ML Wiki

The Memory Hierarchy

Two main categories:

• volatile: forgets what it stores when power goes off
• non-volatile: can persist data for a long time

Normally data is moved only between adjacent levels of hierarchy

Cache

• size: 1 mb or more
• access time: a few nanoseconds

data and instructions are moved to cache from the main memory when they are needed by the CPU

Main Memory

• to more from memory to cache/processor: 10-100 nanoseconds

stores all data and instructions

Secondary Storage

• 10 milliseconds to transfer data from disk to memory

Secondary storage: disks and other devices that can store large amounts of data

Virtual Memory

Virtual Memory is an address space (32 or 64 bits)

• the OS manages VM keeping needed parts at hand (in the main memory)
• and the rest on disk

So beware: data can be moved to and fro by the OS!

Tertiary Storages

• very high read/write times
• may be optical disks stored somewhere

a storage with very large capacity (petabytes, etc)

See also

• Secondary Storage

Sources

• Database Systems: The Complete Book (2nd edition) by H. Garcia-Molina, J. D. Ullman, and J. Widom