Comtrol eCos - Manual (Page 94)

Kernel Real-time Characterization
of objects in the system. For example, the mailbox tests measure the cost of a ’peek’ operation when the mailbox
is empty, has a single item, and has multiple items present. In this way, any effects of the state of the object or how
many items it contains can be determined.
There are a few things to consider about these measurements. Firstly, they are quite micro in scale and only measure
the operation in question. These measurements do not adequately describe how the timings would be perturbed in
a real system with multiple interrupting sources. Secondly, the possible aberration incurred by the real-time clock
(system heartbeat tick) is explicitly avoided. Virtually all kernel functions have been designed to be interruptible.
Thus the times presented are typical, but best case, since any particular function may be interrupted by the clock
tick processing. This number is explicitly calculated so that the value may be included in any deadline calculations
required by the end user. Lastly, the reported measurements were obtained from a system built with all options
at their default values. Kernel instrumentation and asserts are also disabled for these measurements. Any number
of configuration options can change the measured results, sometimes quite dramatically. For example, mutexes
are using priority inheritance in these measurements. The numbers will change if the system is built with priority
inheritance on mutex variables turned off.
The final value that is measured is an estimate of interrupt latency. This particular value is not explicitly calculated
in the test program used, but rather by instrumenting the kernel itself. The raw number of timer ticks that elapse
between the time the timer generates an interrupt and the start of the timer ISR is kept in the kernel. These values
are printed by the test program after all other operations have been tested. Thus this should be a reasonable estimate
of the interrupt latency over time.
Using these Measurements
These measurements can be used in a number of ways. The most typical use will be to compare different real-
time kernel offerings on similar hardware, another will be to estimate the cost of implementing a task using eCos
(applications can be examined to see what effect the kernel operations will have on the total execution time).
Another use would be to observe how the tuning of the kernel affects overall operation.
Influences on Performance
A number of factors can affect real-time performance in a system. One of the most common factors, yet most
difficult to characterize, is the effect of device drivers and interrupts on system timings. Different device drivers
will have differing requirements as to how long interrupts are suppressed, for example. The eCos system has been
designed with this in mind, by separating the management of interrupts (ISR handlers) and the processing required
by the interrupt (DSR—Deferred Service Routine— handlers). However, since there is so much variability here,
and indeed most device drivers will come from the end users themselves, these effects cannot be reliably measured.
Attempts have been made to measure the overhead of the single interrupt that eCos relies on, the real-time clock
timer. This should give you a reasonable idea of the cost of executing interrupt handling for devices.
Measured Items
This section describes the various tests and the numbers presented. All tests use the C kernel API (available by way
of
cyg/kernel/kapi.h ). There is a single main thread in the system that performs the various tests. Additional
threads may be created as part of the testing, but these are short lived and are destroyed between tests unless
otherwise noted. The terminology “lower priority” means a priority that is less important, not necessarily lower in
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