Porting SVR4 MP Modules and Drivers to HP-UX

Differences between SVR4 and HP-UX MP STREAMS

HP-UX STREAMS provides MP scalability differently from SVR4 MP STREAMS. There are two main differences. The first pertains to which STREAMS/UX entities run in parallel. SVR4 MP STREAMS executes put and service routines for the same queue concurrently although only one instance of a service routine can run at a time. HP-UX, unlike SVR4 MP, allows the developer to configure which STREAMS/UX entities run in parallel. The most parallelism that a STREAMS/UX developer can configure is to run entry points for different queues concurrently. Unlike SVR4 MP, HP-UX only allows one entry point for a queue to run at a time. The put and service routines for the same queue cannot run in parallel. Also, multiple instances of a queue's put or service routine cannot execute concurrently.

The second difference has to do with synchronizing access to module and driver private data structures. SVR4 MP STREAMS does not provide protection for private structures. The module or driver code uses spinlocks to synchronize access. STREAMS/UX provides protection for private structures. The developer configures the amount of concurrency for a module or driver based on the entities with which it shares data structures. For example, if all instances of a module access the same table, the programmer can configure the module so that only one instance runs at a time.

Strategies for Porting SVR4 MP Modules and Drivers to HP-UX

The best way to port SVR4 MP scalable modules and drivers to HP-UX is to change the SVR4 MP code to use the STREAMS/UX synchronization levels. First, analyze how the SVR4 MP code shares data structures, and then configure the modules and drivers to use synchronization levels which correctly serialize access to shared data. You can use defines to change module and driver spinlock calls to no-ops. This approach is likely to get the best performance, but may require much effort. Also, the STREAMS/UX synchronization levels may not be suitable for all designs.

To make porting easier, STREAMS/UX will provide support for the SVR4 MP spinlock primitives. SVR4 MP modules and drivers could be ported to HP-UX by configuring them to run with queue synchronization and leaving in the calls to SVR4 MP spinlock routines. A disadvantage of this porting strategy is that it may not achieve as much performance as the first. Some of the synchronization provided by STREAMS/UX will be redundant with the synchronization implemented by module and driver spinlocks. In some cases, a combination of these two strategies may make sense. For example, suppose several modules and drivers share the same structure, but do not access it on the main read and write paths. You can use SVR4 MP spinlocks to protect this data, but use the STREAMS/UX synchronization levels to protect other structures.