Couple of questions about Amazon EC2

Question

Amazon measures their CPU allotment in terms of virtual cores and EC2 Compute Units. EC2 Compute Units are defined as:

The amount of CPU that is allocated to a particular instance is expressed in terms of these EC2 Compute Units. We use several benchmarks and tests to manage the consistency and predictability of the performance from an EC2 Compute Unit. One EC2 Compute Unit provides the equivalent CPU capacity of a 1.0-1.2 GHz 2007 Opteron or 2007 Xeon processor. This is also the equivalent to an early-2006 1.7 GHz Xeon processor referenced in our original documentation.

My question is, say I have a "Large Instance" which comes with "4 EC2 Compute Units (2 virtual cores with 2 EC2 Compute Units each)". Does this mean I essentially have 4 cores in a logical sense? Would I want to spawn 4 CPU-bound threads? Or are the compute units simply a measure of power, and I have 2 cores?

Also, given the scalability of the servers, would it be better to double the computing power of a single box and host the database and server on the same box? Or should I have 2 seperate, weaker boxes?

Answer 1

nicholaides is correct, the small instances are the equivalent of one core, the large two cores. The remainder of the measurement is expressed as Compute Units, which are defined as follows:

One EC2 Compute Unit (ECU) provides the equivalent CPU capacity of a 1.0-1.2 GHz 2007 Opteron or 2007 Xeon processor.

I run my small website on a single small instance, with both web server and database hosted on the one virtual machine. I've been impressed with the performance, but again don't have a tremendous amount of load on it.

If all you're caring for is bang for your buck, I'd try your setup with both servers running on a single small instance (1 core, 1 EC2 unit at $0.10 / hour) and see how that stacks up. The next step up would be a high-CPU medium instance (2 cores, 5 total EC2 units at $0.20 / hour). Unless you're really hammering your servers, I have to believe you'll be able to run them on that single medium instance. For only twice the price of the small instance, you get five times the performance, which is much better than running two small instances.

One thing to be careful of is that the small and high-CPU medium instances are 32-bit, where all others (large, extra large, and high-CPU extra large) are 64-bit. You cannot run a 32-bit Amazon Machine Image on a 64-bit instance, and vice versa. If you're working with a stock AMI, this isn't a problem because you'll usually be able to find both versions of it, but for a custom image it might make you do a little extra work.

Answer 2

"4 EC2 Compute Units (2 virtual cores with 2 EC2 Compute Units each)" simply means you get 2 virtual cpu's, each of which is twice as fast as the basic Small instance.

In total, you get 4 times the power of the Small instance, but since you only get 2 cores, it makes sense to start only two threads.

As for your second question, I think Brad Larson answers it pretty well. The Medium instance has a lot of power for the money. We run our db en web servers on the same host, and it's surprising how many db-heavy sites you can run on a single machine. However, since it depends on your own application your best bet is to benchmark it to see how much load it can handle.

If you must scale up I would suggest separating the two services into different servers, instead of running a larger server, simply because it is easier to optimize each host for the specific service.