I would still set the innodb_buffer_pool_size much higher that 400M. The reason? InnoDB Buffer Pool will still cache the data and index pages you need for tables accessed frequently.
Run this query to get the recommended innodb_buffer_pool_size in MB:
SELECT CONCAT(ROUND(KBS/POWER(1024,IF(pw<0,0,IF(pw>3,0,pw)))+0.49999),SUBSTR(' KMG',IF(pw<0,0,IF(pw>3,0,pw))+1,1)) recommended_innodb_buffer_pool_size FROM (SELECT SUM(data_length+index_length) KBS FROM information_schema.tables WHERE engine='InnoDB') A,(SELECT 2 pw) B;
Simply use either the result of this query or 80% of installed RAM (in your case 19660M) whichever is smaller.
I would also set the innodb_log_file_size to 25% of the InnoDB Buffer Pool size. Unfortunately, the maximum value of innodb_log_file_size is 2047M. (1M short of 2G) Thus, set innodb_log_file_size to 2047M since 25% of innodb_buffer_pool_size of my recommendated setting is 4915M.
Yet another recommedation is to disable ACID compliance. Use either 0 or 2 for innodb_flush_log_at_trx_commit (default is 1 which support ACID compliance) This will produce faster InnoDB writes AT THE RISK of losing up to 1 second's worth of transactions in the event of a crash.