Studying the Applicability of the Scratchpad Memory Management Unit

A combination of a scratchpad and scratchpad memory management unit
(SMMU) has been proposed as a way to implement fast and time-predictable
memory access operations in programs that use dynamic data structures.
A memory access operation is time-predictable if its execution time
is known or bounded -- this is important within a hard real-time task
so that the worst-case execution time (WCET) can be
determined. However, the requirement for time-predictability does not
remove the conventional requirement for efficiency:
operations must be serviced as quickly as possible under
worst-case conditions.

This paper studies the capabilities of the SMMU when applied to a
number of benchmark programs. A new allocation algorithm is
proposed to dynamically manage the scratchpad space. In many cases,
the SMMU vastly reduces the number of accesses to dynamic
data structures stored in external memory
along the worst-case execution path (WCEP). Across all the benchmarks,
an average of 47% of accesses are rerouted to scratchpad, with nearly
100% for some programs. In previous scratchpad-based work, time-predictability
could only be assured for these operations using external memory.
The paper also examines situations in which the SMMU does not perform so
well, and discusses how these could be addressed.