Ghilyn

After sitneg innodb_buffer_pool_size on more than 500MB mysql doesnt start ??? how's that ??  but if i left it set to around 500MB   than in short time it's exceeded  . if i leave it in default works fine   i am confused     shortly i have to leave it on deafult if i dont want to restart mysql every 5 min  i have 4GB RAM in server