AWS EC2 – Placement Groups – Certification

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Placement Groups Overview

  • Placement group determines how instances are placed on underlying hardware
  • AWS now provides two types of placement groups
    • Cluster – clusters instances into a low-latency group in a single AZ
    • Spread – spreads instances across underlying hardware

Cluster Placement Group

  • is a logical grouping of instances within a single Availability Zone
  • don’t span across Availability Zones
  • recommended for applications that benefits from low network latency, high network throughput, or both.
  • To provide the lowest latency, and the highest packet-per-second network performance for the placement group, choose an instance type that supports enhanced networking
  • recommended to launch all group instances at the same time to ensure enough capacity
  • instances can be added later, but there are chances of encountering an insufficient capacity error
  • for moving an instance into the placement group,
    • create an AMI from the existing instance,
    • and then launch a new instance from the AMI into a placement group.
  • stopping and starting an instance within the placement group, the instance still runs in the same placement group
  • in case of an capacity error, stop and start all of the instances in the placement group, and try the launch again. Restarting the instances may migrate them to hardware that has capacity for all requested instances
  • is only available within a single AZ either in the same VPC or peered VPCs
  • is more of an hint to AWS that the instances need to be launched physically close to each together
  • enables applications to participate in a low-latency, 10 Gbps network.

AWS EC2 Placement Group

Spread Placement Groups

  • is a group of instances that are each placed on distinct underlying hardware
  • recommended for applications that have a small number of critical instances that should be kept separate from each other.
  • reduces the risk of simultaneous failures that might occur when instances share the same underlying hardware.
  • provide access to distinct hardware, and are therefore suitable for mixing instance types or launching instances over time.
  • can span multiple AZs, and can have a maximum of seven running instances per AZ per group.
  • If the start or launch an instance in a spread placement group fails cause of insufficient unique hardware to fulfill the request, the request can be tried later as EC2 makes more distinct hardware available over time

Placement Group Rules and Limitations

  • Ensure unique Placement group name within AWS account for the region
  • Placement groups cannot be merged
  • An instance can be launched in one placement group at a time; it cannot span multiple placement groups.
  • Instances with a tenancy of host cannot be launched in placement groups.
  • Cluster Placement group
    • can’t span multiple Availability Zones.
    • supported by Specific Instance types (General Purpose, GPU, Compute, Memory, Storage Optimized – c4.8xlarge, c3.8xlarge, g2.8xlarge, i2.8xlarge, r3.8xlarge, m4.10xlarge, d2.8xlarge) which support 10 Gigabyte network
    • maximum network throughput speed of traffic between two instances in a cluster placement group is limited by the slower of the two instances, so choose the instance type properly.
    • can use up to 10 Gbps for single-flow traffic.
    • Traffic to and from S3 buckets within the same region over the public IP address space or through a VPC endpoint can use all available instance aggregate bandwidth.
    • recommended to use the same instance type i.e. homogenous instance types. Although multiple instance types can be launched into a cluster placement group. However, this reduces the likelihood that the required capacity will be available for your launch to succeed
    • Network traffic to the internet and over an AWS Direct Connect connection to on-premises resources is limited to 5 Gbps.
  • Spread placement groups
    • supports a maximum of seven running instances per Availability Zone for e.g., in a region that has three AZs, then a total of 21 running instances in the group (seven per zone).
    • are not supported for Dedicated Instances or Dedicated Hosts.

AWS Certification Exam Practice Questions

  • Questions are collected from Internet and the answers are marked as per my knowledge and understanding (which might differ with yours).
  • AWS services are updated everyday and both the answers and questions might be outdated soon, so research accordingly.
  • AWS exam questions are not updated to keep up the pace with AWS updates, so even if the underlying feature has changed the question might not be updated
  • Open to further feedback, discussion and correction.
  1. What is a placement group?
    • A collection of Auto Scaling groups in the same Region
    • Feature that enables EC2 instances to interact with each other via high bandwidth, low latency connections
    • A collection of Elastic Load Balancers in the same Region or Availability Zone
    • A collection of authorized Cloud Front edge locations for a distribution
  2. In order to optimize performance for a compute cluster that requires low inter-node latency, which feature in the following list should you use?
    • AWS Direct Connect
    • Placement Groups
    • VPC private subnets
    • EC2 Dedicated Instances
    • Multiple Availability Zones
  3. What is required to achieve gigabit network throughput on EC2? You already selected cluster-compute, 10GB instances with enhanced networking, and your workload is already network-bound, but you are not seeing 10 gigabit speeds.
    1. Enable biplex networking on your servers, so packets are non-blocking in both directions and there’s no switching overhead.
    2. Ensure the instances are in different VPCs so you don’t saturate the Internet Gateway on any one VPC.
    3. Select PIOPS for your drives and mount several, so you can provision sufficient disk throughput
    4. Use a placement group for your instances so the instances are physically near each other in the same Availability Zone. (You are not guaranteed 10 gigabit performance, except within a placement group. Using placement groups enables applications to participate in a low-latency, 10 Gbps network)
  4. You need the absolute highest possible network performance for a cluster computing application. You already selected homogeneous instance types supporting 10 gigabit enhanced networking, made sure that your workload was network bound, and put the instances in a placement group. What is the last optimization you can make?
    1. Use 9001 MTU instead of 1500 for Jumbo Frames, to raise packet body to packet overhead ratios. (For instances that are collocated inside a placement group, jumbo frames help to achieve the maximum network throughput possible, and they are recommended in this case)
    2. Segregate the instances into different peered VPCs while keeping them all in a placement group, so each one has its own Internet Gateway.
    3. Bake an AMI for the instances and relaunch, so the instances are fresh in the placement group and do not have noisy neighbors
    4. Turn off SYN/ACK on your TCP stack or begin using UDP for higher throughput.

References

EC2_User_Guide – Placement_Groups

9 thoughts on “AWS EC2 – Placement Groups – Certification

  1. I have a question on Placement Group. We know all instances must be placed in the same AZ for the PG to work. But AWS docs says that a PG can span peered VPCs. My question is that if peered VPCs are involved then it means a different AZs.

    I mean ….if VPCs peer with each other, then they both have different IP segments. They cannot have overlapping CIDRs. Therefore their subnets and AZs are different from each other. So, how can PG span across two peered VPCs?

    1. Placement group allows instances to be launched within it and cannot span across AZ.
      It works for VPC peered as well, however the instances launched within the peered VPC should also be in the same AZ to be added to the placement group.
      It cannot span across AZ even in different VPC.

      1. This is what I understood –
        We can’t make a general statement saying Placement Group cannot span across AZ. They can if they placed in Spread Placement Groups.
        Please correct me if wrong.

  2. Hi Jayendra,

    >>Placement group is only available for One VPC and One Availability Zone
    Placement groups can span peered VPCs. So I guess the above statement should read “Placement group is only available with in a single Availability Zone and across peered VPCs”, right?

    Cheers,
    Satish

  3. Hi Jayendra,

    Instances in Placement Group should be in same AZ is only applicable for “Cluster” placement group. This is not mandatory for “Spread” placement group. So, I think few points need to be updated accordingly. Or I am missing something,

    Thanks.

    1. Thanks Roy, the Cluster and Spread placement groups are latest AWS enhancements and have not been included. Will update the post accordingly.

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