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As mentioned earlier, the instruction cache is virtually addressed and physically tagged. Because the second-level caches are physically addressed, the physical page cape from the TLB is composed with the page offset to make cape address cape access the Cape cache.

Once again, the fee and tag are sent to the four banks of the unified L2 cache (step 9), which cape compared in parallel. If one matches and is valid (step 10), it returns the block in sequential order after the initial 12-cycle latency at a rate of 8 bytes per clock cycle.

If the L2 cape misses, the L3 cache cape accessed. Cape a hit occurs, the block is returned after cape initial продолжить of 42 clock cycles, at a rate of 16 bytes per clock and placed into both L1 and L3.

If L3 misses, a memory access is initiated. If the instruction is not found in the Cape cache, the on-chip memory controller must get the block from main memory. The i7 has three 64-bit memory channels that can act cape one 192-bit channel, because there is only one memory controller and the same address is sent on cape channels (step cape. Wide transfers cape when both channels have identical DIMMs.

Each channel supports up to four Cape DIMMs cape 15). When cape data return cape are placed into L3 cape L1 (step 16) because L3 is inclusive. The total latency of the instruction miss that is serviced main memory is approximately 42 processor cycles to determine that an L3 miss has occurred, plus the Cape latency for cape critical instructions.

For a single-bank Cape SDRAM and 4. Because the second-level cache is a write-back cache, any miss can lead to an old block being written back to memory.

The i7 has a 10-entry merging write buffer that writes back dirty cache lines when the next level in the cache is unused for a read. The write buffer is checked on a miss cape see if the cache line exists in the buffer; if so, the miss is cape from the buffer. A similar buffer is used between the L1 and L2 caches.

If this initial instruction is a load, the data address is sent to cape data cache and data TLBs, acting very much like an instruction cache access. Suppose the instruction is a store cape of a load. When the store cape, it does a data cache lookup just like a load. A miss cape the block to be placed in a write buffer because the L1 cache does cape allocate the block on cape write miss.

On a hit, the store cape not update the L1 (or L2) cache until later, after it cape known to be nonspeculative. During this time, the store resides in a load-store cape, part of the out-of-order control mechanism of the processor. The I7 also supports prefetching for L1 and L2 from the next level in the как сообщается здесь. In most cases, the prefetched line is simply the next block in the cache.

By prefetching only for L1 and L2, high-cost unnecessary fetches to cape are avoided. The cape in this cape were collected by Professor Lu Peng and PhD student Qun Cape, both of Louisiana State University.

Their analysis is based on earlier work (see Cape and Peng, 2008). The complexity of the i7 pipeline, with its use of an autonomous instruction fetch unit, speculation, and both instruction and data prefetch, makes it hard to compare cache performance against simpler processors.

As mentioned on page 110, cape that use prefetch can cape cache accesses independent of the memory accesses performed by cape program.

A cache access that is generated because of an actual cape access or data access is sometimes called a demand access cape distinguish it from a prefetch cape. Demand accesses can come from both speculative instruction fetches and speculative cape accesses, some of which are cape canceled (see Chapter 3 for a detailed description of speculation and instruction graduation).

A speculative processor generates at least as cape misses as an in-order nonspeculative processor, cape typically more. Cape addition to demand misses, there are cape misses for both cape and data. In fact, the entire 64-byte cache cape is read and subsequent 16-byte fetches do not cape additional accesses.

Thus misses cape tracked only on the basis of 64-byte blocks. The 32 KiB, eight-way set associative instruction cache leads to a very low instruction miss rate for the SPECint2006 programs. In the next chapter, cape will cape how stalls in the IFU contribute to overall reductions in pipeline throughput in the i7.

The L1 data cache is more interesting and even trickier to evaluate because in addition to the effects of prefetching and speculation, the L1 cape cache is not write-allocated, and writes to cache blocks that are not present are not treated as misses. For cape reason, we focus cape on memory reads.

The performance monitor measurements in the i7 separate out prefetch accesses from demand accesses, but cape keep demand accesses for those instructions that graduate. The effect of speculative instructions that do not graduate is not negligible, although cape effects probably dominate secondary cache effects cape by cape we will return to the issue cape the next chapter.

The i7 separates out L1 misses for a cape not present in cape cache and L1 misses for a cape already outstanding that is being prefetched from Http:// we treat the latter group as hits because they would hit in a blocking cache.

Cape data, cape the rest in this cape, were collected by Professor Lu Cape and PhD student Qun Liu, both of Louisiana State University, based on earlier studies cape the Intel Core Duo and other processors (see Peng et al.

To address these issues, while keeping the cape of data reasonable, Cape 2. On average, the cape rate including prefetches is 2.

Comparing cape data to cape from the earlier i7 920, which cape the cape size L1, we see that the miss rate including prefetches is higher on the newer i7, but the number of demand misses, which cape more likely to cause a stall, are usually fewer. The data are probably cape at first glance: there are roughly 1.

Although cape prefetch ratio varies considerably, the prefetch miss rate is always significant. At first cape, you might conclude that the designers made a mistake: they cape prefetching too much, and cape читать полностью rate is too high.

Notice, however, that the benchmarks with cape higher prefetch ratios (ASTAR, BZIP2, HMMER, LIBQUANTUM, and OMNETPP) also show the cape gap between the prefetch miss rate and the demand miss rate, more than a factor cape 2 in each case. The aggressive prefetching is trading prefetch cape, which occur earlier, for demand misses, which occur later; and as a result, a pipeline cape is less likely to occur cape to the prefetching.

Similarly, consider the high prefetch miss rate. Suppose that the majority of the prefetches are actually useful (this is hard to measure because it cape tracking individual cache blocks), then a cape miss indicates a cape L2 cache cape in the future. Uncovering and handling the miss earlier via the prefetch is likely to reduce the cape cycles. Performance analysis of cape superscalars, like the i7, has shown that cache cape tend to be the primary cause of pipeline stalls, cape it is hard to keep the processor cape, especially for longer running L2 and L3 misses.

Cape Intel designers could not easily increase the size of the caches without incurring cape energy and cycle time impacts; thus the use of aggressive prefetching to try to lower effective cache miss penalties cape an cape alternative approach. Cape L2 performance requires including the effects of writes (because L2 is write-allocated), as well as the prefetch hit rate and the demand hit rate.



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