2^x or 10^x?

When working on my Linux QoS project I ran into the confusion over what k,M,G mean in computing again.

Here is the problem. In normal usage k = 1000. Obvious example, kilometer == 1000 meters. However in computing the factor most used is 1024. This means a kilobyte in your computer RAM is 1024 bytes == 2^10 bytes. A megabyte in RAM is 1024 * 1024 == 1048576 bytes == 2^20 bytes. The nice network and hard drive people have decided to use powers of 10 instead of powers of two. This means your 100Mbit network is 100,000,000 bits/sec not 100 * 1024 * 1024 == 104857600 bits/sec. Hard drives are also a fun example of this. A hard drive sold with a capacity of 40 GB can store 40,000,000,000 bits but your computer calculates file sizes with powers of two so this is really 40,000,000,000 / 1024 / 1024 / 1024 == 37.25 GB to your computer.

The networking example is where the problem with Linux QoS came up. The Linux traffic control utilities use a multiplier of 1024 for kbit. My upload data rate is 640kbit. When I specified 640kbit/sec via the Linux QoS utilities I was actually specifying 640 * 1024 == 655360 bits/sec not the actual line rate which is 640,000 bits/sec. Even more fun is calculating download rates from network interface speeds. The network is rated with 10^x but the KB number you see on the screen is 2^x.

Wikipedia has a great article on this whole mess. This article explains the new prefixes that have been introduced to remove this abiguity. So from now on my computer has 512Mib of RAM, the hard drive has 40Gb of capacity and the network interface is 100Mbit/sec.

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