PHP Fatal error: Allowed memory size of 536870912 bytes exhausted

Thanks for all suggestions, I dont think I have endless loop as the same code runs fine under 2.2.x and I migrated to CI 3.1.5
with little changes except class names for my custom controllers/ libraries/ models. I made some changes today and try again to see if this is gone away.

I did use mpdf as libraries as mine is heavily computing data and make pdf output for deliveries but I dont get the point if this
is the contributing factor to the problem. I have multiple users access to app but not like 100s at the same time so on...

Regards