Memory Hierarchy: Performance Optimization

This page describes the memory hierarchy. This hierarchy determines the efficiency of data access.
Memory hierarchy. Optimization involves the memory hierarchy. We see an example memory hierarchy in a fairly modern computer. This guides optimization of computer programs. It also influences how compilers, operating systems and databases work.
Virtual Memory: Disk > 2GB (Typical size) 3-15 ms (Access time) Physical Memory 256MB-2GB 100-150 ns 2nd-Level Cache 128KB-4MB 40-60 ns 1st-Level Cache 16-64KB 5-10 ns Registers: Processor 32 Words 1 ns
The table shows the critical performance aspects of a somewhat recent memory hierarchy on a personal computer. You can see that reading a file from the disk requires about 3-15 milliseconds.Compilers:

So: Reading data from memory requires 100-150 nanoseconds. And reading data from the processor caches requires 1-60 nanoseconds.

How much faster is physical memory than the disk? Here we use the table shown above to compute how much faster you can load a small data object from memory instead of the disk. Parsing time is not included.

Note: Please recall that one millisecond is equal to one million nanoseconds. One nanosecond is quite small.

And: Let us say that the computer is busy and the disk requires 15 milliseconds, and the memory requires 150 nanoseconds.

Disk time 15 ms * 1000000 = 15000000 ns Memory time 150 ns Memory performance Memory is 100,000x (one-hundred thousand) times faster.
Uses. There is more complexity to computers than this memory hierarchy. All modern operating systems use a file cache, which puts disk files into memory, essentially making accesses one-hundred thousand times faster.

However: This is not configurable in many cases and it can be better to use custom file caches that will not be expired.

Summary. We explored the memory hierarchy using a table from the book Compilers: Principles, Techniques and Tools. The memory hierarchy shows us that disk accesses are far more expensive than in-memory reads with similar complexity.

Often: Avoiding accessing virtual memory (disk) is an important optimization to make.

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