How to fix error for wp-includes/plugin.php on line 193 and wp-includes/functions.php

Dear Sir,

How to fix the error?

PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 110 bytes) in /wp-includes/plugin.php on line 193

PHP Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 41635 bytes) in /wp-includes/functions.php on line 252

The site db 3.171G ~ Mem 115.81 of 256M

The site is very slow and have server error.

Thanks!