Thank you very much for the replies. I saw a post from Steffen from 5 days ago, and now have changed my settings to match those (pasted again below). This is helping! Something must have changed in 2020.09, the default settings had always been fine up before. I will post again in the next day to confirm if the issue has been entirely resolved :)

PHP-FPM pool:
pm = dynamic
pm.max_children = 275
pm.start_servers = 24
pm.min_spare_servers = 6
pm.max_spare_servers = 42
pm.max_requests = 1250

@Matt Panhans I experience a similar increase in load. Especially I/O load. It started with/after the 2020.09 release when I remember correctly. But I'm not sure yet what exactly is causing this.

@Matt Panhans Please be careful with my settings. They're good for my machine with 64GB RAM. But they can be too high for a smaller sever. Also, with 275 max. FPM processes you need at least 350 possible DB connections or you will get problems/errors.

My issue appears to be resolve! I want to give a huge thank you to everyone for such prompt and helpful replies. I get better support here than I do from the IT department at my workplace!

My last thought on this issue - and I will leave to someone more technical to make this determination - is that it seems to me that there was an increase in Friendica's required resources/memory when moving from 2020.07 to 2020.09 that caused this issue. That's fine if expected, but if not it might be worth trying to understand the cause better, and perhaps get back to lower requirements. But I will leave to those more knowledgeable whether it's worth opening this as an issue on github.

I had a similar problem on my setup which I also started noticing after the last update of friendica. I'm also serving other service like Nextcloud on the same raspberry. I saw the following lines in my php log:

WARNING: [pool www] child 17597 exited on signal 11 (SIGSEGV) after 1326.443580 seconds from start

opcache.validate_permission=0
opcache.validate_root=0

Good question. I do see a difference in traffic, in the Apache SSL/TLS access log. Sept 2 - Sept 4, 2020, it was logging an average of 32 entries per minute. On Sept 26, 2020, it was logging 171 entries per minute on average. For Sept 26, the entries are mostly many repeated "GET /.well-known/webfinger?" and "GET /friendica HTTP/1.0"

I guess the puzzle to me is what about updating to 2020.09 would have caused an increase in traffic? Any other info I can provide that would be helpful?

I'm uncertain if this answers your question, but the following three request lines were logged >46,000 times each on my server on Sept 26, 2020. The next most frequent request line was less than 5,000 times. (I am running Friendica on the domain sendero.social)

"GET /friendica HTTP/1.0 200 4303 - Friendica 'Red Hot Poker' 2020.09-1368; social.brevsprekk.net
"GET /.well-known/webfinger?resource=acct%3Afriendica%40sendero.social HTTP/1.0 200 836 - Friendica 'Red Hot Poker' 2020.09-1368; social.brevsprekk.net
"GET /friendica HTTP/1.0 200 2511 - Friendica 'Red Hot Poker' 2020.09-1368; social.brevsprekk.net

Just to report back. Since the update to 2021.01 I don't have to restart php constantly any more. #Friendica is faster and makes more fun than ever.

Thanks to all the developers and contributors.