I have been working with Magento for a about 9 months. I have used composer to upgrade Mangento Versions over time as they are available. The last upgrade from 2.1.10 to 2.1.11 worked well but 2.1.11 to 2.1.12 has hit a memory limit issue again. I am using this command:
composer require magento/product-community-edition 2.1.12 --no-update
This is when the memory is exhausted. One thing that has changed is the cron jobs were not running in the past. Could the cron jobs scheduled every minute take memory from the upgrade? Any help would be appreciated.
Create a .bat to run the cron job with this,
C:\xampp\php\php.exe C:\xampp\htdocs\magento\bin\magento cron:run C:\xampp\php\php.exe C:\xampp\htdocs\magento\update\cron.php C:\xampp\php\php.exe C:\xampp\htdocs\magento\bin\magento setup:cron:run
And configure your php.ini with 1024M memory_limit and your .htaccess root file with 1024M as well.
Run below command,
composer require magento/product-community-edition 2.1.12 --no-update composer update
You can also lookup this below link if above trick is not working for you,