![]() The bad thing is that forcing every write to go to main memory, we use up bandwidth between the cache and the memory. This is simple to implement and keeps the cache and memory consistent. 2k Index Tag Data Valid Address (m bits) = Hit k (m-k-n) Tag 2-to-1 mux Data 2n Tag Valid Data 2n 2n = Index Block offset Compare a 2-way cache set associative cache with a fully-associative cache? Only 2 comparators needed Cache tags are a little shorter too … deciding replacement? 2 3 Set associative caches are a general idea By now you have noticed the 1-way set associative cache is the same as a direct-mapped cache Similarly, if a cache has 2k blocks, a 2k-way set associative cache would be the same as a fully- associative cache 0 1 2 3 4 5 6 7 Set 0 1 2 3 Set 0 1 Set 1-way 8 sets, 1 block each 2-way 4 sets, 2 blocks each 4-way 2 sets, 4 blocks each 0 Set 8-way 1 set, 8 blocks direct mapped fully associative 4 Summary Larger block sizes can take advantage of spatial locality by loading data from not just one address, but also nearby addresses, into the cache Associative caches assign each memory address to a particular set within the cache, but not to any specific block within that set Set sizes range from 1 (direct-mapped) to 2k (fully associative) Larger sets and higher associativity lead to fewer cache conflicts and lower miss rates, but they also increase the hardware cost In practice, 2-way through 16-way set-associative caches strike a good balance between lower miss rates and higher costs Next, we’ll talk more about measuring cache performance, and also discuss the issue of writing data to a cache 5 9 Write-through caches A write-through cache solves the inconsistency problem by forcing all writes to update both the cache and the main memory. 1022 1023 Index Tag Data Valid Address (32 bits) = Hit 10 20 Tag 2 bits Mux Data 8 8 8 8 8 2 2-way set associative implementation 0. There is also a 2015 edition of this course freely available on youtube.Download lab on cache direct mapping, set associative and more Computer Science Exercises in PDF only on Docsity!1 1 Review How is this cache different if… - the block is 4 words? - the index field is 12 bits? 0 1 2 3. In addition to other stuff it contains 3 lectures about memory hierarchy and cache implementations. ![]() I would highly recommend a 2011 course by UC Berkeley, "Computer Science 61C", available on Archive. N-way set associative cache pretty much solves the problem of temporal locality and not that complex to be used in practice. The number of "ways" is usually small, for example in Intel Nehalem CPU there are 4-way (L1i), 8-way (L1d, L2) and 16-way (元) sets. Sets are directly mapped, and within itself are fully associative. We are talking about a few dozen entries at most.Įven L1i and L1d caches are bigger and require a combined approach: a cache is divided into sets, and each set consists of "ways". Usually approximation of LRU ( least recently used) is implemented, but it is also adds additional comparators and transistors into the scheme and of course consumes some time.įully associative caches are practical for small caches (for instance, the TLB caches on some Intel processors are fully associative) but those caches are small, really small. Besides in order to maintain temporal locality, it must have an eviction policy. In order to check if a particular address is in the cache, it has to compare all current entries (the tags to be exact). Even if the cache is big and contains many stale entries, it can't simply evict those, because the position within cache is predetermined by the address.įull Associative Cache is much more complex, and it allows to store an address into any entry. A major drawback when using DM cache is called a conflict miss, when two different addresses correspond to one entry in the cache. Given any address, it is easy to identify the single entry in cache, where it can be. These are two different ways of organizing a cache (another one would be n-way set associative, which combines both, and most often used in real world CPU).ĭirect-Mapped Cache is simplier (requires just one comparator and one multiplexer), as a result is cheaper and works faster. In short you have basically answered your question.
0 Comments
Leave a Reply. |