Abstract - Technical Program - 2nd USENIX Windows NT Symposium
Vassal: Loadable Scheduler Support for Multi-Policy Scheduling
George M. Candea
Massachusetts Institute of Technology
Michael B. Jones
Microsoft Research
Abstract
This paper presents Vassal, a system that enables applications to dynamically load and
unload CPU scheduling policies into the operating system kernel, allowing multiple
policies to be in effect simultaneously. With Vassal, applications can utilize scheduling
algorithms tailored to their specific needs and general-purpose operating systems can
support a wide variety of special-purpose scheduling policies without implementing each of
them as a permanent feature of the operating system. We implemented Vassal in the Windows
NT 4.0 kernel.
Loaded schedulers coexist with the standard Windows NT scheduler, allowing most
applications to continue being scheduled as before, even while specialized scheduling is
employed for applications that request it. A loaded scheduler can dynamically choose to
schedule threads in its class, or can delegate their scheduling to the native scheduler,
exercising as much or as little control as needed. Thus, loaded schedulers can provide
scheduling facilities and behaviors not otherwise available. Our initial prototype
implementation of Vassal supports two concurrent scheduling policies: a single loaded
scheduler and the native scheduler. The changes we made to Windows NT were minimal and
they have essentially no impact on system behavior when loadable schedulers are not in
use. Furthermore, loaded schedulers operate with essentially the same efficiency as the
default scheduler. An added benefit of loadable schedulers is that they enable rapid
prototyping of new scheduling algorithms by often removing the time-consuming reboot step
from the traditional edit/compile/reboot/debug cycle.
In addition to the Vassal infrastructure, we also describe a "proof of
concept" loadable real-time scheduler and performance results.
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