Embedded Software for SoC - Grupo de Mecatrônica EESC/USP

Hardware and Software Partitioning of Operating Systems 195
Example 5-2: The following picture shows a database example [28]. Each
database access requires a lock so that no other process can acci-dently
alter the database object at the same time. Note that a “short” re-quest
indicates that no context switch is allowed to occur since all access-es to
the object are very short (e.g., less than the time it takes to switch context).
A “long” request, on the other hand, requires a significant amount of time
holding the database object (and hence the lock); thus, for a “long” request,
if the lock is not available then the requesting process is context switched
out by the RTOS running on the processor.
We ran this example with 40 tasks – 10 server tasks and 30 client tasks –
on the four-MPC750 SoC architecture shown in Figure 15-8 with 10 task
on each MPC750. The results are shown in the following table. Please
note that lock latency is the time required to access a lock in the absence
of contention (i.e., the lock is free); also, please note that the 908 cycles
shown in the following table for lock latency in the “With SoCLC” is
measured from before to after the software application level API call. Thus,
in the “With SoCLC” case, part of the 908 cycles reported are spent
executing API software including specialized assembly instructions which
* Without
SoCLC
With
SoCLC
Speedup
Lock Latency
(clock cycles)
Lock Delay
(clock cycles)
Execution Time
(clock cycles)
1200
47264
36.9M
908
23590
29M
1.32x
2.00x
1.27x
* Semaphores for long CSes and spin-locks for short
CSes are used instead of SoCLC.