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<H1 class=firstHeading>Resource Arbitration and Power Management</H1>
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<LI class=toclevel-1><A
href="http://docs.tinyos.net/index.php/Resource_Arbitration_and_Power_Management#Introduction"><SPAN
class=tocnumber>1</SPAN> <SPAN class=toctext>Introduction</SPAN></A>
<LI class=toclevel-1><A
href="http://docs.tinyos.net/index.php/Resource_Arbitration_and_Power_Management#Working_with_Shared_Resources"><SPAN
class=tocnumber>2</SPAN> <SPAN class=toctext>Working with Shared
Resources</SPAN></A>
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href="http://docs.tinyos.net/index.php/Resource_Arbitration_and_Power_Management#Conclusion"><SPAN
class=tocnumber>3</SPAN> <SPAN class=toctext>Conclusion</SPAN></A>
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Documentation</SPAN></A> </LI></UL></TD></TR></TBODY></TABLE>
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<A name=Introduction></A>
<H1><SPAN class=mw-headline>Introduction</SPAN></H1>
<P>TinyOS distinguishes between three kinds of resource abstractions:
<B>dedicated</B>, <B>virtualized</B>, and <B>shared</B>. </P>
<P>Two fundamental questions must be asked about each type of abstraction. </P>
<OL>
<LI>How can a client gain access to the resource provided through this
abstraction?
<LI>How can the power state of that resource be controlled? </LI></OL>
<P>Components offer resource sharing mechanisms and power mangement capabilites
according to the goals and level of abstraction required by their clients. </P>
<HR>
<P>An abstraction is dedicated if it represents a resource which a subsystem
needs exclusive access to at all times. In this class of resources, no sharing
policy is needed since only a single component ever requires use of the
resource. Resource clients simply call commands from the interfaces provided by
the resource just as they would with any other TinyOS component. Resources of
this type provide either an <TT>AsyncStdControl</TT>, <TT>StdControl</TT>, or
<TT>SplitControl</TT> interface for controlling their power states. The
definition of each of these interfaces can be found in
<TT>tinyos-2.x/tos/interfaces</TT>. </P><PRE>interface AsyncStdControl {
async command error_t start();
async command error_t stop();
}
</PRE><PRE>interface StdControl {
command error_t start();
command error_t stop();
}
interface SplitControl {
async command error_t start();
async command void startDone(error_t error);
async command error_t stop();
async command void stopDone(error_t error);
}
</PRE>
<P>Currently, the power states of all dedicated resources are controlled by one
of these three interfaces. They are only allowed to enter one of two logical
power states (on/off), regardless of the number of physical power states
provided by the hardware on top of which their resource abstraction has been
built. Which of these interfaces is provided by a particular resource depends on
the timing requirements for physically powering it on or off. </P>
<HR>
<P>Virtual abstractions hide multiple clients from each other through software
virtualization. Every client of a virtualized resource interacts with it as if
it were a dedicated resource, with all virtualized instances being multiplexed
on top of a single underlying resource. Because the virtualization is done in
software, there is no upper bound on the number of clients using the
abstraction, barring memory or efficiency constraints. The power states of a
virtualized resource are handled automatically, and no interface is provided to
the user for explicity controlling its power state. As they are built on top of
shared resources, the reason their power states can be automatically controlled
will become clearer after reading the following section. </P>
<HR>
<P>Dedicated abstractions are useful when a resource is always controlled by a
single component. Virtualized abstractions are useful when clients are willing
to pay a bit of overhead and sacrifice control in order to share a resource in a
simple way. There are situations, however, when many clients need precise
control of a resource. Clearly, they can't all have such control at the same
time: some degree of multiplexing is needed. </P>
<P>A motivating example of a shared resource is a bus. The bus may have multiple
peripherals on it, corresponding to different subsystems. For example, on the
Telos platform the flash chip (storage) and the radio (network) share a bus. The
storage and network stacks need exclusive access to the bus when using it, but
they also need to share it with the other subsystem. In this case,
virtualization is problematic, as the radio stack needs to be able to perform a
series of operations in quick succession without having to reacquire the bus in
each case. Having the bus be a shared resource allows the radio stack to send a
series of operations to the radio atomically, without having to buffer them all
up in memory beforehand (introducing memory pressure in the process). </P>
<P>In TinyOS, a resource <B>arbiter</B> is responsible for multiplexing between
the different clients of a shared resource. It determines which client has
access to the resource at which time. While a client holds a resource, it has
complete and unfettered control. Arbiters assume that clients are cooperative,
only acquiring the resource when needed and holding on to it no longer than
necessary. Clients explicitly release resources: there is no way for an arbiter
to forcibly reclaim it. </P>
<P>Shared resources are essentially built on top of dedicated resources, with
access to them being controlled by an arbiter component. In this way, <B>power
managers</B> can be used to automatically control the power state of these
resources through their <TT>AsyncStdControl</TT>, <TT>StdControl</TT>, or
<TT>SplitControl</TT> interfaces. They communicate with the arbiter (through the
use of a <TT>ResourceDefaultOwner</TT> interface), monitoring whether the
resource is being used by any of its clients and powering it on/off accordingly.
The figure below shows how an arbiter component and a power manager can be wired
together to provide arbitration and automatic power management for a shared
resource. </P>
<CENTER><A class=image
title="This is a picture of how an arbiter and power manager work together"
href="http://docs.tinyos.net/index.php/Image:Arbiter_pm_graph.png"><IMG
height=76
alt="This is a picture of how an arbiter and power manager work together"
src="Resource Arbitration and Power Management - TinyOS Documentation Wiki.files/225px-Arbiter_pm_graph.png"
width=225 border=0></A><BR>Figure 1: Arbiters and Power Managers </CENTER>
<P>The arbiter component provides the <TT>Resource</TT>, <TT>ArbiterInfo</TT>,
<TT>ResourceRequested</TT>, and <TT>ResourceDefaultOwner</TT> interfaces and
uses the <TT>ResourceConfigure</TT> interface. The power manager doesn't provide
any interfaces, but uses one of either the <TT>AsyncStdControl</TT>,
<TT>StdControl</TT>, or <TT>SplitControl</TT> interfaces from the underlying
resource, as well as the <TT>ResourceDefaultOwner</TT> interface provided by the
arbiter. The figure below shows how these interface are then wired together with
the implementation of a shared resource. Please refer to TEP 108 for more
information on arbiters and TEP 115 for more information on Power Managers. </P>
<CENTER><A class=image
title="This is a picture of how a shared resource works together with an arbiter and a power manager"
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