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authorJoel Sherrill <joel.sherrill@OARcorp.com>1998-10-19 17:11:10 +0000
committerJoel Sherrill <joel.sherrill@OARcorp.com>1998-10-19 17:11:10 +0000
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downloadrtems-210931dd0fc22080e024138a5d8c8c17e1b4c1ba.tar.bz2
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+@c
+@c COPYRIGHT (c) 1988-1998.
+@c On-Line Applications Research Corporation (OAR).
+@c All rights reserved.
+@c
+@c $Id$
+@c
+
+@chapter Timing Specification
+
+@section Introduction
+
+This chapter provides information pertaining to the
+measurement of the performance of RTEMS, the methods of
+gathering the timing data, and the usefulness of the data. Also
+discussed are other time critical aspects of RTEMS that affect
+an applications design and ultimate throughput. These aspects
+include determinancy, interrupt latency and context switch times.
+
+@section Philosophy
+
+Benchmarks are commonly used to evaluate the
+performance of software and hardware. Benchmarks can be an
+effective tool when comparing systems. Unfortunately,
+benchmarks can also be manipulated to justify virtually any
+claim. Benchmarks of real-time executives are difficult to
+evaluate for a variety of reasons. Executives vary in the
+robustness of features and options provided. Even when
+executives compare favorably in functionality, it is quite
+likely that different methodologies were used to obtain the
+timing data. Another problem is that some executives provide
+times for only a small subset of directives, This is typically
+justified by claiming that these are the only time-critical
+directives. The performance of some executives is also very
+sensitive to the number of objects in the system. To obtain any
+measure of usefulness, the performance information provided for
+an executive should address each of these issues.
+
+When evaluating the performance of a real-time
+executive, one typically considers the following areas:
+determinancy, directive times, worst case interrupt latency, and
+context switch time. Unfortunately, these areas do not have
+standard measurement methodologies. This allows vendors to
+manipulate the results such that their product is favorably
+represented. We have attempted to provide useful and meaningful
+timing information for RTEMS. To insure the usefulness of our
+data, the methodology and definitions used to obtain and
+describe the data are also documented.
+
+@subsection Determinancy
+
+The correctness of data in a real-time system must
+always be judged by its timeliness. In many real-time systems,
+obtaining the correct answer does not necessarily solve the
+problem. For example, in a nuclear reactor it is not enough to
+determine that the core is overheating. This situation must be
+detected and acknowledged early enough that corrective action
+can be taken and a meltdown avoided.
+
+Consequently, a system designer must be able to
+predict the worst-case behavior of the application running under
+the selected executive. In this light, it is important that a
+real-time system perform consistently regardless of the number
+of tasks, semaphores, or other resources allocated. An
+important design goal of a real-time executive is that all
+internal algorithms be fixed-cost. Unfortunately, this goal is
+difficult to completely meet without sacrificing the robustness
+of the executive's feature set.
+
+Many executives use the term deterministic to mean
+that the execution times of their services can be predicted.
+However, they often provide formulas to modify execution times
+based upon the number of objects in the system. This usage is
+in sharp contrast to the notion of deterministic meaning fixed
+cost.
+
+Almost all RTEMS directives execute in a fixed amount
+of time regardless of the number of objects present in the
+system. The primary exception occurs when a task blocks while
+acquiring a resource and specifies a non-zero timeout interval.
+
+Other exceptions are message queue broadcast,
+obtaining a variable length memory block, object name to ID
+translation, and deleting a resource upon which tasks are
+waiting. In addition, the time required to service a clock tick
+interrupt is based upon the number of timeouts and other
+"events" which must be processed at that tick. This second
+group is composed primarily of capabilities which are inherently
+non-deterministic but are infrequently used in time critical
+situations. The major exception is that of servicing a clock
+tick. However, most applications have a very small number of
+timeouts which expire at exactly the same millisecond (usually
+none, but occasionally two or three).
+
+@subsection Interrupt Latency
+
+Interrupt latency is the delay between the CPU's
+receipt of an interrupt request and the execution of the first
+application-specific instruction in an interrupt service
+routine. Interrupts are a critical component of most real-time
+applications and it is critical that they be acted upon as
+quickly as possible.
+
+Knowledge of the worst case interrupt latency of an
+executive aids the application designer in determining the
+maximum period of time between the generation of an interrupt
+and an interrupt handler responding to that interrupt. The
+interrupt latency of an system is the greater of the executive's
+and the applications's interrupt latency. If the application
+disables interrupts longer than the executive, then the
+application's interrupt latency is the system's worst case
+interrupt disable period.
+
+The worst case interrupt latency for a real-time
+executive is based upon the following components:
+
+@itemize @bullet
+@item the longest period of time interrupts are disabled
+by the executive,
+
+@item the overhead required by the executive at the
+beginning of each ISR,
+
+@item the time required for the CPU to vector the
+interrupt, and
+
+@item for some microprocessors, the length of the longest
+instruction.
+@end itemize
+
+The first component is irrelevant if an interrupt
+occurs when interrupts are enabled, although it must be included
+in a worst case analysis. The third and fourth components are
+particular to a CPU implementation and are not dependent on the
+executive. The fourth component is ignored by this document
+because most applications use only a subset of a
+microprocessor's instruction set. Because of this the longest
+instruction actually executed is application dependent. The
+worst case interrupt latency of an executive is typically
+defined as the sum of components (1) and (2). The second
+component includes the time necessry for RTEMS to save registers
+and vector to the user-defined handler. RTEMS includes the
+third component, the time required for the CPU to vector the
+interrupt, because it is a required part of any interrupt.
+
+Many executives report the maximum interrupt disable
+period as their interrupt latency and ignore the other
+components. This results in very low worst-case interrupt
+latency times which are not indicative of actual application
+performance. The definition used by RTEMS results in a higher
+interrupt latency being reported, but accurately reflects the
+longest delay between the CPU's receipt of an interrupt request
+and the execution of the first application-specific instruction
+in an interrupt service routine.
+
+The actual interrupt latency times are reported in
+the Timing Data chapter of this supplement.
+
+@subsection Context Switch Time
+
+An RTEMS context switch is defined as the act of
+taking the CPU from the currently executing task and giving it
+to another task. This process involves the following components:
+
+@itemize @bullet
+@item Saving the hardware state of the current task.
+
+@item Optionally, invoking the TASK_SWITCH user extension.
+
+@item Restoring the hardware state of the new task.
+@end itemize
+
+RTEMS defines the hardware state of a task to include
+the CPU's data registers, address registers, and, optionally,
+floating point registers.
+
+Context switch time is often touted as a performance
+measure of real-time executives. However, a context switch is
+performed as part of a directive's actions and should be viewed
+as such when designing an application. For example, if a task
+is unable to acquire a semaphore and blocks, a context switch is
+required to transfer control from the blocking task to a new
+task. From the application's perspective, the context switch is
+a direct result of not acquiring the semaphore. In this light,
+the context switch time is no more relevant than the performance
+of any other of the executive's subroutines which are not
+directly accessible by the application.
+
+In spite of the inappropriateness of using the
+context switch time as a performance metric, RTEMS context
+switch times for floating point and non-floating points tasks
+are provided for comparison purposes. Of the executives which
+actually support floating point operations, many do not report
+context switch times for floating point context switch time.
+This results in a reported context switch time which is
+meaningless for an application with floating point tasks.
+
+The actual context switch times are reported in the
+Timing Data chapter of this supplement.
+
+@subsection Directive Times
+
+Directives are the application's interface to the
+executive, and as such their execution times are critical in
+determining the performance of the application. For example, an
+application using a semaphore to protect a critical data
+structure should be aware of the time required to acquire and
+release a semaphore. In addition, the application designer can
+utilize the directive execution times to evaluate the
+performance of different synchronization and communication
+mechanisms.
+
+The actual directive execution times are reported in
+the Timing Data chapter of this supplement.
+
+@section Methodology
+
+@subsection Software Platform
+
+The RTEMS timing suite is written in C. The overhead
+of passing arguments to RTEMS by C is not timed. The times
+reported represent the amount of time from entering to exiting
+RTEMS.
+
+The tests are based upon one of two execution models:
+(1) single invocation times, and (2) average times of repeated
+invocations. Single invocation times are provided for
+directives which cannot easily be invoked multiple times in the
+same scenario. For example, the times reported for entering and
+exiting an interrupt service routine are single invocation
+times. The second model is used for directives which can easily
+be invoked multiple times in the same scenario. For example,
+the times reported for semaphore obtain and semaphore release
+are averages of multiple invocations. At least 100 invocations
+are used to obtain the average.
+
+@subsection Hardware Platform
+
+Since RTEMS supports a variety of processors, the
+hardware platform used to gather the benchmark times must also
+vary. Therefore, for each processor supported the hardware
+platform must be defined. Each definition will include a brief
+description of the target hardware platform including the clock
+speed, memory wait states encountered, and any other pertinent
+information. This definition may be found in the processor
+dependent timing data chapter within this supplement.
+
+@subsection What is measured?
+
+An effort was made to provide execution times for a
+large portion of RTEMS. Times were provided for most directives
+regardless of whether or not they are typically used in time
+critical code. For example, execution times are provided for
+all object create and delete directives, even though these are
+typically part of application initialization.
+
+The times include all RTEMS actions necessary in a
+particular scenario. For example, all times for blocking
+directives include the context switch necessary to transfer
+control to a new task. Under no circumstances is it necessary
+to add context switch time to the reported times.
+
+The following list describes the objects created by
+the timing suite:
+
+@itemize @bullet
+@item All tasks are non-floating point.
+
+@item All tasks are created as local objects.
+
+@item No timeouts are used on blocking directives.
+
+@item All tasks wait for objects in FIFO order.
+
+@end itemize
+
+In addition, no user extensions are configured.
+
+@subsection What is not measured?
+
+The times presented in this document are not intended
+to represent best or worst case times, nor are all directives
+included. For example, no times are provided for the initialize
+executive and fatal_error_occurred directives. Other than the
+exceptions detailed in the Determinancy section, all directives
+will execute in the fixed length of time given.
+
+Other than entering and exiting an interrupt service
+routine, all directives were executed from tasks and not from
+interrupt service routines. Directives invoked from ISRs, when
+allowable, will execute in slightly less time than when invoked
+from a task because rescheduling is delayed until the interrupt
+exits.
+
+@subsection Terminology
+
+The following is a list of phrases which are used to
+distinguish individual execution paths of the directives taken
+during the RTEMS performance analysis:
+
+@table @b
+@item another task
+The directive was performed
+on a task other than the calling task.
+
+@item available
+A task attempted to obtain a resource and
+immediately acquired it.
+
+@item blocked task
+The task operated upon by the
+directive was blocked waiting for a resource.
+
+@item caller blocks
+The requested resoure was not
+immediately available and the calling task chose to wait.
+
+@item calling task
+The task invoking the directive.
+
+@item messages flushed
+One or more messages was flushed
+from the message queue.
+
+@item no messages flushed
+No messages were flushed from
+the message queue.
+
+@item not available
+A task attempted to obtain a resource
+and could not immediately acquire it.
+
+@item no reschedule
+The directive did not require a
+rescheduling operation.
+
+@item NO_WAIT
+A resource was not available and the
+calling task chose to return immediately via the NO_WAIT option
+with an error.
+
+@item obtain current
+The current value of something was
+requested by the calling task.
+
+@item preempts caller
+The release of a resource caused a
+task of higher priority than the calling to be readied and it
+became the executing task.
+
+@item ready task
+The task operated upon by the directive
+was in the ready state.
+
+@item reschedule
+The actions of the directive
+necessitated a rescheduling operation.
+
+@item returns to caller
+The directive succeeded and
+immediately returned to the calling task.
+
+@item returns to interrupted task
+The instructions
+executed immediately following this interrupt will be in the
+interrupted task.
+
+@item returns to nested interrupt
+The instructions
+executed immediately following this interrupt will be in a
+previously interrupted ISR.
+
+@item returns to preempting task
+The instructions
+executed immediately following this interrupt or signal handler
+will be in a task other than the interrupted task.
+
+@item signal to self
+The signal set was sent to the
+calling task and signal processing was enabled.
+
+@item suspended task
+The task operated upon by the
+directive was in the suspended state.
+
+@item task readied
+The release of a resource caused a
+task of lower or equal priority to be readied and the calling
+task remained the executing task.
+
+@item yield
+The act of attempting to voluntarily release
+the CPU.
+
+@end table
+