Cloud Compromises: Constrained and Optimized CPUs

Imagine the scenario where you wonder into a clothing store to buy a t-shirt. You find a design you like in size “Medium” but it’s too tight (I guess #lockdown has been unkind to us all…) so you ask for the next size up. But when it arrives, you notice something bizarre: the “Large” is not only wider and longer, it also has an extra arm hole. Yes, there are enough holes for three arms as well as your head. Even more bizarrely, the “XL” size has four sets of sleeves, while the “Small” has only one and the “XS” none at all!

Surprisingly, this analogy is very applicable to cloud computing, where properties like compute power, memory, network bandwidth, capacity and performance are often tied together. As we saw in the previous post, a requirement for a certain amount of read I/O Operations Per Second (IOPS) can result in the need to overprovision unwanted capacity and possibly even unnecessary amounts of compute power.

But there is one situation where this causes extra levels of pain: when the workload in question is database software which is licensable by CPU cores (e.g. Oracle Database, Microsoft SQL Server).

To extend the opening analogy into total surrealism, imagine that the above clothing store exists in a state which collects a Sleeve Tax of %100 of the item value per sleeve. Now, your chosen t-shirt might be $40 but the Medium size will cost you $120, the Large $160 and the XXXXXL (suitable for octopods) a massive $360.

Luckily, the cloud providers have a way to help you out here. But it kind of sucks…

Constrained / Optimized VM Sizes

If you need large amounts of memory or I/O, the chances are you will have to pick a VM type which has a larger number of cores. But if you don’t want to buy databases licenses for these additional cores (because you don’t need the extra CPU power), you can choose to restrict the VM instance so that it only uses a subset of the total available cores. This is similar to the concept of logical partitioning which you may already have used on prem. Here are two examples of this practice from the big hyperscalers:

Microsoft Azure: Constrained vCPU capable VM sizes

Amazon Web Services: Introducing Optimize CPUs for Amazon EC2 Instances

As you can see, Microsoft and AWS have different names for this concept, but the idea is the same. You provision, let’s say, a 128 vCPU instance and then you restrict it to only using, for example, 32 vCPUs. Boom – you’ve dropped your database license requirement to 25% of the total number of vCPUs. Ok so you only get the compute performance of 25% too, but that’s still a big win on the license cost… right?

Well yes but…

There’s a snag. You still have to pay the full cost of the virtual machine despite only using a fraction of its resources. The monthly cost from the cloud provider is the same as if you were using the whole machine!

To quote Amazon (emphasis mine):

Please note that CPU optimized instances will have the same price as full-sized EC2 instances of the same size.

Or to quote the slightly longer version from Microsoft (emphasis mine):

The licensing fees charged for SQL Server or Oracle are constrained to the new vCPU count, and other products should be charged based on the new vCPU count. This results in a 50% to 75% increase in the ratio of the VM specs to active (billable) vCPUs. These new VM sizes allow customer workloads to use the same memory, storage, and I/O bandwidth while optimizing their software licensing cost. At this time, the compute cost, which includes OS licensing, remains the same one as the original size.

It’s great to be able to avoid the (potentially astronomical) cost of unnecessary database licences, but this is still a massive compromise – and the cost will add up over each month you are billed for compute cores that you literally cannot use. Again, this is the public cloud demonstrating that inefficiency and overprovisioning are to be accepted as a way of life.

Surely there must be a better way?

Spoiler alert: there IS a better way and I’ll be writing about it very soon.

 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.