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  1. (www.norvig.com)
    Teach Yourself Programming in Ten Years

    execute single instruction 1 nanosec = (1/1,000,000,000) sec
    fetch word from L1 cache memory 2 nanosec
    fetch word from main memory 10 nanosec
    fetch word from consecutive disk location 200 nanosec
    fetch word from new disk location (seek) 8,000,000 nanosec = 8 millisec

     
 

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Why remember basic computer access times?

 

when you're in the imagination stage for complex systems, doing simple arithmetic on these numbers can help you predict the difference between an app that takes 2 seconds and one that takes 200 milliseconds.

This point was first made for me a long time ago by a programmer mentor of mine, he made it in passing and I didn't think to pay more attention. Fast forward a few years and I at least knew enough to pay attention when I saw the same advice in a computer architecture book I was reviewing. Since then, I've been collecting mnemonics to help me never forget. Here is my list to add to Norvig's:

First, the useful units (though there are more):

1 ns = 1 billionth 1 us = 1 millionth 1 ms = 1 thousandth

And the measurements to build intuition:

1 ns = execute single instruction 2 ns = fetch word from L1 cache 10 ns = fetch a word from main memory 200 ns = fetch a word from consecutive disk location (bulk read) 8,000,000 ns (8ms) = seek to a new disk location and fetch a word

To be continued...

| posted by seemore | to programming