Operating system issues

The thin layer of C/C++ code that interfaces between the RVM and the operating system was ported from AIX to Linux either without change or by replacing the invocation of a system function on AIX with its Linux equivalent. We rewrote for IA32 less than half a page of assembly code which transfers initial execution to the RVM image. The only system service which proved troublesome was the Linux POSIX thread (pthread) library where we had troubles both with the earlier implementation of the library and with differences between the AIX and later Linux implementations.

We started this work on the 2.2 Linux kernel and associated libraries. For this Linux release, the pthread library computes the identity of a thread as a function of the ESP (stack pointer) machine register. Since the RVM virtual processors multiplex several Java threads, each with its own stack (none of them beginning on the large power of two boundary expected by the library), and since our implementation used the ESP register to address the stack of the running thread, we cannot run with this pthread library. (On 2.2 Linux, the RVM only runs with a single virtual processor.)

Fortunately, the 2.4 Linux/IA32 release (and some earlier development releases) resolved this problem.⁸ We also found some differences in the behavior of POSIX threads on AIX and Linux. On AIX, one process may have many pthreads. On Linux, each pthread looks and acts like a separate process. One area where this difference manifests is the behavior of the system with respect to signal handling. The RVM uses two signal handlers which are not reentrant and cannot execute simultaneously. On AIX, we specified that these signals were to be masked for the duration of the signal handler, and as expected pending signals wait until earlier executions of a signal handler finish before executing. Linux did not follow this behavior, so the Linux signal handlers need to provide their own explicit synchronization.