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Sector Cache
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Background
IBM Model 85
One early cache organization<ref name="Rothman"></ref>
technique was sector cache. It was first used on the IBM System/360 Model 85, which was perhaps the earliest commercial system that used a CPU data cache of any kind<ref name="Rothman">http://www.eecs.berkeley.edu/Pubs/TechRpts/1999/CSD-99-1034.pdf</ref>. The Model 85 was the one of the first computers to have a CPU data cache, which at the time was known as "buffer storage", come as a standard feature. As with most caches, the cache on the Model 85 attempts to fetch from the cache first before fetching from main memory. This allowed for much faster data fetching and storage, reducing the average system storage cycle time to about one third or one quarter what it would be if using main memory alone<ref name="IBM">http://www.bitsavers.org/pdf/ibm/360/funcChar/A22-6916-1_360-85_funcChar_Jun68.pdf
Template:Cite manual</ref>.
Sector cache allowed for smaller tag sizes, which made searching in cache easier and quicker, without requiring cache lines to be excessively long.<ref name="rot99"/> It was also easier to build with the circuit technology of the time.<ref name="rot99"/> In testing during the design of the IBM 360/85, it was found to run at 81% of the maximum ideal efficiency calculated by IBM's designers.<ref name="lip68">Liptay, J. S. Structural Aspects of the System/360 Model 85, Part II: The Cache.</ref> Sector caches were later replaced by set associative caches, which were found to be more efficient.<ref name="rot99"/>
One early cache organization technique was sector cache. It was used on the IBM 360/85, which was one of the earliest commercial CPUs<ref name="rot99">Rothman, Jeffrey B. and Alan Jay Smith. Sector Cache Design and Performance</ref>. Sector cache allowed for smaller tag sizes, which made searching in cache easier and quicker, without requiring cache lines to be excessively long.<ref name="rot99"/> It was also easier to build with the circuit technology of the time.<ref name="rot99"/> In testing during the design of the IBM 360/85, it was found to run at 81% of the maximum ideal efficiency calculated by IBM's designers.<ref name="lip68">Liptay, J. S. Structural Aspects of the System/360 Model 85, Part II: The Cache.</ref> Sector caches were later replaced by set associative caches, which were found to be more efficient.<ref name="rot99"/>
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Organization
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The cache is divided into sectors, which correspond to logical sectors on the main storage device.<ref name="lip68"/> When sectors are needed, they are not loaded into cache all at once, but in smaller pieces known as subsectors.<ref name="lip68"/> Subsectors are similar to the lines in a direct mapped cache, and are only loaded into cache when needed. This prevents large amounts of data from having to be transferred for every memory reference. Subsectors have a validity bit that indicates whether they are loaded with data or not.<ref name="lip68"/>
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Implementation
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Advantages/Disadvantages
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Miss Handling
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When a process requests data from a disk sector that is not in the cache, a cache sector is assigned to the sector on the main storage device where the requested data is stored.<ref name="lip68"/> Then the subsector where the data is located is loaded into the cache. The subsector's validity bit is then set to reflect that it has been loaded from the main storage.<ref name="lip68"/> When data from other subsectors within a loaded sector are requested, the system loads those subsectors into the cache sector and sets their validity bits.<ref name="lip68"/> Sectors are not removed from the cache until the system needs to reclaim the space to process another request.<ref name="lip68"/>
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Architectures
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Parameters
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Evolution
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Victim Cache
Background
Victim caches were first proposed by Norman P. Jouppi in 1990. Victim caching implements a small, fully-associative cache between direct-mapped L1 memory and the next level of memory. The cache allows lines evicted from the L1 cache a “second-chance” by loading them into the victim cache. Victim caches decrease the overall conflict miss rate (Jouppi).
Direct-mapped caches can especially benefit from victim caching due to their large miss rates. Victim caching allows direct-mapped caches to still be used in order to take advantage of their speed while decreasing the miss rate to an even lower rate than the miss rate found in set-associative caches (Jouppi).
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Victim caches were first proposed by Norman P. Jouppi in 1990. Victim caching implements a small, fully-associative cache between direct-mapped L1 memory and the next level of memory. The cache allows lines evicted from the L1 cache a “second-chance” by loading them into the victim cache. Victim caches decrease the overall conflict miss rate (Jouppi). Direct-mapped caches can especially benefit from victim caching due to their large miss rates. Victim caching allows direct-mapped caches to still be used in order to take advantage of their speed while decreasing the miss rate to an even lower rate than the miss rate found in set-associative caches (Jouppi).
Victim caches were first proposed by Norman P. Jouppi in 1990 at the 17th Annual International Symposium on Computer Architecture. During this time the performance of processors was increasing drastically, however memory hierarchies had not advanced in the same manner. Jouppi observed that if this trend continued the newer machines would easily lose half of their potential performance due to the short comings of the state of memory hierarchy at the time. This is illustrated in the figure 1.1 which is a table from Jouppi's original paper that shows how the cost of cache misses were dramatically increasing.
Handling Misses
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The proposed victim cache is fully-associative and lies between the L1 memory and the next level of memory. While Jouppi proposed a victim cache with 1 to 5 entries, Naz et al. proposed that the victim caches should be 4 to 16 cache lines.<ref>http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=134547</ref> Regardless of the size, when a miss occurs in the L1 cache, the victim cache is then scanned for the wanted line. If a miss occurs in both the L1 and victim cache, the needed line is then pulled from the next level, and the line evicted from the L1 cache is then placed in the victim cache. If a miss occurs in the L1 cache but hits in the victim cache, the two lines are swapped between the two caches. Thus, this eliminates the majority of conflict misses that occur due to temporal locality.
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Implementation
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Victim caching can be found in AMD's Opteron processor series produced specifically for servers and workstations. <ref>http://en.wikipedia.org/wiki/Opteron</ref> Opteron processors use a victim cache that is capable of holding eight victim blocks. <ref> Hennessy, John L., and David A. Patterson. Computer Architecture: A Quantitative Approach. Elsevier, Inc., 2012, p. B-14.</ref>
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References
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Topic 1b: Sectored Caches & Victim Caches
Sectored Cache:
Links: Wikipedia page saying IBM 360/85 --Taolande 11:31, 3 September 2013 (EDT)