以前没有注意，ubuntu的系统的源原来每次在GUI操作后都会追加在后面而不是覆盖，比如先添加了上海交大的源那么后来如果发现很慢下载不了，而这个包已经在apt-cache里面了，下次如果要install这个软件，那么他会在之前出现的地点找，这样就会导致下载非常慢，现在经验总结如下，下次如果装ubuntu系统一定要在/etc/apt/sources.list更改。首先要将sources.list备份为sources.list.back一次，将之前的源都要删掉，切记，不然重复的包会首先调用前面的
比如添加163的源：
deb http://mirrors.163.com/ubuntu/ precise main universe restricted multiverse deb-src http://mirrors.163.com/ubuntu/ precise main universe restricted multiverse deb http://mirrors.163.com/ubuntu/ precise-security universe main multiverse restricted deb-src http://mirrors.163.com/ubuntu/ precise-security universe main multiverse restricted deb http://mirrors.163.com/ubuntu/ precise-updates universe main multiverse restricted deb http://mirrors.163.com/ubuntu/ precise-proposed universe main multiverse restricted deb-src http://mirrors.163.com/ubuntu/ precise-proposed universe main multiverse restricted deb http://mirrors.163.com/ubuntu/ precise-backports universe main multiverse restricted deb-src http://mirrors.163.com/ubuntu/ precise-backports universe main multiverse restricted deb-src http://mirrors.163.com/ubuntu/ precise-updates universe main multiverse restricted
这样就okay啦！