Jolyon

I believe I have found the cause of all these issues!
CloudLinux (excellent idea and product) limits my RAM to 256MB.
My host has PHP’s memory_limit set to 128MB.
When a PHP page is first called, the memory_limit of 128MB is assigned (for ~100 milliseconds), then reduced to the actual RAM required (in my case, 524KB).
I’m not using WordPress, but have AJAX code calling multiple PHP pages simultaneously – wham, I hit the limit.
I have some (relatively) simple demo code, which you are free to copy and use to test with at:
https://DingoSeatCovers.com.au/demo
The PHP pages are one-liners.
I’m trying to persuade my Website Host to reduce PHP’s memory_limit from 128MB to reduce this issue.
Anyone have contacts with Apache/PHP to see if the “assign memory_limit” could be changed?
Contacted CloudLinux, but no success, as they do not consider it their problem – fair enough, I think.

Most aggravating thing is, Website Hosts see this as a “we have found the heavy users, and they are the problem”. No, the heavy users are more likely to see this issue, but the problem is a very high PHP memory use (very brief, but actual) and CloudLinux seeing that breaching the limit.

My code (not fancy) did not automatically retry when finding 500 errors, so I saw the exact problem much earlier and more easily.
Most good code (jQuery and WP) is likely to retry the 500 errors, and so mask this problem from sites with low usage.