Intro to Computer Systems

Chapter 12: System Integration

Price, Performance, Power

In any discipline, the choice of components and materials always involves compromises - very rarely is there one clearly superior product or substance that is appopriate for all use cases. In mechanical and building design, a common mantra is "Light, cheap, strong. Pick two!"

When it comes to selecting computer subsystems, it could be paraphrased as, "Price, performance, power consumption. Pick two!"

A "Balanced System"

Occasionally, the concept of a "balanced system" is bandied about - that a system of a certain processor performance (to take one yardstick) somehow requires a particular graphics card, memory capacity or mass storage system to achieve a sense of "balance".

This is a misnomer as a system is built not as a monument to itself, but to serve a need - and very few computing needs stress each subsystem an equal and balanced amount. Most often a computer that is "great at everything" is overpriced and expensive to run, compared to a properly optimised system.

To achieve the optimal balance of price, performance, power consumption (and perhaps also reliability) the ideal situation is to analyse and select each subsystem to serve its intended purpose, and forget the idea of "balance".

Compromise with Components

Taking each of the eight subsystems identified earlier, the major compromises in selection are summarised below.

CPU

• CPUs come in a variety of sockets and TDP ratings across a family. The lower TDP versions tend to be more expensive; it may or may not make fiscal sense to use the lower TDP parts, depending on the usage profile.
• Although a CPU family may have the same TDP, faster processors will use more power - however they are likely to finish heavy computing jobs (e.g. video rendering) sooner.
• The highest rated processors of a CPU family are usually very expensive; most often there is a "sweet spot" in the range which offers the best compromise between price and performance.
• Some low-end processors do not support certain features, such as SMT/Hyperthreading, hardware virtualisation and encryption.
• If a system is to be built with integrated graphics, the CPU family will have a significant effect on this as it is where the graphics processing unit is housed.

Memory

• Most systems support some form of memory interleaving; either dual-channel (laptop and desktop systems) or triple-channel (workstations and servers). Where prudent it's best to take advantage of this where possible.
• However, it may be the case that having a greater total memory amount offers a greater performance benefit than running less memory in interleaved mode.
• Most memory controllers will support a number of memory speeds. It is generally only necessary to optimise this for memory-bound computational tasks.

Motherboard and Chipset

• Generally speaking, the chosen CPU will considerably narrow down the chipset options (and hence the motherboard selection).
• Within a particular CPU/chipset family there are usually a number of parts designed for various use cases; e.g. general purpose, office, enthusiast, etc.
• A wide variety of motherboards are available from many manufacturers, each with a different level of integrated peripherals, form factors and expansion options.
• It's not uncommon for motherboards to be available across the entire spectrum for a given chipset, from an $80 basic board to a $400 enthusiast board with high-end features and several expansion options.

Graphics and Display

• Modern integrated graphics systems are "fine enough" for basic computing and productivity tasks, including high definition media decoding. The first question to ask is, does the system actually need discrete graphics?
• Discrete graphics processors are primarily sought after for gaming, 3D rendering, video rendering, and specialist computing tasks.
• Unlike CPUs, power consumption and efficiency has not been a major design criteria for GPUs until recently, so some graphics cards use immense amounts of power and generate a huge amount of heat.
• Gaming, 3D and video rendering are very commonly benchmarked activities and finding performance data to make an informed decision should not be difficult.
• It is important to verify that display interfaces will match what the system will be used for; for example:
  • if there is an HDMI port, will audio pass through it?
  • if there is a DVI port, does it support dual-link DVI for older 27" and 30" monitors?
  • if there are only DisplayPort connectors, what adapters does it/does it not support? (For example, it may not support audio passthrough.)

Mass Storage

• SSDs are much more expensive than hard disks, but they offer significant performance advantages. Many consider a solid-state disk to be even more significant upgrade than CPU or memory.
• There are compromise solutions to a pure SSD approach, which is quite expensive:
  • use two disks: an SSD for the operating system and applications, and a larger hard disk for data storage;
  • consider the use of operating system-level optimisation such as Apple's Fusion Drive or Intel's Smart Response Technology.
  • specify a hybrid solid-state hard disk (SSHD), which uses a small internal SSD as a cache of sorts for the hard disk.
• Mechanical hard disks often come in a number of performance tiers, optimised for a particular use case:
  • Enterprise-grade drives maximise speed and reliability, but are expensive. They also often have greater power consumption.
  • Consumer performance grade drives offer enterprise-grade speed, but consumer-grade reliability.
  • Archival or "NAS" drives are lower performance, but have greater than average reliability, and recommended for use in network-attached storage devices and in consumer grade RAID arrays.
  • "Green" or "Eco" drives minimise power consumption, at the cost of performance. They are also often the cheapest drives available, and useful for bulk data storage.

Expansion Options

• Extensive expansion options are often only available on higher-end enthusiast and enterprise grade motherboards. It is prudent to consider whether the expansion interfaces are really necessary.
• Be careful to check the electrical interface as well as physical, for example:
  • an mSATA slot looks identical to a mini-PCI Express slot, but may not support mini-PCI Express cards.
  • most motherboards with multiple PCI Express x16 slots only have four or eight lanes attached to the second slot, not the full 16.
• If a feature is required, it's typically more power-efficient for it to be an integrated peripheral (preferably on the chipset, or as discrete logic on the motherboard), rather than on a card in an expansion slot.

Power Supply

• Most modern general-purpose systems use less than 100 watts of power, even under load. Remember the efficiency of most power supplies is relatively poor at very low load - more is sometimes not better when it comes to power supply specification.
• Low quality power supplies are less likely to cope well with extreme conditions, such as 'dirty' mains power, brownouts and blackouts, or heavy power loads.
• If using power-hungry equipment, like a high end CPU or discrete graphics card, or a large number of hard disks, special attention must be paid to the power supply specification. It is necessary to consider not just the total wattage rating of the power supply, but the current it can provide on each voltage rail.

Enclosure and Cooling

• Cheaper cases will typically be made of thinner metal, and have rough-cut edges to the case that are likely to cause injury to computer technicians.
• The case design will dictate what sort of approach to cooling can be sought, with considerations such as:
  • size and number of fan mounts, and their placement
  • placement of mass storage, power supply
  • ducting and thermal zoning for each subsystem
  • clearances for heatsink/fan units
• Name-brand cases - even the less expensive models - are generally well engineered in terms of physical and thermal design.
• Cable routing can have a significant impact on airflow within the case. Well-designed cases usually have this in mind, offering channels along walls or behind the motherboard for interference-free cable routing.
• To achieve a given airflow, larger diameter fans can spin at lower rotational speeds, minimising noise.
• Specialty cases are available from many manufacturers for specific use cases, such as noise minimisation, lots of hard disk bays, cooling for extreme multiple graphics card setups, etc.
• For extreme computing needs, water cooling can be an option. However water cooling itself will only transport the heat - it will be passed to a radiator which will still need cooling.