Planning Based On Performance Characteristics

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Planning Based On Performance Characteristics
An important class of prioritized scheduling algorithms is algorithms in
which decisions about the choice of flow to execute are made on the basis
of knowledge or evaluation of the characteristics of its further execution.
First of all, it should be noted that in the " first - with the shortest time of
execution " algorithm ( Shortest Time to Completion First, STCF ), when
associated with each flow duration interval following the use of the CPU,
each is selected to perform at the stream in which this short interval. As a
result, streams that capture the CPU for a shorter time get an advantage
during scheduling and leave the system faster .
The STCF algorithm is theoretically optimal by the criterion of the average
response time, that is, it can be proved that for the selected group of flows,
the average response time in the application of this algorithm will be
minimal compared to any other algorithm. Unfortunately, for short-term
planning, it is impossible to implement it because this implementation
requires anticipation of the expected characteristics. For long-term
planning, it is used quite often (in this case, when setting the task for the
operator, the operator must indicate the expected time of completion, which
the system will take into account when choosing). Note also that the
optimality of such an algorithm is inseparable from its "unfairness" to flows
with longer CPU usage intervals.
For short-term scheduling, an approximation to this algorithm may be
implemented based on an estimate of the length of the next processor usage
interval, taking into account previous intervals of the same flow. You can
use recursion in the formula to calculate this estimate
t
n +1
= aT
n
+ (1- a ) t
n
, 0 < a < 1, t
0
= T
0,

where t
n +1
- estimate the length of the interval;
t
n
is an estimate of the length of the previous interval;
T
n
is the true length of the previous interval .
The most commonly used values are a = 0.5, in this case, it is sufficient to
calculate the average between the previous estimate and the actual interval
value.
Shortest Remaining Time to Completion First, SRTCF.
Its difference from STCF is that when a new thread is queued for ready
threads, the next CPU usage time is shorter than the time remaining before
the current thread completes, the current thread is interrupted and a new
thread becomes in its place.

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