roughnecks via Mailman-users writes:
if you look at this graph you'll notice how my VPS "reacted" after mailman3 install.
Looks right to me. Mailman 3 was designed at a time when it was reasonable to expect that a serious list host would be actual hardware, configured pretty much exclusively for the use of the MTA, an httpd, an RDBMS, and the Mailman suite on at least consumer-grade hardware (at that time one-core CPU, 4GB RAM, 25+GB HDD). Sooo ...
By default Mailman core will be running about 15 processes, Postorius about 3, HyperKitty up to about 8 I think, the webserver will be running a bunch, if your MTA is Postfix that will be running a bunch of daemons and firing up worker processes whenever there's something to dov, you'll probably be running a bunch of ancillary daemons for mail (opendkim, Spamassassin, clamav, Amavis) and your RDBMS will be running a couple. Mailman's queue runners (about half of its processes) all wake up frequently and check for work. Janitorial processes like crond and logrotate will have more to do (even if all they do is check that there's nothing to do). You'll be responding to the Internet on multiple ports (most likely all of 22, 25, 80, 443, and 587, maybe a couple more) which will attract the usual conglomeration of Internet busybodies and felons unless you're more diligent than I am about "iptables -A -s x.y.z.w -j DROP" for miscreant x.y.z.w :-).
Somebody posted a couple months back that they were working on a fork of Mailman 3 that would use threads rather than processes as much as possible, but that sounds quite hard (so we're unlikely to do it ourselves) and I haven't heard anything since. And although that would definitely help get LOADAVG down, it wouldn't help that much with %CPU since it doesn't do anything to deal with the busybodies and miscreants pounding on your open ports.
-- GNU Mailman consultant (installation, migration, customization) Sirius Open Source https://www.siriusopensource.com/ Software systems consulting in Europe, North America, and Japan