After determining the minimum acceptable performance, perform
external observations of turnaround time on benchmarks and specify a
point beyond which the aQQ1t10n of more users would be However, when that point is reached, more sophisticated internal
measurement is required to deter.ine the scarcest resource ana how the
bottleneck can be relieved by additional hardware.
Several possible
different bottlenecks.
conditions
They are:
can be identified resulting from • Real storage levels of multiprogramming are low compared with the
number of contending users. Hence, each user is dispatched so
infrequently that running time or response time may become
intolerable. • Storage may be adequate to contain the working sets of contending nsers# but the processor is being shared among so many users that
each is receiving inadequate attention for good throughput. • Real storage space may be adequate for the processor, and a high
speed drum is used for paging; however, some virtual storage pages of
some users have spilled onto slower paging devices because the drum
is With low levels of multiprogramming, user page wait can
become a significant portion of system wait time. Consequently,
processor utilization falls and throughput deteriorates. • Storage, processor, and paging resources are adequate, yet several
users are heavily I/O-bound on the same disk, control unit, or channel. In these circumstances, real storage may be fully committed
because the correct level of multiprogramming is selected, yet device
contention is forcing high I/O wait times and unacceptable processor
utilization.
Estimates ef typical working set sizes are needed to determine how
well an application may run in a multiprogramming environment on a given
virtual storage system. A measure of the application's processor
requirements may be required for similar reasons. Measurements may be
required on the type and density of privileged instructions a certain
programming system may execute, because, in the virtual machine
environment, privileged instruction execution may be a major source of
overhead. If the virtual machine environment is used for programming
development, where the improvement in programmer productivity outweighs
the disadvantages of extra overheads, the above points may not be teo
critical. However, if throughput and turnaround time are important,
then the converse is true, and the points need close evaluation before
allocating resources to a virtual machine operation.
High levels of multiprogramming and overcommitment of real storage
space lead to high paging rates. High paging rates can indicate a
healthy condition; but be concerned about page stealing and get evidence
that this rate is maintained at an acceptable level. A system with a
high rate of page stealing is probably thrashing. Part 2. Control Program (CP) 125

Most of the conditions for good performance, established for the
time-shared batch systems, apply equally well to mixed mode systems.
However, two major factors make any determination more difficult to
make. First, get evidence to show that, in all circumstances, priority
is given to maintaining good interactive response, and that nontrivial tasks truly take place in the background. Second, background tasks, no
matter how large, inefficient, or demanding should not be allowed to
dominate the overall utiliZation of the time-sharing system. In other
words, in mixed mode operation, get evidence that users with poor
characteristics are discriminated against for the sake of maintaining a
healthy system for the remaining users.
A number of other conditions are more obvious and straightforward. You need to measure response and determine at what point it becomes
unacceptable and why. Studies of time-sharing systems have shown that a
user's rate of working is closely correlated with the system response. When the system responds quickly, the user is alert, ready for the next
interaction, and thought processes are uninterrupted. When the system
response is poor, the user becomes sluggish.
For interactive environments, a need exists to analyze command usage.
Average execution time of the truly interactive commands can provide
data for validation of the Queue 1 execution time.
126 IBM 1M/370 System Programmerls Guide