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D.12 Other Optimizations and Determinism Rules
1
[This clause describes various requirements for
improving the response and determinism in a real-time system.]
Implementation Requirements
2
If the implementation blocks interrupts (see
C.3)
not as a result of direct user action (e.g. an execution of a protected
action) there shall be an upper bound on the duration of this blocking.
2.a
Ramification: The implementation
shall not allow itself to be interrupted when it is in a state where
it is unable to support all the language-defined operations permitted
in the execution of interrupt handlers. (see 9.5.1).
3
The implementation shall recognize entry-less
protected types. The overhead of acquiring the execution resource of
an object of such a type (see
9.5.1) shall
be minimized. In particular, there should not be any overhead due to
evaluating
entry_barrier conditions.
3.a
Implementation Note: Ideally
the overhead should just be a spin-lock.
4
Unchecked_Deallocation shall be supported for
terminated tasks that are designated by access types, and shall have
the effect of releasing all the storage associated with the task. This
includes any run-time system or heap storage that has been implicitly
allocated for the task by the implementation.
Documentation Requirements
5
The implementation shall document the upper bound
on the duration of interrupt blocking caused by the implementation. If
this is different for different interrupts or interrupt priority levels,
it should be documented for each case.
5.a
Implementation defined: The
upper bound on the duration of interrupt blocking caused by the implementation.
Metrics
6
The implementation
shall document the following metric:
7
- The overhead associated with obtaining
a mutual-exclusive access to an entry-less protected object. This shall
be measured in the following way:
8
For a protected object
of the form:
9
protected Lock is
procedure Set;
function Read return Boolean;
private
Flag : Boolean := False;
end Lock;
10
protected body Lock is
procedure Set is
begin
Flag := True;
end Set;
function Read return Boolean
Begin
return Flag;
end Read;
end Lock;
11
The execution time, in processor clock cycles, of
a call to Set. This shall be measured between the point just before issuing
the call, and the point just after the call completes. The function Read
shall be called later to verify that Set was indeed called (and not optimized
away). The calling task shall have sufficiently high priority as to not
be preempted during the measurement period. The protected object shall
have sufficiently high ceiling priority to allow the task to call Set.
12
For a multiprocessor, if supported, the metric shall
be reported for the case where no contention (on the execution resource)
exists [from tasks executing on other processors].
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