Thank you so much for your detailed reply and reminding.
My server's RAM has been upgraded to 4G. So I suppose it's enough to change the Max_connections into 200 at least.
The reason for this changing is that there are more than 100 people log on my Linux+apache+mysql+php system simultaneously and edit their courses. They often meet the error: database connection failed.
But I worry about slowing down or even collapsing because of the excessive memory consumption. So I have not taken acton yet.