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NAME="OVERVIEW.DEGRESS">Degrees of Configurability</H1
><P
>Components can support configurability in varying degrees. It is not
necessary to have any configuration options at all, and the only user
choice is whether or not to load a particular package. Alternatively
it is possible to implement highly-configurable code. As an example
consider a typical facility that is provided by many real-time
kernels, mutex locks. The possible configuration options include:</P
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>If no part of the application and no other component requires mutexes
then there is no point in having the mutex code compiled into a
library at all. This saves having to compile the code. In addition
there will never be any need for the user to configure the detailed
behavior of mutexes. Therefore the presence of mutexes is a
configuration option in itself.</P
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><P
>Even if the application does make use of mutexes directly or
indirectly, this does not mean that all mutex functions have to be
included. The minimum functionality consists of lock and unlock
functions. However there are variants of the locking primitive such as
try-lock and try-with-timeout which may or may not be needed.</P
><P
>Generally it will be harmless to compile the try-lock function even if
it is not actually required, because the function will get eliminated
at link-time. Some users might take the view that the try-lock
function should never get compiled in unless it is actually needed, to
reduce compile-time and disk usage. Other users might argue that there
are very few valid uses for a try-lock function and it should not be
compiled by default to discourage incorrect uses. The presence of a
try-lock function is a possible configuration option, although it may
be sensible to default it to true.</P
><P
>The try-with-timeout variant is more complicated because it adds a
dependency: the mutex code will now rely on some other component to
provide a timer facility. To make things worse the presence of this
timer might impact other components, for example it may now be
necessary to guard against timer interrupts, and thus have an
insidious effect on code size. The presence of a lock-with-timeout
function is clearly a sensible configuration option, but the default
value is less obvious. If the option is enabled by default then the
final application image may end up with code that is not actually
essential. If the option is disabled by default then users will have
to enable the option somehow in order to use the function, implying
more effort on the part of the user. One possible approach is to
calculate the default value based on whether or not a timer component
is present anyway.</P
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><P
>The application may or may not require the ability to create and
destroy mutexes dynamically. For most embedded systems it is both less
error-prone and more efficient to create objects like mutexes
statically. Dynamic creation of mutexes can be implemented using a
pre-allocated pool of mutex objects, involving some extra code to
manipulate the pool and an additional configuration option to define
the size of the pool. Alternatively it can be implemented using a
general-purpose memory allocator, involving quite a lot of extra code
and configuration options. However this general-purpose memory
allocator may be present anyway to support the application itself or
some other component. The ability to create and destroy mutexes
dynamically is a configuration option, and there may not be a sensible
default that is appropriate for all applications.</P
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><P
>An important issue for mutex locks is the handling of priority
inversion, where a high priority thread is prevented from running
because it needs a lock owned by a lower priority thread. This is only
an issue if there is a scheduler with multiple priorities: some
systems may need multi-threading and hence synchronization primitives,
but a single priority level may suffice. If priority inversion is a
theoretical possibility then the application developer may still want
to ignore it because the application has been designed such that the
problem cannot arise in practice. Alternatively the developer may want
some sort of exception raised if priority inversion does occur,
because it should not happen but there may still be bugs in the code.
If priority inversion can occur legally then there are three main ways
of handling it: priority ceilings, priority inheritance, and ignoring
the problem. Priority ceilings require little code but extra effort on
the part of the application developer. Priority inheritance requires
more code but is automatic. Ignoring priority inversion may or may not
be acceptable, depending on the application and exactly when priority
inversion can occur. Some of these choices involve additional
configuration options, for example there are different ways of raising
an exception, and priority inheritance may or may not be applied
recursively.</P
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>As a further complication some mutexes may be hidden inside a
component rather than being an explicit part of the application. For
example, if the C library is configured to provide a
<TT
CLASS="FUNCTION"
>malloc</TT
> call then there may be an associated mutex
to make the function automatically thread-safe, with no need for
external locking. In such cases the memory allocation component of the
C library can impose a constraint on the kernel, requiring that
mutexes be provided. If the user attempts to disable mutexes anyway
then the configuration tools will report a conflict.</P
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><P
>The mutex code should contain some general debugging code such as
assertions and tracing. Usually such debug support will be enabled or
disabled at a coarse level such as the entire system or everything
inside the kernel, but sometimes it will be desirable to enable the
support more selectively. One reason would be memory requirements: the
target may not have enough memory to hold the system if all debugging
is enabled. Another reason is if most of the system is working but
there are a few problems still to resolved; enabling debugging in the
entire system might change the system's timing behavior too much, but
enabling some debug options selectively can still be useful. There
should be configuration options to allow specific types of debugging
to be enabled at a fine-grain, but with default settings inherited
from an enclosing component or from global settings.</P
></LI
><LI
><P
>The mutex code may contain specialized code to interact
with a debugging tool running on the host. It should be
possible to enable or disable this debugging code, and there may
be additional configuration options controlling the detailed
behavior.</P
></LI
></OL
><P
>Altogether there may be something like ten to twenty configuration
options that are specific to the mutex code. There may be a similar
number of additional options related to assertions and other debug
facilities. All of the options should have sensible default values,
possibly fixed, possibly calculated depending on what is happening
elsewhere in the configuration. For example the default setting for
an assertion option should generally inherit from a kernel-wide
assertion control option, which in turn inherits from a global option.
This allows users to enable or disable assertions globally or at
a more fine-grained level, as desired.</P
><P
>Different components may be configurable to different degrees, ranging
from no options at all to the fine-grained configurability of the
above mutex example (or possibly even further). It is up to component
writers to decide what options should be provided and how best to
serve the needs of application developers who want to use that
component.</P
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