D.9 Delay Accuracy
1
[This clause specifies performance requirements for
the delay_statement. The rules apply both
to delay_relative_statement and to delay_until_statement.
Similarly, they apply equally to a simple delay_statement
and to one which appears in a delay_alternative.]
Dynamic Semantics
2
The effect of the delay_statement
for Real_Time.Time is defined in terms of Real_Time.Clock:
3
- If C1
is a value of Clock read before a task executes a delay_relative_statement
with duration D, and C2
is a value of Clock read after the task resumes execution following that
delay_statement, then C2
– C1
>= D.
4
- If C is a value of Clock read after
a task resumes execution following a delay_until_statement
with Real_Time.Time value T, then C >= T.
5
{potentially blocking
operation (delay_statement) [partial]} {blocking,
potentially (delay_statement) [partial]} A
simple
delay_statement with a negative or
zero value for the expiration time does not cause the calling task to
be blocked; it is nevertheless a potentially blocking operation (see
9.5.1).
6
When a delay_statement
appears in a delay_alternative of a timed_entry_call
the selection of the entry call is attempted, regardless of the specified
expiration time. When a delay_statement appears
in a selective_accept_alternative, and a call
is queued on one of the open entries, the selection of that entry call
proceeds, regardless of the value of the delay expression.
6.a
Ramification: The effect of these requirements
is that one has to always attempt a rendezvous, regardless of the value
of the delay expression. This can be tested by issuing a timed_entry_call
with an expiration time of zero, to an open entry.
Documentation Requirements
7
The implementation shall document the minimum value
of the delay expression of a delay_relative_statement
that causes the task to actually be blocked.
7.a/2
Documentation Requirement:
The minimum value of the delay expression
of a delay_relative_statement that causes
a task to actually be blocked.
8
The implementation shall document the minimum difference
between the value of the delay expression of a delay_until_statement
and the value of Real_Time.Clock, that causes the task to actually be
blocked.
8.a/2
This paragraph
was deleted.Implementation defined:
Implementation-defined aspects of delay_statements.
8.b/2
Documentation Requirement:
The minimum difference between the value
of the delay expression of a delay_until_statement
and the value of Real_Time.Clock, that causes the task to actually be
blocked.
Metrics
9
The implementation
shall document the following metrics:
10
- An upper bound on the execution time,
in processor clock cycles, of a delay_relative_statement
whose requested value of the delay expression is less than or equal to
zero.
11
- An upper bound on the execution time,
in processor clock cycles, of a delay_until_statement
whose requested value of the delay expression is less than or equal to
the value of Real_Time.Clock at the time of executing the statement.
Similarly, for Calendar.Clock.
12
- {lateness}
{actual duration}
An upper bound on the lateness of a delay_relative_statement,
for a positive value of the delay expression, in a situation where the
task has sufficient priority to preempt the processor as soon as it becomes
ready, and does not need to wait for any other execution resources. The
upper bound is expressed as a function of the value of the delay expression.
The lateness is obtained by subtracting the value of the delay expression
from the actual duration. The actual duration is measured from
a point immediately before a task executes the delay_statement
to a point immediately after the task resumes execution following this
statement.
13
- An upper bound on the lateness of
a delay_until_statement, in a situation where
the value of the requested expiration time is after the time the task
begins executing the statement, the task has sufficient priority to preempt
the processor as soon as it becomes ready, and it does not need to wait
for any other execution resources. The upper bound is expressed as a
function of the difference between the requested expiration time and
the clock value at the time the statement begins execution. The lateness
of a delay_until_statement is obtained by
subtracting the requested expiration time from the real time that the
task resumes execution following this statement.
13.a/2
Documentation Requirement:
The metrics for delay statements.
14/2
This paragraph was
deleted.40 {
AI95-00355-01}
The execution time of a delay_statement
that does not cause the task to be blocked (e.g. “delay
0.0;” ) is of interest in situations where delays are used to achieve
voluntary round-robin task dispatching among equal-priority tasks.
Wording Changes from Ada 83
14.a
The rules regarding a timed_entry_call
with a very small positive Duration value, have been tightened to always
require the check whether the rendezvous is immediately possible.
Wording Changes from Ada 95
14.b/2
{
AI95-00355-01}
The note about “voluntary round-robin’,
while still true, has been deleted as potentially confusing as it is
describing a different kind of round-robin than is defined by the round-robin
dispatching policy.
Sponsored by Ada-Europe