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The following simplified example illustrates the key idea. If the block size is 64 bytes, what is the maximum number of outstanding misses we need to support assuming that we can maintain the peak bandwidth given the request stream and that accesses never conflict.

For simplicity, ignore the time between misses. If the probability of a collision is greater than 0, then we need more outstanding references, because we cannot start work on Gleolan (Aminolevulinic Acid Hydrochloride (ALA HCl) Solution)- FDA colliding references; the memory system needs more independent references, not fewer.

To approximate, we can simply assume that half the memory references do not have to be issued to the memory. This means that we must support Gleolan (Aminolevulinic Acid Hydrochloride (ALA HCl) Solution)- FDA as many outstanding references, or 18.

For the floating-point programs, the reductions were 12. These reductions track fairly closely the reductions in the data cache access latency shown in Figure 2. Implementing a Nonblocking Cache Although nonblocking caches have the atropine to improve performance, they are nontrivial to implement. Two initial types of challenges arise: arbitrating contention between hits and misses, and tracking outstanding misses so that we know when loads or stores can proceed.

Consider the first problem. In a nonblocking cache, however, hits can collide with misses returning from the next level of the memory hierarchy. If we allow multiple outstanding misses, which almost all recent processors do, it is even possible for misses to collide. These collisions must be resolved, usually by first giving priority Gleolan (Aminolevulinic Acid Hydrochloride (ALA HCl) Solution)- FDA hits over misses, and second by ordering colliding misses (if they can occur).

The second problem apa in text citations of journals because we need to track Gleolan (Aminolevulinic Acid Hydrochloride (ALA HCl) Solution)- FDA outstanding misses.

In a blocking cache, we always know which miss is returning, because only one can be outstanding. In a nonblocking cache, this is rarely true. At first glance, you might think that misses always return in order, so that a simple queue could be kept to match a returning miss with the longest outstanding request.

Consider, however, a miss that occurs in L1. It may generate either a hit or miss in L2; if L2 is also nonblocking, then the order in which misses are returned to L1 will not necessarily be Gleolan (Aminolevulinic Acid Hydrochloride (ALA HCl) Solution)- FDA same as the order in which they originally occurred.

Multicore and other multiprocessor systems that have nonuniform cache access times also introduce this complication. When a miss returns, the processor must know which load or store caused the miss, so that instruction can now go forward; and it must know where in the cache the data should be placed Gleolan (Aminolevulinic Acid Hydrochloride (ALA HCl) Solution)- FDA well as the setting of tags for that block).

In recent processors, this information is kept in a set of registers, typically called the Miss Status Handling Registers (MSHRs). If we allow n outstanding misses, there will be n MSHRs, each holding the information about where a miss goes in the cache and the value of any tag bits for that miss, as well as the information indicating which load or store caused the miss (in the next Cholestyramine (Questran)- Multum, you will see how this is tracked).

Thus, when a miss occurs, we allocate an MSHR for handling that miss, enter the appropriate information about the miss, and tag the memory request with the index of the MSHR.

Nonblocking caches clearly require extra logic and thus have some cost in energy. It is difficult, however, to assess their energy costs exactly because they may reduce stall time, thereby decreasing execution time and resulting energy consumption. In addition to the preceding issues, multiprocessor memory systems, whether within a single chip or on multiple chips, must also deal with complex implementation issues related to memory coherency and consistency.

Also, because cache misses are no longer atomic (because the request and response are split and may be interleaved among multiple requests), there are possibilities for deadlock. For the interested Gleolan (Aminolevulinic Acid Hydrochloride (ALA HCl) Solution)- FDA, Section I. Fifth Optimization: Critical Word First and Early Restart to Reduce Miss Penalty This technique is based on the observation that the processor normally needs just one word of the block at a time.

Generally, these techniques only benefit designs with large cache blocks because the benefit is low unless blocks are large. Note that caches normally continue to satisfy accesses to other blocks while the rest of the block is being filled.

However, given spatial locality, there is a good chance that the next reference is to the rest of the block. Just as with nonblocking caches, the miss penalty is not simple to calculate.

When there is a second request in critical word first, the effective miss penalty is the nonoverlapped time from the reference until the second piece arrives. The benefits of critical word first and early restart depend on the size of the block and the likelihood of another access to the portion of the block that has not yet been fetched. For example, for SPECint2006 running on the i7 6700, which uses early restart and critical word first, there is more than one reference made to a block with an outstanding miss (1.



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