Multithreaded STREAMS

A new SCO OpenServer 5 Release 5 feature is support for multithreaded STREAMS drivers. This allows STREAMS drivers to run on all CPUs in a multiprocessing platform, which can significantly improve system performance.

The specifics of the multithreaded STREAMS interface are summarized below:

• The following can execute on any processor in a multithreaded driver that is registered as a distributed driver:

  • STREAMS head code.

  • servicing of STREAMS service routines. Note, however, that only a single instance of the service routine for any given STREAMS queue is active at any given time, just as it is in a single-threaded driver.

  • put routines of multithreaded drivers and modules.

• Initialization, open, and close routines will run only on the default CPU even when the driver is registered as distributed. Most STREAMS drivers do their interesting work at put, service, and interrupt time, so this does not have much impact on system performance.

• The put and service routines of STREAMS drivers that do not declare themselves as multithreaded execute on the default CPU.

• The putnext(D3str) macro is now a function that checks the next queue. If the queue is not registered as multithreaded, the message is placed on a global queue called the putnext queue which is serviced only by the default CPU.

• The STREAMS queue_t queue data structure and the STREAMS struct stdata structure have been extended with a parallel set of data structures that store locks and other information that is required to support multithreaded modules and drivers. These structures are called equeue_t and struct estdata.

To register the task-time level of a STREAMS driver as multithreaded, issue the sdistributed(D3oddi) function from the driver's xxopen( ) routine after all other initialization is complete. Arguments to sdistributed( ) indicate which put and service routines are multithreaded. Use the MPSTR_QLOCK(D3str) and MPSTR_QRELE(D3str) functions to protect critical code that accesses the STREAMS queue_t data structure. Use the MPSTR_STPLOCK(D3str) and MPSTR_STPRELE(D3str) functions to protect critical code that accesses the Stream head.

To multithread the interrupt level of a STREAMS driver, issue the idistributed(D3oddi) function from the driver's open(D2str) routine.

Multithreaded drivers must use locks rather than the spl(D3oddi) functions to protect critical code sections. For more information about the SCO multiprocessing architecture and protecting critical code sections in multithreaded drivers, see ``Multithreading device drivers''.