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2001 FREENIX Track Technical Program - Abstract
Improving the FreeBSD SMP implementation
Greg Lehey, IBM LTC Ozlabs
Abstract
UNIX-derived operating systems have traditionally have a simplistic approach to
process synchronization which is unsuited to multiprocessor application.
Initial FreeBSD SMP support kept this approach by allowing only one process to
run in kernel mode at any time, and also blocked interrupts across multiple
processors, causing seriously suboptimal performance of I/O bound systems. This
paper describes work done to remove this bottleneck, replacing it with
fine-grained locking. It derives from work done on BSD/OS and has many
similarities with the approach taken in SunOS 5. Synchronization is performed
primarily by a locking construct intermediate between a spin lock and a binary
semaphore, termed mutexes. In general, mutexes attempt to block rather
than to spin in cases where the likely wait time is long enough to warrant a
process switch. The issue of blocking interrupt handlers is addressed by
attaching a process context to the interrupt handlers. Despite this process
context, an interrupt handler normally runs in the context of the interrupted
process and is scheduled only when blocking is required.
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