This set of blocks is called the working set of the

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Unformatted text preview: hnology trends. ¯ DRAM and disk access times are lagging behind CPU cycle times. As we see in Figure 6.15(d), CPU cycle times improved by a factor of 600 between 1980 and 2000. While SRAM performance lags, it is roughly keeping up. However, the gap between DRAM and disk performance and CPU performance is actually widening. The various trends are shown quite clearly in Figure 6.16, which plots the access and cycle times from Figure 6.15 on a semi-log scale. As we will see in Section 6.4, modern computers make heavy use of SRAM-based caches to try to bridge the processor-memory gap. This approach works because of a fundamental property of application programs known as locality, which we discuss next. 6.2. LOCALITY 100,000,000 10,000,000 1,000,000 100,000 ns 10,000 1,000 100 10 1 1980 1985 1990 Year 1995 2000 Disk seek time DRAM access time SRAM access time CPU cycle time 295 Figure 6.16: The increasing gap between DRAM, disk, and CPU speeds. 6.2 Locality Well-written computer programs tend to exhibit good locality. That is, t...
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