Thresholds in computing: Part 3 – The compact desktop

(Part 3 in a series of posts on small-form-factor computing)

In the last post, we stopped short of crossing any thresholds; all we did was zap empty space inside the case. But that only got us so far.

Bitfenix Prodigy M [Anandtech, labelled]
Bitfenix Prodigy M [Anandtech, labelled]

We were looking at systems that could support two graphics cards or more—gaming builds, basically. But not all gaming needs or builds rely on more than one graphics card. Let’s look at what’s possible when we shift to only using one graphics card.

Motherboards #


Motherboard form factors [Wikipedia]

Motherboards come in an array of standard sizes, known as form factors. The PC builds in the previous post were based on Standard-ATX and micro-ATX motherboards, the mainstay form factors of the past decade.

Gaming motherboard with two graphics card slots (yellow, below) [Anandtech]
Gaming motherboard with two graphics card slots (yellow, below) [Anandtech]

A micro-ATX gaming board will often have two graphics card slots (the thick horizontal yellow slots near the bottom, in the above image); these are PCI Express (PCIe) slots, and can be used for many other peripherals (audio cards, networking cards, storage expansion cards, …) as well. If we only use one graphics card, we only need one of those. And if we’re not using the other slots, everything below the first graphics slot can effectively be eliminated. What would this look like?

Mini-ITX motherboard with one PCIe card slot (black, below) [Anandtech]
Mini-ITX motherboard with one PCIe card slot (black, below) [Anandtech]

Shifting to only one graphics/expansion card lets us go from using a micro-ATX to a mini-ITX motherboard (refer to the form factors image above). That’s our first threshold: a smaller motherboard means that the minimum dimensions of our PC can be smaller. A micro-ATX motherboard typically measures 24×24cm (9.6″), while mini-ITX measures 17×17cm (6.7″). That’s about a 50% reduction in area!

Power supplies #

Just like motherboards, power supply units (PSUs) also come in various form factors.

ATX PSU: 15×8.6×14 cm (5.9×3.4×5.5 in), 400–1200W (approx.)
SFX PSU: 12.5×6.4×10 cm (5×2.5×4 in), 200–600W (approx.)

ATX (left) and SFX (right) PSUs side by side
ATX (left) and SFX (right) PSUs side by side [bit-tech]

How much power does an ITX PC use? With a typical mid-end graphics card (GTX 660, $199) and a high-end CPU (i7-3960X), the entire system draws a load power of 287W. This is a ridiculously high upper bound for power consumption;1 few people are going to have an i7-3960X ($999) in their PC build; a typical quad-core is going to use much less power (with an older i5-2500 quad-core, the GTX 660 uses 202W at load).

If we are realistic with ourselves, an 800W ATX power supply is way beyond our needs. At the same time, we don’t want to stretch our power supply too much and have it running near 100% load constantly. For optimal efficiency (to produce minimal waste heat), we want the system’s range of power usage to cover 25–75% of the power supply’s rated load. In our case, with a mid-/high-end quad-core CPU and a mid-end graphics card, let’s assume a 240W power consumption at load. This means a 320W power supply should be sufficient for our needs. Even factoring for user paranoia (“but what if that’s still not enough?!”), a 400W power supply is more than sufficient for our needs. So, now that we’ve gotten over our power supply paranoia, let’s switch to a SFX PSU. (That’s a volume reduction of >50% over an ATX PSU, by the way.) What does our system look like now?

Case/Chassis #

We run into a slight complication here: changing our choice of motherboard or power supply is not as simple as doing a mental switch. Our computer case of choice must also be able to mount and secure it. This is a bit easier on the motherboard side: for ATX, micro-ATX and mini-ITX, anything that can mount a larger motherboard form factor can also mount a smaller one. ATX cases can mount all three form factors (albeit with an accompanying waste of space), micro-ATX cases can mount m-ATX and ITX, while ITX cases can only mount ITX motherboards.

This is not the case with PSUs. A case that can mount an ATX PSU typically can’t hold an SFX PSU, and vice-versa. Sad but true. Let’s switch to one such ITX case that can mount an SFX PSU: the Silverstone SG08.

Silverstone SG08 [Anandtech]
Silverstone SG08 [Anandtech]

The SG08 measures 22.2×35.1×19 cm, while the Prodigy M measures 25×40.4×35.9 cm. Even accounting for the soft rubber carrying handles on top and at the bottom of the Prodigy M, that’s a volume reduction of over 50%! Where did all that space go?

Bitfenix Prodigy M (without CPU cooler) [Overclock3D]
Bitfenix Prodigy M (without CPU cooler) [Overclock3D]

Silverstone SG08, left and right view (without CPU cooler) [Anandtech]
Silverstone SG08, left and right view (without CPU cooler) [Anandtech]

Most noticeably, we’ve eliminated all the empty space from the unused PCIe slots (above the graphics card, in the Prodigy M), by switching to a smaller motherboard. We’ve also saved quite a bit of space by using the SFX PSU. The CPU + heatsink area has shrunk considerably, and we’ve shaved off yet more space by fitting hard drives above the PSU in the SG08 instead of layering them above the CPU cooler in the Prodigy M. And perhaps less noticeably, the SG08 has much fewer fans than the Prodigy M. The SG08 has barely any empty space inside it; it is virtually a shell wrapped around its internal components. Isn’t our ITX system in the SG08 going to overheat, with so little space for moving air? This is one of the cool things about ITX, in both senses of the word. We prevent cooling performance from dropping too much by ensuring proximity to outside air. Notice that, in the SG08,

  • The graphics card is right next to the side panel.
  • The case fan feeds the CPU cooler directly.

This differs noticeably from the usual cooling setup we see in ATX and micro-ATX systems:

  1. Individual component fans (for PSU, graphics card, CPU) blow air in the case over the hot components, transferring thermal energy from them to the air in the case.
  2. Case fans pull in cool air from the surroundings into the case, and exhaust warm air from the case to the surroundings.
    By shrinking the case so much that the hot components are practically right next to the side panels, we eliminate the need for many case fans. The individual component fans pull air directly from the surroundings, through strategically placed vents, and blow them directly over the hot components.

Silverstone SG08 panel vents, left and right view [Anandtech]
Silverstone SG08 panel vents, left and right view [Anandtech]

A typical case fan is about 12×12×2 cm (4.8×4.8×0.8 in), which does not seem like much, but the space they take up can be significant in a case that has 4 or more case fans. The SG08 gets by with a single case fan (above the CPU). Does that make the SG08 a better case than the Prodigy M, in terms of cooling performance? No. The Prodigy M still has more airflow overall (because it has more fans), but the SG08’s airflow will be sufficient for a typical quad-core setup. The question here is not whether it is powerful, but whether it will be enough. And the answer to that is yes.

Thresholds redux #

I promised, in the last post, that I’d talk about thresholds. Here we’ve already crossed a few:

  • Going from multiple graphics cards to one graphics card
  • Going from a >600W ATX PSU to a 400W SFX PSU

The effect of these two threshold crossings is a shrink in case dimensions, which leads us to a major threshold:

When the case dimensions are very close to internal system dimensions, airflow needs are greatly simplified.

What this means, for us, is that when the case is shrunk so small that it practically becomes a shell tightly wrapped around the internal components, our need for case fans is greatly reduced. Which lets us shrink the case even further.

Allometry in PCs #

This threshold-crossing has close parallels with an idea in biology: the “quarter-power scaling rule”, otherwise known as allometry, the study of how biological processes change with organism size. In biology, the larger an organism grows, the more complex its internal circulation systems (blood, excretory, lymphatic, etc) become. For a tiny organism like a bacterium, simple diffusion (for the most part) is sufficient for it to take in nutrients and excrete waste. Its energy consumption is thus minimal—no need for specialised mouth parts, or an anus, or circulatory vessels. Go a bit larger, and we get to shellfish, nematodes (worms), and the like. Simple mouth, simple anus, rudimentary digestive+circulatory system, but still nothing like what comes at the next level, in reptiles and such. By the time we get to human-sized organisms and larger, just keeping the various circulatory systems running already uses up a significant proportion of rest metabolic energy.

Laptop heatsink [Instructables]
Laptop heatsink [Instructables]

Here, we can start to see such effects in familiar computing systems. Our small smartphones and tablets have no moving parts; they dissipate heat through sheer conduction to the surroundings (see Part 1). Our laptops are not so simple, they need (relatively) small heatsinks and cooling fans to remain at a safe operating temperature. A major leap upwards, and we get to tower desktops, which have anywhere from 3 to 8 cooling fans, or even more!

Project Steampunk’d: Silverstone TJ11 case mod [Pureoverclock Forums]
Project Steampunk’d: Silverstone TJ11 case mod [Pureoverclock Forums]

Our study of thresholds is effectively a study of allometric scaling in computer systems. (allo-, “other” (i.e. not same), -meter, related to measurement) What this means for us is that when we shrink a desktop by 50%, it does not necessarily become 50% slower, nor does it use 50% less power—power consumption and processing speed do not scale with size.

Caveats: This decrease in size does not come without tradeoffs. By placing fans right next to vents, we also make the system louder, since sound is projected from vents more readily. There are ways to sidestep these issues though. More on this later.

Conclusion #

If the mental image that comes to mind when you hear “desktop” is more like the Dell Inspiron instead of the SG08, I hope this post has gone some way to changing that impression. PCs do not have to be big and bulky to be performant, and at the same time making them larger does not necessarily make them more powerful—from our perspective of PC allometry, we end up making some tradeoffs when we scale PCs beyond certain size thresholds. I have a mini-rant coming up about proclamations of the “death of the desktop”, and then it’s back to our regular programme on thresholds.

See also: Thresholds in computing: Part 2 – Does it need to take up so much space?

On to Thresholds in computing: Part 4 – Death of the desktop


  1. Take this up with the hardware reviewers; I have no idea why most of them don’t use more realistic hardware builds for power consumption numbers …