B. Performance Evaluation
As most of PARSEC-2 programs have large working set
size [3], larger cache size can considerably decrease references
to the main memory. This is also observed in Figure 6 that
reports number of misses per thousand instructions (MPTI) in
L2 cache. Although the 8MB STT-RAM L2 cache results in
lower MPTI for most programs, this improvement should not
directly be translated into IPC improvement. Figure 7 confirms
these findings. It shows only 13% IPC improvement for the
STT-RAM architecture and 5% IPC improvement for the
proposed hybrid architecture. Here we point out that, although
the 8MB STT-RAM cache increases L2 hit rate and hence the
average IPC, due to the program behavior and its memory
references, it cannot last for a long time and some lines face
quick wear-out. Besides, size of the evaluated hybrid cache is
conservatively fixed at 4MB. If we consider a hybrid cache
with equal area of baselines, higher L2 cache hit rate and more
IPC improvement is expected. In short, our proposal guarantees
no degradation in performance with respect to the traditional
SRAM cache especially for applications with large WWS.