Hardware factors that influence performance

Your system's hardware has the greatest influence on its performance. It is the ultimate limiting factor on how fast a process will run before it has to start sharing what is available with the operating system and other user processes.

Performance tuning can require you to add hardware or upgrade existing hardware if a system's physical subsystems are unbalanced in power, or insufficiently powerful to satisfy the demands being put on them. There may come a time when, despite your best efforts, you cannot please enough people enough of the time with the hardware resources at your disposal. If so, you will have to go and buy some more hardware. This is one reason why monitoring and recording your system's performance is important if you are not the person spending the money. With the information that you have gathered, you can make a strong case for upgrading your system.

It is important to balance the power of your computer's subsystems with each other; the power of the CPU(s) is not enough in itself. If the other subsystems are slow relative to the available processing power, they will act to constrain it. If they are more powerful, you have possibly overspent, although you should be able to upgrade processing power without much extra expenditure.

There are many hardware factors that can limit the overall system performance:

• The speed and width of the system's address and data buses.

• The model, clock speed, and the size of the internal level-one (L1) memory cache of the system's CPU or CPUs.

• The size of the level-two (L2) cache memory which is external to the CPU. This should be capable of working with all of physical memory.

• The amount of memory, the width of its data path, and its access time. The time that the CPU has to wait for memory to be accessed limits its performance.

• The speed and width of a SCSI bus controlled by a host adapter.

• The width of the data path on peripheral controller cards (32, 16, or 8-bit).

• Whether controllers have built-in cache. This is particularly important for disk and network controllers.

• Access time for hard disks.

• Whether intelligent or dumb serial cards are used; intelligent cards offload much of the work that would otherwise be performed by the CPU.

• Write-back L2 cache (for instructions and data) with cache coherency on each CPU to reduce the number of accesses to main memory. This has the benefit of improving CPU performance as well as improving general system performance by reducing contention for the system bus.

• Support for fully distributed interrupts to allow any CPU to service interrupts from I/O devices such as network and disk controllers.

• The memory and I/O subsystems must be as fast as possible to keep up with the demands of the enhanced CPU performance. Use of intelligent peripheral controllers is particularly desirable.