User level DB: a debugging API for user-level thread libraries

With the advent of the multicore era, parallel programming is becoming ubiquitous. Multithreading is a common approach to benefit from these architectures. Hybrid M:N libraries like MultiProcessor Communication (MPC) or MARCEL reach high performance expressing fine-grain parallelism by mapping M user-level threads onto N kernel-level threads. However, such implementations skew the debuggers' ability to distinguish one thread from another, because only kernel threads can be handled. SUN MICROSYSTEMS' THREAD_DB API is an interface between the debugger and the thread library allowing the debugger to inquire for thread semantics details. In this paper we introduce the USER LEVEL DB (ULDB) library, an implementation of the THREAD_DB interface abstracting the common features of user-level thread libraries. ULDB gathers the generic algorithms required to debug threads and provide the thread library with a small and focused interface. We describe the usage of our library with widely-used debuggers (GDB, DBX) and the integration into a user-level thread library (GNUPTH) and two high-performance hybrid libraries (MPC, MARCEL).