Number of threads
Elasticsearch uses a number of thread pools for different types of operations. It is important that it is able to create new threads whenever needed. Make sure that the number of threads that the Elasticsearch user can create is at least 4096.
This can be done by setting ulimit -u 4096 as root before starting Elasticsearch, or by setting nproc to 4096 in /etc/security/limits.conf.
The package distributions when run as services under systemd will configure the number of threads for the Elasticsearch process automatically. No additional configuration is required.
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https://www.elastic.co/guide/en/elasticsearch/reference/7.17/max-number-of-threads.html
