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ENA Linux Driver Best Practices and Performance Optimization Guide

This document attempts to answer the most frequent ENA users’ questions and provides guidelines for achieving the best performance with ENA network device at EC2 instances.

This document covers ENA LINUX DRIVER ONLY

General FAQs

Q: I’m using a 1-year old driver version. Shall I upgrade?

A: Definitely! There are number of reasons why you should upgrade ASAP

  • Outdated drivers are prone to functional bugs, they lack performance optimizations and mechanisms for efficient management of system resources that were introduced in later versions
  • Old and outdated driver versions might not be compatible with future generations of ENA devices
  • Outdated drivers miss important features

Always aim to use the latest and greatest ENA driver in your AMI. The driver can be downloaded from GitHub, Linux upstream tree, and also included with major Linux distributions.

Q: What is ENAv3? How do I know that my instance type uses ENAv3 device? Do I have to upgrade my driver in order to take advantage of ENAv3?

A: ENAv3 is a new version of ENA device that introduces additional improvements in latency and performance. ENAv3 is available on the majority of the 6th generation instance types. ENAv3 can be differentiated from ENAv2 by the presence of an additional BAR (BAR1) on the PCI bus.

Important! ENAv3 is supported by the ENA driver starting with v2.2.9. Instances using AMIs with older drivers would experience performance degradation on ENAv3 devices. Instances with very old drivers (older than v1.2) will fail to attach ENAv3 ENI. If you use the default ENA driver preinstalled with the Linux distributions AMIs, these are the distribution and kernel versions where ENAv3 support was introduced:

  • Amazon Linux - 4.14.186
  • RHEL8.3 - 4.18.0-240.1.1.el8_3
  • Ubuntu 20.04 - 5.4.0-1025-aws
  • SLES 15 - 5.3.18-24.15

Q: How do I make sure I’m using the latest/correct driver?

A: Please follow one of the options below:

  • Make sure your AMI is up-to-date with the latest updates of the Linux distro you use
  • Install the latest version of the ENA GitHub driver

Q: What is ENA queue?

A: ENA queue is composed of Tx Submission and Completion ring and Rx Submission and Completion ring

Q: How many ENA queues are available?

A: It depends on the instance type and instance size you are using. Usually the number of queues exposed per ENI is calculated as MIN(MAX_NUM_QUEUES_PER_ENI, NUM_OF_VCPUS) MAX_NUM_QUEUES_PER_ENI is 8 for most of the instance types and is 32 only for network accelerated instances.

Q: What is ENA LLQ aka ENAv2?

A: ENA LLQ is a Tx submission ring that is located in ENA device DRAM behind PCI BAR2. The driver maps this area as Write Combined, and pushes there the Tx submission ring descriptors along with packet headers. LLQs improves latency and PPS. Starting with ENA driver v2.0 LLQ is the default mode of operation of majority of the instance types.

Q: How many IRQs does the driver allocate for each ENI?

A: For each ENI the driver allocates 1 IRQ for management (Admin CQ, AENQ) and one IRQ for each ENA queue. Please note ENA queue an IRQ is shared between Tx and Rx Completion rings of the same queue.

Q: What are default Rx and Tx ring sizes? Can they be changed?

A: The default size is 1K entries for both Tx and Rx. Ring size can be adjusted using ethtool -G command. Rx ring sizes can vary between 256 to up to 16K entries (max value depends on instance type/size). Tx ring size varies between 256 to 1K entries. Ring size value must be a power of 2. Please run

$ ethtool -g DEVNAME to see the

to check the supported Tx/Rx rings max size.

Q: What offloads are enabled on an ENA device ?

A: This might depend on instance type. Please run ethtool -k to determine.

Q: Is custom RSS hash key supported?

A: This might depend on the instance type. Please run ethtool -x to determine. If RSS key modification is supported please refer to Configuring RSS section below, explaining how ENA calculates RSS hash.

Performance Optimizations FAQs

Q: How much bandwidth shall I expect for single TCP/UDP flow?

A: EC2 infrastructure limits single flow BW to 5-10 Gbps (depending on protocol type and placement group settings). Please refer to ec2 instance network bandwidth for further details.

Q: ifconfig shows increasing number of dropped Rx packets. What should I do?

A: ENA device would drop packets if Rx ring becomes full. This usually happens because instance's vCPUs don't keep up with the incoming traffic. There are several options you might want to consider. Please refer to CPU Starvation section below.

Q: I spotted ENA driver “Tx Completion Timeout” messages in the kernel logs. What does it mean and what should I do?

A: It means that Tx packets weren’t completed within reasonable time. If a certain threshold of such uncompleted packets is crossed, the driver resets the device. It’s part of ENA self-healing mechanism and would cure the situation for extremely rare cases of an unresponsive device. However usually this message is an indicator of vCPU starvation in your instance. Therefore it is advised to investigate vCPU load. Please refer to CPU starvation section below.

Q: What is ENA device reset?

A: ENA device reset is a self healing mechanism that is triggered when the driver detects unexpected device behavior. Example of such behavior could be an unresponsive device, missing keep-alive events from the device, Tx completions timeouts, netdev timeout etc. The device reset is a rare event, lasts less than a millisecond and might incur loss of traffic during this time, which is expected to be recovered by the transport protocol in the instance kernel.

Q: I want fewer ENA queues, I’d prefer only a portion of my instance's vCPUs to handle network processing.

A: No problem, please use ethtool -l option to see the number of available ENA queues. To adjust the number of queues to N instantaneously, please use:

$ sudo ethtool -L DEVNAME combined N

Please note that changing the number of queues, as well as the rings' sizes might cause a short-lasting (less than a millisecond) traffic interruption.

Q: I want more ENA queues, I’d prefer to expose a dedicated ENA queue for each instance vCPU?

A: Depending on the instance type ENA ENI supports up to 32 queues. If you desire to expose more ENA queues to the instance, please attach to it an additional ENI.

Q: Host vCPU utilization by ENA IRQ processing seems to be too high. I suspect high interrupt rate.

A: Interrupt moderation is supported on the majority of Nitro powered instances types. For Tx, the static interrupt delay is set to 64 usec by default. As for Rx moderation rate, its settings might vary depending on the instance type. On some instance types Rx moderation is disabled by default, on others it is enabled in adaptive mode. Please use

$ ethtool -c DEVNAME

to determine interrupt moderation mode on your instance. If you suspect high interrupt rate, we recommend to enable adaptive Rx moderation. The ENA device implements Dynamic Interrupt Moderation (DIM) mechanism (more details can be found here: net_dim.rst). To enable adaptive Rx interrupt moderation:

$ sudo ethtool -C DEVNAME adaptive-rx on

Q: I notice low BW and throughput. What could be possible reasons?

A: Please check vCPUs utilization (top/htop) on your instance and refer to CPU Starvation section below. Also we recommend to validate that egress traffic is evenly distributed across Tx rings: ethtool -S can be used to observe per ring stats.

Q: Where can I see the ENA device stats

A: ethtool -S DEVNAME

Q: I noticed multiple queue_stops reported by device stats. What does it mean?

A: There might be various reasons for that:

  1. Packets were submitted to the Tx rings faster than they can be processed. This usually happens if the submission rate across your instance queues exceeds PPS rate limit. If this happens and Tx packets are dropped pps_allowance_exceeded/bw_out_allowance_exceeded stats would indicate it. Consider moving to a larger instance size or to a newer generation of the instance family.
  2. Tx Completions weren’t processed in time by the driver and hence Tx submission ring entries weren’t freed. Please refer to CPU Starvation section below for potential causes of vCPU starvation and ways to handle it.
  3. Packets were submitted to a certain Tx ring at a higher rate than it can process it. In this case try to take advantage of multi-queue ENA capability and distribute traffic across multiple Tx queues

Q: What are the optimal settings for achieving the best latency

A: These are the measures that help improve latency:

  1. Make sure CPU power state is set to avoid deep sleep states (see CPU Power State section for the details)

  2. Consider enabling busy poll mode:

    $ echo 70 > /proc/sys/net/core/busy_read
$ echo 70 > /proc/sys/net/core/busy_poll

  3. If possible consider setting the affinity of your program to the same vCPU as the ENA IRQ processing its traffic.

  4. Make sure vCPUs handling ENA IRQs are not overloaded with other unrelated tasks (use taskset or numactl to move heavy tasks to other vCPUs)

  5. Disable DIM (Dynamic Interrupt Moderation):

    $ sudo ethtool -C DEVNAME adaptive-rx off rx-usecs 0 tx-usecs 0

$ ethtool -c eth0 | grep -E 'Adaptive|usecs|frames'
Adaptive RX: off  TX: off
rx-usecs: 0
rx-frames: 0
tx-usecs: 0
tx-frames: 0

Q: Part of my network traffic uses IPv6 header with extensions and also TCP header with options. I suspect my Tx packets are not sent out.

A: ENA LLQs in default mode support network headers size up to 96 bytes. If header size is larger, the packet will be dropped. To resolve this issue, we recommend to reload the ENA driver with module parameter force_large_llq_header=1. This will increase the supported header size to a maximum of 224 bytes. Please note that this option reduces the max Tx ring size form 1K to 512. An example of such use case is IPv6 protocol with TCP SACK enabled, which might result in the packet header exceeding 96 bytes. An alternative solution for this particular use-case would be to disable TCP SACK:

$ echo 0 > /proc/sys/net/ipv4/tcp_sack

Please also note that this feature is only supported by the GitHub version of ENA driver and by AL2 distro.

CPU starvation

Overloaded or unevenly used instance vCPUs might cause delays in network traffic processing leading to packet drops on the Rx side and completion timeouts on the Tx side. This will result in low performance and increased and highly variable latency.

In order to achieve high and stable performance, the user should make sure the instance vCPUs in charge of the network traffic are available and given sufficient processing time for this task. Most of the network processing happens in NAPI routine that runs in softirq context. vCPUs involved in NAPI processing can be identified by running

$ sudo cat /proc/interrupts | grep Tx-Rx

vCPU starvation can be caused by multiple reasons. The following course of actions is recommended if network performance degrades:

  1. Check kernel log for vCPU lockups or other signs of vCPU starvation. ENA packet drops might be a side effect of the global system issue that consumes vCPUs. Usually utilities like htop help observe this. Users can also use linux perf tool to determine where vCPUs spend most of their time.

  2. Sometimes CPU utilization has a spiky nature resulting in short-lasting peaks. This might be enough to cause ingress packet drops for network intensive workloads. In this case we recommend to increase the size of the Rx ring in order to compensate for temporary vCPU unavailability. This would compensate for vCPU short-lasting unavailability. The default size of the ENA Rx ring is 1K entries, however it can be dynamically increased up to 16K entries using ethtool -G option. For example to increase the Rx ring size on eth0 interface to 4096, please run

    $ sudo ethtool -G eth0 rx 4096

    Please note, ring resize operation might cause short-lasting packet drops, that are expected to be recovered by the transport protocol in the instance kernel.

  3. If vCPUs responsible for network processing are constantly overloaded and approach 100% utilization this might indicate uneven load distribution across available vCPUs. The following options might be considered to improve load balancing:

    1. Reassign other tasks running on the overloaded vCPUs to other less loaded vCPUs that don’t participate in network processing. This can be achieved by taskset or numactl Linux utilities

    2. Alternatively steer away network interrupts from already overloaded vCPU. It can be done by:

      1. setting IRQBALANCE_BANNED_CPUS variable in /etc/sysconfig/irqbalance to the CPU mask indicating CPUs that you want to exclude

      2. restarting irqbalance service

        $ sudo service irqbalance restart

      3. Exampe: IRQBALANCE_BANNED_CPUS=00000001,00000f00 will exclude CPUs 8-11 and 33

      4. Note: we do not recommend disabling irqbalance service. ENA driver doesn’t provide affinity hints, and if device reset happens while irqbalance is disabled, this might cause undesirable IRQ distribution with multiple IRQs landing on the same CPU core.

    3. If there are more vCPUs in your instance than ENA queues, consider enabling receive packet steering (RPS) in order to offload part of the Rx traffic processing to other vCPUs. It is advised to keep RPS vCPU cores at the same NUMA node as the vCPU nodes processing ENA IRQs. Also avoid having RPS vCPU on sibling cores of IRQ vCPUs.

      1. To figure out NUMA cores distribution:

        $ lscpu | grep NUMA

The output:
NUMA node(s): 2
NUMA node0 CPU(s): 0-15,32-47 //cores 32-47 are siblings of cores 0-15
NUMA node1 CPU(s): 16-31,48-63 //cores 48-63 are siblings of cores 16-31

      2. Example of RPS activation:

        $ for i in `seq 0 7`; do echo $(printf "00000000,0000ff00") | sudo tee /sys/class/net/eth0/queues/rx-$i/rps_cpus; done

        This would assign cores 8-15 to RPS.

        Please note that if irqbalance service is enabled, IRQ processing might migrate to different vCPUs and make RPS less effective. We do not recommend disabling irqbalance service (See FAQ above), but rather indicate what CPU cores should be excluded by irqbalance service from IRQs processing (please see the point above)

    4. Instances with multiple ENIs and intensive traffic might encounter cases where vCPUs get heavily contended by skbuf allocation/deallocation mechanism. This would usually manifest in a way of native_queued_spin_lock_slowpath() function consuming most of processing time. To overcome this issue ENA driver introduces Local Page Cache (LPC) that allocates a page cache for each Rx ring and helps relieve allocation contention. LPC size by default is 2K pages, however it might be increased using module load parameter. Please see Local Page Cache (LPC) section below for more for more details.

    5. If you suspect elevated CPU utilization due to high interrupt rate please enable Rx adaptive moderation as explained in the FAQs above:

      $ sudo ethtool -C DEVNAME adaptive-rx on

    6. For some workloads it makes sense to reduce the number of vCPUs handling ENA IRQs, and thus free up more vCPU resources for other purposes. This can be achieved by reducing the number of ENA queues

       $ sudo ethtool -L DEVNAME combined N

where N is a desired number of queues.

Reserving sufficient kernel memory

Ensure that your reserved kernel memory is sufficient to sustain a high rate of packet buffer allocations (the default value may be too small).

  • Open (as root or with sudo) the /etc/sysctl.conf file with the editor of your choice.

  • Add the vm.min_free_kbytes line to the file with the reserved kernel memory value (in kilobytes) for your instance type. As a rule of thumb, you should set this value to between 1-3% of available system memory, and adjust this value up or down to meet the needs of your application.

  • Apply this configuration with the following command:

    $ sudo sysctl -p

  • Alternatively one can use the below command, but it will not persist after reboot:

    $ sudo sysctl -w vm.min_free_kbytes=1048576

  • Verify that the setting was applied with the following command:

    $ sudo sysctl -n vm.min_free_kbytes

Local Page Cache (LPC)

ENA Linux driver allows to reduce lock contention and improve CPU usage by allocating Rx buffers from a page cache rather than from Linux memory system (PCP or buddy allocator). The cache is created and bound to Rx queue, and pages allocated for the queue are stored in the cache (up to cache maximum size).

To set the cache size, one can specify lpc_size module parameter, which would create a cache that can hold up to lpc_size * 1024 pages for each Rx queue. Setting it to 0, would disable this feature completely (fallback to regular page allocations).

The feature can be toggled between on/off state using ethtool private flags, e.g.

$ ethtool --set-priv-flags eth1 local_page_cache off

The cache usage for each queue can be monitored using ethtool -S counters. Where:

  • rx_queue#_lpc_warm_up - number of pages that were allocated and stored in the cache
  • rx_queue#_lpc_full - number of pages that were allocated without using the cache because it didn't have free pages
  • rx_queue#_lpc_wrong_numa - number of pages from the cache that belong to a different NUMA node than the CPU which runs the NAPI routine. In this case, the driver would try to allocate a new page from the same NUMA node instead

Note that lpc_size is set to 2 by default and cannot exceed 32. Also LPC is disabled when using XDP or when using less than 16 queues. Increasing the cache size might result in higher memory usage, and should be handled with care.

CPU Power State

If your instance type is listed as supported on Processor state control for your EC2 instance, one can prevent the system from using deeper C-states to ensure low-latency system performance. For more information, see High performance and low latency by limiting deeper C-states.

  • Edit the GRUB configuration and add intel_idle.max_cstate=1 and processor.max_cstate=1 to the kernel boot options For Amazon Linux 2, edit the /etc/default/grub file and add this option to the GRUB_CMDLINE_LINUX_DEFAULT line, as shown below:

    > GRUB_CMDLINE_LINUX_DEFAULT="console=tty0 console=ttyS0,115200n8 net.ifnames=0 biosdevname=0 nvme_core.io_timeout=4294967295 xen_nopvspin=1 clocksource=tsc intel_idle.max_cstate=1 processor.max_cstate=1"

> GRUB_TIMEOUT=0

    For Amazon Linux AMI, edit the /boot/grub/grub.conf file and add this option to the kernel line, as shown below:

    > kernel /boot/vmlinuz-4.14.62-65.117.amzn1.x86_64 root=LABEL=/ console=tty1 console=ttyS0 selinux=0 nvme_core.io_timeout=4294967295 xen_nopvspin=1 clocksource=tsc intel_idle.max_cstate=1 processor.max_cstate=1

  • (Amazon Linux 2 only) Rebuild your GRUB configuration file to pick up these changes:

    $ sudo grub2-mkconfig -o /boot/grub2/grub.cfg

Configuring RSS

The ENA device supports RSS, and depending on the instance type, allows to configure the hash function, hash key and indirection table. Please note that hash function/key configuration is supported by the 5th generation network accelerated instances (c5n, m5n, r5n etc) and all 6th generation instances (c6gn, m6i etc). Also Linux kernel 5.9 or newer is required for hash function/key configuration support but the major Linux distributions ported the driver support to kernels older than v5.9 (For example Amazon Linux 2 supports it since kernel 4.14.209). You can also manually install GitHub driver v2.2.11g or newer to get this support if your instance doesn't come with it.

The device supports Toeplitz and CRC32 hash functions and ethtool -X command can be used to modify hash function/key and indirection table.

To calculate the Toeplitz hash value for a given flow (identified by a 4-tuple: source/destination ip and source/destination port) one can use toeplitz_calc.py script which simulates the hash calculation that is done in HW. Example usage (more information can be found by running python3 toeplitz_calc.py --help):

$ python3 toeplitz_calc.py -t 1.2.3.4 -T 7000 -r 1.2.3.5 -R 7000 -k 77:d1:c9:34:a4:c9:bd:87:6e:35:dd:17:b2:e3:23:9e:39:6d:8a:93:2a:95:b4:72:3a:b3:7f:56:8e:de:b6:01:97:af:3b:2f:3a:70:e7:04
Sending traffic from 1.2.3.4:7000 to 1.2.3.5:7000
to an instance which supports changing the key

Should result in the following hash for each driver:
DPDK                                                    0xa9828bd4 (RSS table entry: 84)
FreeBSD                                                 0xa9828bd4 (RSS table entry: 84)
Linux (before setting the key with ethtool)             0xa4a1471a (RSS table entry: 26)
Linux (after setting the key with ethtool)              0x5b5eb8e5 (RSS table entry: 101)
Windows                                                 0x5b5eb8e5 (RSS table entry: 101)

Please note the Linux driver contains a bug in versions v2.2.11g-v2.6.0g which makes the hash calculated with initial value of 0x0 before setting Toepltiz key manually, and 0xffffffff afterwards. Both cases are printed in the provided script's output.

The script provides the hash value and the RSS table entry for an incoming packet. To retrive the RX queue number on which the packet is received please use ethtool -x [interface number] to find out what queue number each RSS table entry points to.