Sometime on the 12th my site started to produce a series of PHP Fatal errors saying “Allowed memory size of 41943040 bytes exhausted.”. This was a bit of a surprise. It means that my memory limit was 40M, a lot lower than the expected value of 256M.

Other times the response was

[13-Jul-2016 02:54:59 UTC] PHP Fatal error:  Unknown: 
Cannot use output buffering in output buffering display handlers in Unknown on line 0

Problem recovery

To recover from this problem I used ftp to rename Jetpack to Jetpack-f.
I was then able to access the site.

Problem explanation

• The site was migrated from one host to another.
• The PHP version on the new host was 5.4.45.
• The memory_limit was set to 25M.
• For PHP 5.3, 5.5 and 5.6 the memory_limit would have been 256M; a much nicer number.
• By the time WordPress started up the memory limit became 40M.
• It’s WordPress that sets this limit, depending on the site. For WPMS it’s 64M.
  See wp_initial_constants().

I’ve yet to track down the specific plugin/module combination that causes WP to exceed the 4OM limit.

Other notes

• My oik-bwtrace plugin reports the memory limit on its admin page.
• This is 256M, since WordPress sets it higher in admin pages. Using admin_memory_limit in wp-admin/admin.php.
• I’ve changed my [wp] shortcode to display the memory limit on the front-end.
• For this site the memory_limit has already been set by the hosting company to a high value.
  
  WordPress 5.2.1. PHP: 7.3.7. Memory limit: 768M
• I’m also experimenting with code that displays the current memory_limit in trace records produced by oik-bwtrace. They already optionally display current and peak memory usage, so adding the current memory limit is no big deal.