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Performance Tuning

KumoMTA performance can be optimized by fine tuning system parameters. The settings below are examples only but have helped optimize test and development servers. This example represents a starting off point, you should research these and tune as needed for your own system.

KumoMTA makes heavy use of files, RAM, CPU and network resources. Setting these can be helpful as a default Linux install assumes the need to share resources with many applications, but we need to allow KumoMTA to use as much of the resource pool as possible.

Tuning sysctl.conf

The following tuning parameters can help KumoMTA fully leverage its host server resources.

These parameters should be added or updated in /etc/sysctl.conf:

vm.max_map_count = 768000
net.core.rmem_default = 32768
net.core.wmem_default = 32768
net.core.rmem_max = 262144
net.core.wmem_max = 262144
fs.file-max = 250000
net.ipv4.ip_local_port_range = 5000 63000
net.ipv4.tcp_tw_reuse = 1
kernel.shmmax = 68719476736
net.core.somaxconn = 1024
vm.nr_hugepages = 20
kernel.shmmni = 4096

After editing, the changes can be implemented without a restart with the sysctl -p command.

Performance Testing

Performance testing must not be performed against the public internet, as large volumes of test message can be catastrophic for sending reputation. Instead, a second instance of KumoMTA should be installed that uses the sink.lua policy script found at

Write the script to /opt/kumomta/etc/policy/sink.lua and start the sink server using the following command:

sudo KUMOD_LOG=kumod=info /opt/kumomta/sbin/kumod \
   --policy /opt/kumomta/etc/policy/sink.lua --user kumod

With the sink server configured and running, you can send test messages to the sink, knowing that they will be discarded and not relayed to the public Internet. You may want to block outbound traffic on port 25 from your testing servers to help ensure no messages are relayed externally.

Included with the packaged KumoMTA builds is a "Traffic Generator" that can be use to send volume test mail for this purpose. The traffic-gen appends a known domain to all outbound mail that resolves to your own loopback address so that mail can be delivered, but will never deliver to real addresses:

sudo /opt/kumomta/sbin/traffic-gen --target <your.sink.server>:25 --concurrency 20000 --message-count 100000 --body-size 100000

For additional parameters for the traffic-gen utility see:

sudo /opt/kumomta/sbin/traffic-gen --help

The traffic-gen script is used internally by the KumoMTA team to test performance before each release.

It is helpful to use custom routing to configure the test server to route all messages to the sink server, with the sink configured to dev/null all messages. Modify the init.lua on the test server with the following:

kumo.on('smtp_server_message_received', function(msg)
    msg:set_meta('queue', 'my.sink.server')

Sample Test Results

The hardware configuration used in this example is one "sending" configured KumoMTA instance hosted on AWS (variable CPU and RAM) and one "sink" KumoMTA instance hosted on Azure (8 CPU/16GB RAM) using a payload of 100KB messages sent in a loop 100,000 times.

The test utilized the included traffic-gen utility as described above.

2 4 2.7 MMH
4 16 4.4 MMH
8 30 4.9 MMH
16 64 5.1 MMH

NOTE that these numbers are NOT guaranteed and are for informational purposes only. Your results may vary considerably.

In July 2023 another round of testing was done with more detailed results. those results are shown in the table below and were documented in the blog post How we built the most performant Message Transfer Agent on the planet: