Optimizing applications for NSPR

NetScape Portable Runtime (NSPR) tries to provide a consistent level of service across the platforms it supports. This has proven to be quite challenging, a challenge that was met to a large degree, but there is always room for improvement. The casual client may not encounter a need to know the details of the shortcomings to the level described here, but if and when clients become more sophisticated, these issues will certainly surface.

This memo is by no way complete.

Multiplatform

  • Do not call any blocking system call from a local thread. The only exception to this rule is the <tt>select()</tt> and <tt>poll()</tt> system calls on Unix, both of which NSPR has overridden to make sure they are aware of the NSPR local threads.

  • In the combined (MxN) model, which includes NT, IRIX (sprocs), and pthreads-user, the primordial thread is always a local thread. Therefore, if you call a blocking system call from the primordial thread, it is going to block more than just the primordial thread and the system may not function correctly. On NT, this problem is especially obvious because the idle thread, which is in charge of driving the asynch io completion port, is also blocked. Do not call blocking system calls from the primordial thread. Create a global thread and call the system call in that thread, and have the primordial thread join that thread.

  • NSPR uses timer signals to implement thread preemption for local threads on some platforms. If all the software linked into the application is not ported to the NSPR API, the application may fail because of threads being preempted during critical sections. To disable thread preemption call <tt>PR_DisableClockInterrupts()</tt>during initialization.

  • Interrupting threads (via <tt>PR_Interrupt()</tt>) on threads blocked in I/O functions is implemented to various degrees on different platforms. The UNIX based platforms all implement the function though there may be up to a 5 second delay in processing the request.

  • The mechanism used to implement <tt>PR_Interrupt()</tt> on the pthreads versions of NSPR is flawed. No failure attributable to the flaw has shown up in any tests or products - yet. The specific area surrounding pthread’s continuation thread has been both observed and empirically proven faulty, and a correction identified.