Manual:$wgMemoryLimit

Performance hacks and limits: $wgMemoryLimit
The minimum amount of memory that MediaWiki "needs"; MediaWiki will try to raise PHP's memory limit if it's below this amount.
Introduced in version:	1.16.0 (r54339)
Removed in version:	Still in use
Allowed values:	Shorthand byte notation
Default value:	'50M'
Other settings: Alphabetical | By function

Details

The minimum amount of memory that MediaWiki "needs"; MediaWiki will try to raise PHP's memory limit if it's below this amount.

Notes

A check in Setup.php gets run on every page view to make sure that PHP's memory limit is at least this amount. If it's lower, MediaWiki will use ini_set() to raise it if it can.

The installation script used to insert ini_set( 'memory_limit', '20M' ); in LocalSettings.php if the memory limit was below 20M. This led to problems when people raised the memory limit for their server later and found that it was mysteriously overridden.