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Comment by rryan | Hacker News Reader

Actually, this sandwich analogy is not as good as the library/book one.

In this analogy, you are busy while you are making your sandwich (because you are putting it together from stuff you got from your fridge).

In the library-with-free-delivery-service analogy, you can do other work while you wait for data from the library/RAM to be delivered.

Modern superscalar processors can re-order non-dependent instructions while waiting on a memory lookup, and that is what the free-delivery-service aspect of the analogy illustrates.

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Very much agree. The analogy I'm used to (and use) is:

CPU = person (researcher at library) RAM = bounded physical space on desk Swap = cart for stacks Disk = the stacks (requiring scheduling of the elevator)

In my mind, the discussion of second/nanosecond is unimportant and makes this seem more technical than it needs to be to illustrate the point that "fetches from (non-SSD) disk are very slow and waste a lot of time." But this doesn't seem to be quite as focused as "A complete idiot's guide to the main components in a computer and what they do." (though I'm not sure that it's either time scales or components or SSD)

> In my mind, the discussion of second/nanosecond is unimportant

I disagree. The second/nanosecond discussion illustrates the scale of the difference. I can understand the difference between a few seconds and a few years (just about), and it puts it into perspective.

> "fetches from (non-SSD) disk are very slow and waste a lot of time."

Saying "It's very slow" isn't very helpful. I want perspective. Also the point is that the time is perhaps not wasted is an interesting one.

The original analogy is intelligent, (accurate?) and puts this into perspective for developers (and potential SSD customers!).

No! That version of the library analogy makes the ratios much too small.

CPU to main RAM: actually about 150:1; more like 30:1 in your analogy.

Main RAM to HD: actually about 200,000:1; more like 120:1 in your analogy.

The reason why "the discussion of second/nanosecond" is worth having is precisely that if you just say "very slow" and "a lot of time" then you're likely to think about ratios of the sort in your analogy, when the reality is much much much worse. (Extreme case: HD to CPU registers. Actual ratio: about 30 million to 1. Ratio in your analogy: about 4000 to 1.)

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