The 5 minute rule for trading memory for disc accesses and the 10 byte rule for trading memory for CPU time

If an item is accessed frequently enough, it should be main memory resident, and the results depend on current price ratios of processors, memory and disc accesses, and hence the constants in the rules are changing.

Abstract

If an item is accessed frequently enough, it should be main memory resident. For current technology, “frequently enough” means about every five minutes.Along a similar vein, one can frequently trade memory space for CPU time. For example, bits can be packed in a byte at the expense of extra instructions to extract the bits. It makes economic sense to spend ten bytes of main memory to save one instruction per second.These results depend on current price ratios of processors, memory and disc accesses. These ratios are changing and hence the constants in the rules are changing.