Should you be saving up for a quad-core Smartphone?

The short answer is probably no.  At least I wouldn’t, considering that I’m likely to be worse today off with a quad-core smartphone than a dual-core one. Let me explain.

Smartphones are being caught up in the “processor race”, a battle we have seen for a many years with our PCs. Gigahertz and multi-cores becomes part of the buying criteria for your phone alongside the operating system and design ­- the more, the better, right?. Well, maybe not in this case.

It’s all about the software

The dilemma comes from the fact that the additional cores come at the expense of lower peak frequencies per core, since there is more overhead in the communication and synchronization among the cores.  So the extra cores are useful only if the software uses them so effectively that the overhead is more than compensated by the gains.  However, in practice only a few software applications can take advantage of multi-core chips.

You are maybe reading this blog on a quad-core PC using only a small fraction of a single-core. The same is true for most office PC usage. Over time, more software will evolve to make use of more cores, it will however not come as quickly as we might want. Some recent thoughts on that subject here.

Today, only a few applications for video and image editing, and possibly some of the most recent video games, take advantage of the quad-core ­- despite all years we have had multi-core PCs. On a mobile device, the opportunities for parallelism are even smaller since many computing intensive tasks such as video encoding and decoding and graphic processing are done with dedicated hardware to drastically lower power consumption.

So what can I expect from multi-core on mobile with today’s Software?

Let’s take a typical use case where we would like things to be as fast as possible on a mobile device – Web browsing. The browser is an application with very limited parallelism and by itself benefits little from multi-core. However the browser, like some other complex applications, induces several background concurrent activities on the mobile device, such as network connection, multimedia and user interface, providing enough parallelism to accelerate the browsing experience by about 1.5 times on a dual-core versus a single-core at the same frequency. That is good and I am certainly happy to have a dual-core phone.

So doubling the number of cores gives me 1.5 better browser performance, so if I double again what do I get? Well as you have already guessed you would never get four times better performance than a single-core. The calculation is complicated enough that there is a law for it – the well known Amdahl law for it. With quad core, we would at best, according to the theory, get a speed-up of 2x vs one core – again at the same processor speed. In practice the speed up will probably be less.

Now, to the question, why would I be worse off with a quad core than my dual core?

As I mentioned above the dual core can run at a higher peak frequency than the quad core. If only one or two cores of the in total four are being used, the dual core will outperform the quad core. The Gigahertz will speak.

But, to be fair, I could justify a quad-core processor in my smartphone or a tablet. For instance if I’m streaming a video to a TV, where the video transcoding is done on the mobile device CPU (without hardware acceleration), while I’m busy with web browsing or playing a video on the device.  If you foresee you will use your smartphone or tablet like this, then you definitely need to save up to the quad core. The rest of us will do just fine, or even better, with a dual core.