Motivation[ edit ] There is an inherent trade-off between size and speed given that a larger resource implies greater physical distances but also a tradeoff between expensive, premium technologies such as SRAM vs cheaper, easily mass-produced commodities such as DRAM or hard disks. The buffering provided by a cache benefits both bandwidth and latency: This is mitigated by reading in large chunks, in the hope that subsequent reads will be from nearby locations.
The tag contains the most significant bits of the address, which are checked against all rows in the current set the set has been retrieved by index to see if this set contains the requested address. If it does, a cache hit occurs.
The tag length in bits is as follows: The valid bit indicates whether or not a cache block has been loaded with valid data.
The timing of this write is controlled by what is known as the write policy. There are two basic writing approaches: A write-back cache is more complex to implement, (also called write-no-allocate or write around): data at the missed-write location is not loaded to cache, and is written directly to the backing store. In this approach. How to check the performance of a hard drive (Either via terminal or GUI). The write speed. The read speed. Cache size and speed. Random speed. Suppose we have a direct mapped cache and the write back policy is used. So we have a valid bit, a dirty bit, a tag and a data field in a cache line. Suppose we have an operation: write A (where A is mapped to the first line of the cache).
On power-up, the hardware sets all the valid bits in all the caches to "invalid". Some systems also set a valid bit to "invalid" at other times, such as when multi-master bus snooping hardware in the cache of one processor hears an address broadcast from some other processor, and realizes that certain data blocks in the local cache are now stale and should be marked invalid.
Having a dirty bit set indicates that the associated cache line has been changed since it was read from main memory "dirty"meaning that the processor has written data to that line and the new value has not propagated all the way to main memory.
Associativity[ edit ] An illustration of different ways in which memory locations can be cached by particular cache locations The replacement policy decides where in the cache a copy of a particular entry of main memory will go.
If the replacement policy is free to choose any entry in the cache to hold the copy, the cache is called fully associative. At the other extreme, if each entry in main memory can go in just one place in the cache, the cache is direct mapped.
Many caches implement a compromise in which each entry in main memory can go to any one of N places in the cache, and are described as N-way set associative.
Choosing the right value of associativity involves a trade-off. If there are ten places to which the replacement policy could have mapped a memory location, then to check if that location is in the cache, ten cache entries must be searched.
Checking more places takes more power and chip area, and potentially more time. On the other hand, caches with more associativity suffer fewer misses see conflict misses, belowso that the CPU wastes less time reading from the slow main memory.
The general guideline is that doubling the associativity, from direct mapped to two-way, or from two-way to four-way, has about the same effect on raising the hit rate as doubling the cache size.
However, increasing associativity more than four does not improve hit rate as much,  and are generally done for other reasons see virtual aliasing, below. Some CPUs can dynamically reduce the associativity of their caches in low-power states, which acts as a power-saving measure.
Therefore, a direct-mapped cache can also be called a "one-way set associative" cache. It does not have a replacement policy as such, since there is no choice of which cache entry's contents to evict. This means that if two locations map to the same entry, they may continually knock each other out.
Although simpler, a direct-mapped cache needs to be much larger than an associative one to give comparable performance, and it is more unpredictable. Two-way set associative cache[ edit ] If each location in main memory can be cached in either of two locations in the cache, one logical question is: The simplest and most commonly used scheme, shown in the right-hand diagram above, is to use the least significant bits of the memory location's index as the index for the cache memory, and to have two entries for each index.
One benefit of this scheme is that the tags stored in the cache do not have to include that part of the main memory address which is implied by the cache memory's index.
Since the cache tags have fewer bits, they require fewer transistors, take less space on the processor circuit board or on the microprocessor chip, and can be read and compared faster. Also LRU is especially simple since only one bit needs to be stored for each pair.
Speculative execution[ edit ] One of the advantages of a direct mapped cache is that it allows simple and fast speculation. Once the address has been computed, the one cache index which might have a copy of that location in memory is known.
That cache entry can be read, and the processor can continue to work with that data before it finishes checking that the tag actually matches the requested address. The idea of having the processor use the cached data before the tag match completes can be applied to associative caches as well.
A subset of the tag, called a hint, can be used to pick just one of the possible cache entries mapping to the requested address. The entry selected by the hint can then be used in parallel with checking the full tag. The hint technique works best when used in the context of address translation, as explained below.
Two-way skewed associative cache[ edit ] Other schemes have been suggested, such as the skewed cache,  where the index for way 0 is direct, as above, but the index for way 1 is formed with a hash function.
A good hash function has the property that addresses which conflict with the direct mapping tend not to conflict when mapped with the hash function, and so it is less likely that a program will suffer from an unexpectedly large number of conflict misses due to a pathological access pattern.
The downside is extra latency from computing the hash function. Nevertheless, skewed-associative caches have major advantages over conventional set-associative ones. A pseudo-associative cache tests each possible way one at a time. A hash-rehash cache and a column-associative cache are examples of a pseudo-associative cache.
In the common case of finding a hit in the first way tested, a pseudo-associative cache is as fast as a direct-mapped cache, but it has a much lower conflict miss rate than a direct-mapped cache, closer to the miss rate of a fully associative cache.
There are three kinds of cache misses: Cache read misses from an instruction cache generally cause the largest delay, because the processor, or at least the thread of executionhas to wait stall until the instruction is fetched from main memory.
Cache read misses from a data cache usually cause a smaller delay, because instructions not dependent on the cache read can be issued and continue execution until the data is returned from main memory, and the dependent instructions can resume execution.The baseline cache configuration will be byte line size, direct-mapped, 16 KB cache size, write-through and write-allocate.
Assume a default clock rate of 1 GHz.
Memory access time for a load hit is 0 cycles (the In write-around policy, the processor still does not stall for stores, and a store miss does not change the contents of the cache. cache that uses a fetch-on-write policy must wait for a missed cache line to be fetched from a lower level of the memory hierarchy, while a cache using no-fetch-on-write can proceed im- mediately.
Suppose we have a direct mapped cache and the write back policy is used. So we have a valid bit, a dirty bit, a tag and a data field in a cache line.
Suppose we have an operation: write A (where A is mapped to the first line of the cache). When a system writes data to cache, it must at some point write that data to the backing store as well.
The timing of this write is controlled by what is known as the write policy. I have a server with a LSI MegaRAID SAS i controller, RAID-5 with 3 x 2 TB disks. I did some performance testing (with iozone3) and the numbers show clearly that the write cache policy affects the read performance as well.
The timing of this write is controlled by what is known as the write policy. There are two basic writing approaches: A write-back cache is more complex to implement, (also called write-no-allocate or write around): data at the missed-write location is not loaded to cache, and is written directly to the backing store.
In this approach.