第5章アルゴリズムのハードウェアによる実現：コンピューターシステム設計

Hard disks were the main storage solutions for all computers in the 1980s, 1990s, and until the mid-2000s. A hard disk drive (HDD) stores data on a rotating platter, has a very large capacity, and uses a small motor to rotate platters to get the data. You can easily buy an 8TB disk for a modest amount of money. The industry took about 25 years to move from 5MB disk to 1TB, and only two years to go from 1TB to 2TB, and, after that, the capacity increased by a whopping 60% per year. So, we have a very ample size with a low price but also a very slow disk. It is several orders of magnitude slower than the DRAM memory. The reason is shown in Figure 5.24. The main reason the disk is slow is due to the mechanical movement. Therefore, computer designers have been looking for storage that does not need mechanical movement drives and that does not have any moving parts.

The term solid state means that a system does not have moving parts and is expected to be fast. A solid-state drive (SSD) stores data on a chip and is two to three orders of magnitude faster than HDD. SSD has two main parts: the storage itself and the circuitry that accesses the storage. Nowadays SSDs use a storage technology called flash memory, which is a type of nonvolatile storage that can be electronically erased and reprogrammed. It is what you use in your thumb drives, just a bit faster. Flash memory in SSDs is based on NAND gates. A NAND gate is a type of logic gate used to store bits in flash memory. Flash memory is organized as pages, and a group of pages is called a block.

The circuitry that controls the flash memory, called the translation layer, has an important function. It maps the addresses to pages. One of the disadvantages of storage cells that make a page is that they wear out after 100 thousand to 1 million accesses. So, the translation layer tries to change the mapping to ensure that accesses are equally distributed among different cells. This is a complicated process and is one of the reasons SSDs are more expensive than HDDs. The sequential read from SSD reaches 7000 MB/s and the sequential write reaches 5000 MB/s. Random access to SSD has lower speed for reads and writes.

Most laptops now have SSD storage, so it is good to know how SSD really works. Watch this video for a succinct explanation of how SSD works (https://openstax.org/r/76SSDHowItWorks) in the context of smartphones.

As you have learned, cache memory is used as a fast, small memory inside the processor to close the gap between the processor speed and the memory speed. Given that latency is the rate of data transfer, assume DRAM memory’s access latency is M cycles and the cache access latency is m. Also assume that for a specific program the probability of cache hits = p. Then the average latency of the combined cache + Memory = mp + (1 – p)(M + m) = m + (1 – p)M.

Remember that whenever there is a cache miss, we have already spent m cycles searching the cache, then we go to the memory, which takes another M cycles. If we look at the equation m + (1 – p)M, we see that to get good performance, we need to do one or more of the following things:

• Have a faster cache and lower m.

• Increase cache hit rate and reduce p.

• Have a faster memory and lower M.

To have a faster cache, we must make it smaller in capacity, but smaller cache decreases hit rate, which is the number, usually a percentage, of times the cache was used to retrieve data. We cannot easily have faster memory to reduce M so the solution is to have more than one level of caches. Level 1 (L1), closest to the processor, is small in capacity and very fast but with a potentially low hit rate. When there is L1 cache miss, instead of going to the memory off-chip, we go to L2 cache, which is still on chip and bigger in capacity than L1. Most processors now have up to three levels of caches, as shown in Figure 5.25.