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Abstracts - 1997 ANNUAL TECHNICAL CONFERENCE
A Revisitation of Kernel Synchronization Schemes
Christopher Small and Stephen Manley
Harvard University
Abstract
In an operating system kernel, critical sections of code
must be protected from interruption. This is traditionally
accomplished by masking the set of interrupts whose
handlers interfere with the correct operation of the criti
cal section. Because it can be expensive to communicate
with an off-chip interrupt controller, more complex opti
mistic techniques for masking interrupts have been pro
posed.
In this paper we present measurements of the
behavior of the NetBSD 1.2 kernel, and use the
measurements to explore the space of kernel
synchronization schemes. We show that (a) most critical
sections are very short, (b) very few are ever
interrupted, (c) using the traditional synchronization
technique, the synchronization cost is often higher than
the time spent in the body of the critical section, and (d)
under heavy load NetBSD 1.2 can spend 9% to 12% of
its time in synchronization primitives.
The simplest scheme we examined, disabling all
interrupts while in a critical section or interrupt handler,
can lead to loss of data under heavy load. A more
complex optimistic scheme functions correctly under
the heavy workloads we tested and has very low
overhead (at most 0.3%). Based on our measurements,
we present a new model that offers the simplicity of the
traditional scheme with the performance of the
optimistic schemes.
Given the relative CPU, memory, and device
performance of today's hardware, the newer techniques
we examined have a much lower synchronization cost
than the traditional technique. Under heavy load, such
as that incurred by a web server, a system using these
newer techniques will have noticeably better
performance.
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