Elyze: enabling safe parallelism in event-driven servers

It is increasingly necessary for applications to take advantage of concurrency in order to increase performance. Unfortunately, it is notoriously difficult to write correct concurrent applications as they are subject to a variety of subtle bugs that can be difficult to reproduce. We advocate an approach to developing these applications, particularly servers, in which a conservative static analysis determines when code segments may safely run in parallel, and a runtime scheduler respects these constraints, an approach that is safe by default. We have built an analyzer for event-driven servers written in C that detects unsafe data sharing between event handlers. When the analysis determines that two event handlers might access the same data, whether through a global variable or through the request-specific data structure passed to the handlers, it produces a constraint on the concurrent execution of those handlers. We are building a complementary runtime system that will use these constraints to safely run event handlers concurrently. We have analyzed thttpd, an event-driven web server, and show that our analyzer finds safe parallelism in this server, enabling increased performance without the hazards of threads and locks.