Chapter 4. Memory

Ideally one would desire an indefinitely large memory capacity such that any particular. . . word would be immediately available. . . .

”A. W. Burks, H. H. Goldstine, and J. von Neumann: Preliminary Discussion of the Logical Design of an Electronic Computing Instrument, 1946

Physical memory is a set of integrated circuits designed to store binary data. This storage has two characteristic properties: it is transient , as all stored information vanishes when electrical power is lost, and randomly accessible , meaning any bit can be accessed as fast as any other bit. In addition to physical memory, most systems implement virtual memory, which acts to manage physical memory and provide a simple interface to application developers. Virtual memory is consumed by the system kernel, filesystem caches, intimately shared memory, and processes.

Memory performance begins to affect overall system performance in two instances. The first instance occurs when the system is unable to retrieve and store data from physical memory fast enough, or when the system is forced to travel to main memory frequently. This sort of problem can be attacked by tuning the algorithm that is responsible or by buying a system with faster access to main memory. The second, and more likely, case is that the demand for physical memory by all currently running applications, including the kernel, exceeds the available amount. The system is then forced to begin paging , or writing unused pieces of memory to disk. If the low memory condition worsens, the memory consumed by entire processes will be written to disk, which is called swapping . ^[1] Memory conditions fall into four categories:

^[1] Paging and swapping are often used interchangeably. However, they mean very different things in practice, so we'll be careful not to mix them.

• Sufficient memory is available, and the system performs optimally.

• Memory is constrained (one likely culprit, especially on older Solaris systems, is the filesystem cache). Performance begins to suffer as the system attempts to scavenge memory that is not in active use.

• The system is legitimately short of memory. Performance suffers, especially on interactive processes.

• Memory is critically scarce . Swapping begins to take place. System performance takes a heavy hit, and interactive performance becomes abysmal.

In this chapter, I'll describe how memory is physically implemented, the mechanics of how the system manages memory, and how paging and swapping function at a system level. I'll give you some tools you can use to analyze your memory use, and explain how to work with swap space. Finally, I'll cover mechanisms to address some common memory performance problems.