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Similarly, 2 CPUs reading 20 disks will not scale to a
20-fold performance improvement. The system hardware must be balanced for parallelism
to scale.
Most large systems have far more disks than CPUs. In these systems, parallelism
results in a randomization of I/O across the I/O subsystem. This is useful for concurrent
access to data as PE processes for different users read from different disks at
different times, resulting in I/O that is distributed across the available disks.
A useful analogy for dynamic parallelism is eating a pie. The pie is the set of blocks
to be read for the operation, and the goal is to eat the pie as quickly as possible using
a certain number of people. Oracle serves the pie in helpings, and when a person finishes
his first helping, he can come back for more. Not everyone eats at the same
rate, so some people will consume more pie than others. While this approach in the
real world is somewhat unfair, it??™s a good model for parallelism because if everyone
is eating all the time, the pie will be consumed more quickly.


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