In my view, pricing is one of the important aspects of rendering in a farm (others being the ability to actually deliver the project, the level of support and how easy it is to use them). So let’s see how render farm pricing works.
What is a GHz-hour? What about a core-hour?
You have probably seen some online render farms that are showing a price per GHz-hour or per core-hour for their rendering time. It’s a system that can be confusing for a first-time user. They usually do this because they have servers from several generations of hardware. And with the hardware performance being uneven, they are trying to find a common denominator to build a pricing strategy. The result: these two units — GHz and core-hour — which unfortunately require some effort and attention from users to get the price right.
In a price calculated in GHz-hour, they measure your computing time as a sum of GHz-hour from the servers you use.
For instance: you use one server for two hours. The server has a 6 core CPU. Each core has 2.4 GHz. The price per GHz-hour is $0.16. You will have to pay:
6 x 2.4 x 2 x $0.16 = $4.60
If the server has 2 identical CPUs, the price will double.
The pricing model based on core-hour is very similar. You get billed by the number of cores in the server. So for the same server, that has a 6 core CPU, for two hours of usage at the price of $0.5 per core-hour, you will pay:
6 x 2 x $0.5 = $6
Why do I think this model is not a good one?
First of all, it’s difficult to understand. And because of that, it’s difficult to estimate the rendering costs. Plus, the definitions can be ambiguous or misleading. Let me delve some more into this subject:
- Not all cores are created equal. Let’s take for instance two server CPUs from Intel, from the Xeon line: Xeon X5670 @ 2.93GHz and Xeon E5-2667 @ 2.90GHz. They have the same number of cores and almost the same core speed. You will notice that the second one is 23% faster than the first one, even if it has a slightly lower frequency. The first one is from the 2010 generation (and it’s likely you can still find it in some farms), and the second one has been released in 2012. Going further, the AMD Opteron 8439 SE @ 2.80GHz CPU has almost the same characteristics with the two Intel CPUs in terms of cores and speed, but its performance is half of the Xeon X5670.
- The definition of a core can vary from farm to farm. Some of them don’t even tell what kind of machines they have, but only the number of cores and the core speed. For instance one of the farms says they have ‚Xeon 8-core and 12-core machines with 3.0 GHz speed’, while another specifies they have ‚Intel Xeon E5645 * 2, 16 cores’. Some of the farms consider one core with hyperthreading to be two separate cores, so you’ll need to pay attention to that as well.
- Same logic applies to GHz-hour. Obviously, taking the two Intel CPUs compared above, one GHz from the second machine is 23% faster than a GHz from the first machine. So the actual value is 23% higher. Moving to the AMD CPU, the same GHz has only half of the performance.
What does all this mean?
What looks to be the better price isn’t always so. You may get more rendering per buck from a farm that seems to be more expensive, but in fact gives you more performance for the same amount. And that’s strictly in terms of hardware.
We’ll talk about the ability to deliver in another article.