Nitro system considerations for performance tuning - Amazon Elastic Compute Cloud
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Nitro system considerations for performance tuning

The Nitro System is a collection of hardware and software components built by Amazon that enable high performance, high availability, and high security. The Nitro System provides bare metal-like capabilities that eliminate virtualization overhead and support workloads that require full access to host hardware. For more detailed information, see Amazon Nitro System.

All current generation EC2 instance types perform network packet processing on EC2 Nitro Cards. This topic covers high level packet handling on the Nitro card, common aspects of network architecture and configuration that impact packet handling performance, and what actions you can take to achieve peak performance for your Nitro based instances.

Nitro Cards handle all input and output (I/O) interfaces, such as those needed for Virtual Private Clouds (VPCs). For all of the components that send or receive information over the network, the Nitro cards act as a self-contained computing device for I/O traffic that's physically separate from the system main board on which customer workloads run.

Network packet flow on Nitro cards

EC2 instances built on the Nitro system have hardware acceleration capabilities that enable faster packet processing, as measured by packets per second (PPS) throughput rates. When a Nitro card performs the initial evaluation for a new flow, it saves information that's the same for all packets in the flow, such as security groups, access control lists, and route table entries. When it processes additional packets for the same flow, it can use the saved information to reduce overhead for those packets.

Your connection rate is measured by the connections per second (CPS) metric. Each new connection requires additional processing overhead that must be factored into workload capability estimates. It's important to consider both the CPS and PPS metrics when you design your workloads.

How a connection is established

When a connection is established between a Nitro based instance and another endpoint, the Nitro card evaluates the full flow for the first packet that's sent or received between the two endpoints. For subsequent packets of the same flow, full reevaluation is usually not necessary. However, there are exceptions. For more information about the exceptions, see Packets that don't use hardware acceleration.

The following properties define the two endpoints and the packet flow between them. These five properties together are known as a 5-tuple flow.

  • Source IP

  • Source port

  • Destination IP

  • Destination port

  • Communication protocol

The direction of the packet flow is known as ingress (inbound) and egress (outbound). The following high level descriptions summarize end to end network packet flow.

  • Ingress – When a Nitro card handles an inbound network packet, it evaluates the packet against stateful firewall rules and access control lists. It tracks the connection, meters it, and performs other actions as applicable. Then it forwards the packet to its destination on the host CPU.

  • Egress – When a Nitro card handles an outbound network packet, it looks up the remote interface destination, evaluates various VPC functions, applies rate limits, and performs other actions that apply. Then it forwards the packet to its next hop destination on the network.

Design for optimal performance

To take advantage of your Nitro system's performance capabilities, you must understand what your network processing needs are and how those needs affect the workload for your Nitro resources. Then you can design for optimal performance for your network landscape. Your infrastructure settings and application workload design and configuration can impact both the packet processing and connection rates. For example, if your application has a high rate of connection establishment, such as a DNS service, firewall, or virtual router, it will have less opportunity to take advantage of the hardware acceleration that only occurs after the connection is established.

You can configure application and infrastructure settings to streamline workloads and improve network performance. However, not all packets are eligible for acceleration. The Nitro system uses the full network flow for new connections and for packets that aren't eligible for acceleration.

The remainder of this section will focus on application and infrastructure design considerations to help ensure that packets flow within the accelerated path as much as possible.


When you configure network traffic for your instance, there are many aspects to consider that can affect PPS performance. After a flow is established, the majority of packets that regularly come in or go out are eligible for acceleration. However, exceptions exist to ensure that infrastructure designs and packet flows continue to meet protocol standards.

To get the best performance from your Nitro card, you should carefully consider the pros and cons of the following configuration details for your infrastructure and applications.

Infrastructure considerations

Your infrastructure configuration can affect your packet flow and processing efficiency. The following list includes some important considerations.

Network interface configuration with asymmetry

Security groups use connection tracking to track information about traffic that flows to and from the instance. Asymmetric routing, where traffic comes into an instance through one network interface and leaves through a different network interface, can reduce the peak performance that an instance can achieve if flows are tracked. For more information about security group connection tracking, untracked connections, and automatically tracked connections, see Security group connection tracking.

Network drivers

Network drivers are updated and released on a regular basis. If your drivers are out of date, that can significantly impair performance. Keep your drivers up to date to ensure that you have the latest patches and can take advantage of performance improvements, such as the accelerated path feature that's only available for the latest generation of drivers. Earlier drivers don't support the accelerated path feature.


To take advantage of the accelerated path feature, we recommend that you install the latest ENA driver on your instances.

Linux instances: ENA Linux driver 2.2.9 or later. To install or update the ENA Linux driver from the Amazon Drivers GitHub repository, see the Driver compilation section of the readme file.

Windows instances: ENA Windows driver 2.0.0 or later. To install or update the ENA Windows driver, see Install or upgrade Elastic Network Adapter (ENA) driver.

Distance between endpoints

A connection between two instances in the same Availability Zone can process more packets per second than a connection across Regions as a result of TCP windowing at the application layer, which determines how much data can be in flight at any given time. Long distances between instances increase latency and decrease the number of packets that the endpoints can process.

Application design considerations

There are aspects of application design and configuration that can affect your processing efficiency. The following list includes some important considerations.

Packet size

Larger packet sizes can increase throughput for the data that an instance can send and receive on the network. Smaller packet sizes can increase the packet process rate, but this can reduce the maximum achieved bandwidth when the number of packets exceed PPS allowances.

If the size of a packet exceeds the Maximum Transmission Unit (MTU) of a network hop, a router along the path might fragment it. The resulting packet fragments are considered exceptions, and are processed at the standard rate (not accelerated). This can cause variations in your performance. Amazon EC2 supports jumbo frames of 9001 bytes, however not all services support it. We recommended that you evaluate your topology when you configure MTU.

Protocol trade-offs

Reliable protocols like TCP have more overhead than unreliable protocols like UDP. The lower overhead and simplified network processing for the UDP transport protocol can result in a higher PPS rate, but at the expense of reliable packet delivery. If reliable packet delivery isn’t critical for your application, UDP might be a good option.


Micro-bursting occurs when traffic exceeds allowances during brief periods of time rather than being evenly distributed. This typically happens on a microsecond scale.

For example, say that you have an instance that can send up to 10 Gbps, and your application sends the full 10 Gb in half a second. This micro-burst exceeds the allowance during the first half second and leaves nothing for the remainder of the second. Even though you sent 10Gb in the 1 second timeframe, allowances in the first half second can result in packets being queued or dropped.

You can use a network scheduler such as Linux Traffic Control to help pace your throughput and avoid causing queued or dropped packets as a result of micro-bursting.

Number of flows

A single flow is limited to 5 Gbps unless it's inside of a cluster placement group that supports up to 10 Gbps, or if it uses ENA Express, which supports up to 25 Gbps.

Similarly, a Nitro card can process more packets across multiple flows as opposed to using a single flow. To achieve the peak packet processing rate per instance, we recommend at least 100 flows on instances with 100 Gbps or higher aggregate bandwidth. As aggregate bandwidth capabilities increase, the number of flows needed to achieve peak processing rates also increases. Benchmarking will help you determine what configuration you need to achieve peak rates on your network.

Number of Elastic Network Adapter (ENA) queues

By default, the maximum number of ENA queues are allocated to a network interface based on your instance size and type. Reducing the queue count can reduce the maximum PPS rate achievable. We recommend using the default queue allocation for best performance.

For Linux, a network interface is configured with the maximum by default. For applications based on the Data Plane Development Kit (DPDK), we recommend that you configure the maximum number of queues available.

Feature process overhead

Features like Traffic Mirroring and ENA Express can add more processing overhead, which can reduce absolute packet processing performance. You can limit feature use or disable features to increase packet processing rates.

Connection tracking to maintain state

Your security groups use connection tracking to store information about traffic to and from the instance. Connection tracking applies rules against each individual flow of network traffic to determine if the traffic is allowed or denied. The Nitro card uses flow tracking to maintain state for the flow. As more security group rules are applied, more work is required to evaluate the flow.


Not all network traffic flows are tracked. If a security group rule is configured with Untracked connections, no additional work is required except for connections that are automatically tracked to ensure symmetric routing when there are multiple valid reply paths.

Packets that don't use hardware acceleration

Not all packets can take advantage of hardware acceleration. Handling these exceptions involves some processing overhead which is necessary to ensure the health of your network flows. Network flows must reliably meet protocol standards, conform to changes in the VPC design, and route packets only to allowed destinations. However, the overhead reduces your performance.

Packet fragments

As mentioned under Application considerations, packet fragments that result from packets that exceed network MTU are handled as exceptions, and can't take advantage of hardware acceleration.

Idle connections

When a connection has no activity for a while, even if the connection hasn't reached its timeout limit, the system can de-prioritize it. Then, if data comes in after the connection is de-prioritized, the system needs to handle it as an exception in order to reconnect.

To manage your connections, you can use connection tracking timeouts to close idle connections. You can also use TCP keepalives to keep idle connections open. For more information, see Idle connection tracking timeout.

VPC mutation

Updates to security groups, route tables, and access control lists all need to be reevaluated in the processing path to ensure that route entries and security group rules still apply as expected.

ICMP flows

Internet Control Message Protocol (ICMP) is a network layer protocol that network devices use to diagnose network communication issues. These packets always use the full flow.

Maximize network performance on your Nitro system

Before you make any design decisions or adjust any network settings on your instance, we recommend that you take the following steps to help ensure that you have the best outcome:

  1. Understand the pros and cons of the actions that you can take to improve performance by reviewing Considerations.


    For more considerations and best practices for your instance configuration, see:

    Linux instances: ENA Linux Driver Best Practices and Performance Optimization Guide on the GitHub website.

    Windows instances: Best practices for configuring network interfaces.

  2. Benchmark your workloads with peak active flow count to determine a baseline for your application performance. With a performance baseline, you can test variations in your settings or application design to understand which considerations will have the most impact, especially if you plan to scale up or scale out.

The following list contains actions that you can take to tune your PPS performance, depending on your system needs.

  • Reduce the physical distance between two instances. When sending and receiving instances are located in same Availability Zone or use cluster placement groups, you can reduce the number of hops a packet needs to take to travel from one endpoint to another.

  • Use Untracked connections.

  • Use the UDP protocol for network traffic.

  • For EC2 instances with aggregate bandwidth of 100 Gbps or more, distribute the workload over 100 or more individual flows to spread the work evenly across the Nitro card.

Monitor performance on Linux instances

You can use Ethtool metrics on Linux instances to monitor instance networking performance indicators such as bandwidth, packet rate, and connection tracking. For more information, see Monitor network performance for your EC2 instance.