Apache processes were not exist regularly.

Now there are about 200 apache prcoess were running at the same time in our server. But I check the process list, I found many of them are created days ago, only 60% apache processes was created today. I google this problem, but I didn't find a solution for this. I really have no idea why the apache process was not killed every time when the we request was end.

Is there any body can give some suggestions?