dnsdist: Investigate and document OpenSSL 3.x performance

[rgacogne]

• Program: dnsdist
• Issue type: Feature request

Short description

The performance regression in OpenSSL 3.x is a very real problem that keeps showing up when DNSdist users upgrade to a more recent version of their distribution of choice. I'm afraid we cannot do anything to fix it, but we might at least be able to fix the pain by documenting it. Moving to GnuTLS is known to help in some cases. The problem is especially visible when a lot of threads / CPU cores are in use.

Related issues:

• Massive performance degradation in OpenSsl 3.0 if used in a heavy multi threaded server application openssl/openssl#17064
• 3.0 Performance problems openssl/openssl#17627
• HAProxy 2.9.5 performance  haproxy/haproxy#2454

On the same topic, it would be nice to investigate whether DNSdist can use https://github.com/aws/aws-lc or https://github.com/rustls/rustls-openssl-compat (although I'm not sure the latter would help, since it uses libcrypto internally).