I run a phpBB car forum ([Citroën Akuten|https://citroenakuten.com] with more than 21800 registered users.
Lately I have seen the error message "PHP Warning: Unknown: Unable to allocate memory for pool. in Unknown on line X" (X being all sorts of line numbers)
All over assorted help fora's the standard fix are always "increase memory setting" here and there. This however only over time worsen the problem.
I believe I have found the root cause of these "brown outs";
Table sessions_keys allows multiple data for the same user_id from the same last_ip (see my attached short-list from my own site "sessions_keys_bugreport.xlxs"). Note that I have added some columns to identify doublets and count the occurrence.
I suggest that the database may not contain doublets in this aspect which I manually cleaned out and now are trying. I have had no errors at all since then but the doublets will of course slowly grow back. When first examined my sessions_keys it contained around 35-36.000 lines. It's now at 8.650 lines and the site has never felt faster!