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Reduce Latency with EC2 placement groups

Multi-tier applications are common in large organizations. These applications span multiple servers and communications between them can be sensitive to latency.
This has been historically a trivial problem for applications that run on-premises. It is easy to provide high speed connectivity between physical and virtual machines (VMs) so it is not a problem. However, dealing with latency issues in the public cloud can be more difficult.
Placement groups are an Amazon Web Services (AWS) solution for reducing latency between instances. A placement group is exactly what its name suggests, a group. AWS instances that are located in the same availability zone can be placed into a group. Group members can communicate with each other in a way that is low latency and high throughput.
It’s easy to create a placement group. Log in to AWS and open the Elastic Compute Cloud dashboard (EC2). Next, click the Placement Groups link found in the console tree.
Click the Create Placement Groups button shown in Figure 1. AWS will open the Create Plament Group dialog box. Figure 2 shows this. Click the Create button to enter a name for your placement group.
[Click on the image to see a larger version.] Figure 1: Click the Create Placement Group button.
Figure 2: Enter a name for the new placement group and click on the Create button. That’s it. It is important to note that the process of creating a group takes some time, so results may not be immediate.
AWS makes it easy to create a place group, as you can see. Placement groups do have limitations, but they are important to remember. You will likely encounter a capacity limit as the primary limitation. You are creating a placement group by creating a capacity reservation for EC2 instances in an availability zone. AWS doesn’t charge any fees for setting up this capacity reservation.
AWS doesn’t give you any control over how a placement group establishes a capacity reservation. This is the thing that is interesting about using a placement team to do this. There is no way to control the amount of resources you want to make available for a placement group. It is important to consider how you will use a placement group before you use it.
This raises the question of how to utilize a placement group. A placement group is nothing more than a container that acts to group AWS instances in a logical way. A placement group cannot be used to move an existing instance. The placement group must be explicitly specified at the time the instance is created.
Not all instances can be launched into a group. AWS keeps a list of all the instances that can be launched to a placement group. While there are some general-purpose instances types that are supported, the supported instance types are large instances that have been optimized for compute, storage, or accelerated computing.
AWS recommends that every instance that is placed in a placement group has the same instance type. AWS recommends that all instances be created simultaneously. Although there is nothing that prevents you from adding an instance to a place group later, you risk running into an error due to insufficient capacity. It is safer to create a batch of identical-sized instances.

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