Modern Computers

2009-05-14T07:57:00.000-07:00

There is this common belief among PC enthusiasts that bandwidth, or million transfers per second or megabytes per second, is the most important thing that a good memory system should aim for. Such a belief is so deep-rooted that even the professionals (i.e., AMD & Intel) began to calibrate & market their products based on the memory bandwidth values.

First lets look at the data bandwidth. A 3.33GHz, 3 IPC processor would execute up to 10G I/s (giga-instructions per second). Suppose 1 out of 3 instructions has a load or store, which is supported by the fact both Core 2 and Barcelona have 6-issue (micro-op) engines and perform up to 2 loads or stores per cycle. Thus,10G I/s * 0.333 LS/I = 3.33G LS/s (giga-load/store per second, per core)Multiply this number by 4 cores, the total is 13.33G LS/s. According to Figure 5.10 of Computer Architecture AQA on page 416, a 4MB cache has miss rate about 1%. Lets make it 2% to be conservative. Thus the number of memory accesses going to the memory bus is13.33G LS/s * 2% MA/LS = 0.267G MA/s (giga-memory accesses per second)Each memory access is at most 16-byte, but mostly likely 8-byte or less in average. This makes the worst-case memory bandwidth requirement 0.267G*16 = 4.27GB/s, and the average-case 2.14GB/s. Note that a single channel of DDR2 memory can support up to 6.4GB/s, much more than the numbers above.