Information Technology Reference
In-Depth Information
FIGURE 4.20: Accounting cache results. From [ 68 ]. Copyright 2002 IEEE.
both cache levels, savings range from 35% to 53% depending on the tolerance settings while
performance loss ranges from less than 1% to less than 4%.
4.8.4 CAM-Tag Cache Resizing
Zhang and Asanovic point out that while considerable effort is expended on proposals to resize
high-performance RAM-tag caches, many real low-power processors actually use the more
power-efficient CAM-tag design (see, for example, Section 4.2.4 for a commercial CAM-tag
cache). Resizing, however, is equally—if not more—advantageous for highly-associative CAM-
tag caches. To fill this gap, Zhang and Asanovic proposed the first technique for CAM-tag
cache resizing, called Miss-Tag Resizing (MTR) [ 243 ].
There are distinct advantages in resizing a highly-associative CAM-tag cache. To begin
with, resizing at a granularity of an associative way is finer-grain in CAM-tag caches than in
RAM-tag caches. The latter have fewer but larger ways (see, for example, the Selective Cache
Ways in Section 4.8.2). Moreover, in CAM-tag caches resizing can be done individually per set ,
meaning that an associative way need not be disabled in its entirety across all sets. Rather, in
each set, one can disable any associative way without regard to what is happening in other sets.
To do this, Zhang and Asanovic rely on the bit-line segmentation technique (see “Sidebar:
Bit-line Segmentation”). But for bit-lines, there is an important difference between RAM-tag
and CAM-tag organizations.
Bit-lines in RAM-tag caches run across the sets of the cache. In contrast, bit-lines in
CAM-tag caches run across the ways of a set. This is shown in Figure 4.21 for a RAM-tag
cache of N sets and 4 ways and a CAM-tag cache of N ways and 4 sets. This figure shows
that bit-line segmentation in a RAM-tag cache partitions the sets while in a CAM-tag cache
partitions the ways. It is also evident, that in the CAM-tag organization each set can be resized
Search WWH ::




Custom Search