(I'm the post author.) We did not intend for readers to think that hardware configurations were changing while the instance (virtual machine) was running. As

(I'm the post author.) We did not intend for readers to think that hardware configurations were changing while the instance (virtual machine) was running. As EC2 does not support live migration, this is not possible. When you use EC2 (or any cloud infrastructure) a lot, you stop thinking within the constraints of physical servers. The very nature of EC2 is the first letter of its acronym: Elasticity. You are able to bring up functionally equivalent duplicate servers when the demand arises and terminate them when the demand no longer exists. This exists not only for HPC applications (our domain), but even horizontally scaling web servers based on demand. EC2 allows you to request different "instance types". A given class offers the same cpu speed[1], memory, and disk usage. To make scaling tractable, you need to make the assumption that if your program operates correctly on a single (or even a few), say, m2.4xlarge instance types, it will do so on all of them. Customers do not use us solely because we provide an excellent distributed computing interface. They also use PiCloud, because we (thanks to the capabilities of Amazon EC2) are highly elastic. This customer doesn't want to continually run several dozen m2.4xlarge equivalent instances, because their utilization doesn't justify doing so. Instead, PiCloud allows them to run massive computation for a short time and not incur charges when the servers are no longer needed. (Resulting in far lower average costs than having their own on-site cluster.) Think about how this customer would have used EC2 directly within these constraints. Their workload would be something like deploy a few dozen servers, run computation on said servers, and terminate servers when complete. After getting this application to work correctly, they may conclude they are "done". It might work for months, and then one day, he would find that some "jobs" raised Illegal Instructions. And there would have been no way to anticipate this happening. And from their perspective, where the "requesting dozens of servers" is part of the application, the hardware had changed underneath them. [1] Yes, performance will differ a bit across different instances. However, you expect that functionality is identical across all instances of the same type (e.g. m2.4xlarge).