dynamic Software Updates in Multi-threaded C-Language Programs

Abstract

A Dynamic Software Update (DSU) system enables an operator to modify a running program without interrupting its execution. However, creating a DSU system to allow programs written in the C programming language to be modified while they are executing is challenging. This paper presents the Portable Software Update (PSU) system, a new framework that allows the creation of C-language DSU programs. PSU offers a simple programming interface to build DSU versions of existing C programs. Once a program is built using PSU, updates can be applied by background threads that have negligible impact on the execution of the program. PSU supports multi-threaded and recursive programs without the use of safe points or thread blocking. PSU uses function indirection to redirect DSU functions calls to the newest version of the function code. Once a DSU function is invoked in a PSU program, it executes to completion using the version of the function that was active when it was invoked. However, if a new version is installed, any future calls to the same function always execute the newest version. This simple mechanism allows for quick loading of updates in PSU. PSU unloads obsolete version of DSU functions after they are no longer executing. This mechanism makes PSU the first DSU system for C-language programs that is able to unload older versions of code. This efficient use of resources enables many patches to be applied to a long-running application. A suite of specialized custom synthetic programs, and a DSU-enabled version of the MySQL database storage engine, are used to evaluate the overhead of the DSU-enabling features. The MySQL storage engine maintains over 95% of the performance of the non-DSU version and allows the entire storage engine to be updated while the database continues executing. PSU includes a simple and straightforward process for the modification of the storage engine that enables DSU.