centos7无法安装桌面，1核，内存1G

输入这个命令：yum groups install "MATE Desktop"
[root@iZrj90zkx5pcka ~]# yum groups install "MATE Desktop"
已加载插件：langpacks
没有安装组信息文件
Maybe run: yum groups mark convert (see man yum)
base                                                     | 3.6 kB     00:00    
epel                                                     | 4.3 kB     00:00    
extras                                                   | 3.4 kB     00:00    
nginx                                                    | 2.9 kB     00:00    
Could not retrieve mirrorlist h-t-t-p://poptop.sourceforge.net/yum/stable/mirrorlist-poptop-stable-rhel7 error was
（h-t-t-p是我写的，因为不能发链接）
14: HTTP Error 404 - Not Found




One of the configured repositories failed (未知),
and yum doesn't have enough cached data to continue. At this point the only
safe thing yum can do is fail. There are a few ways to work "fix" this:


     1. Contact the upstream for the repository and get them to fix the problem.


     2. Reconfigure the baseurl/etc. for the repository, to point to a working
        upstream. This is most often useful if you are using a newer
        distribution release than is supported by the repository (and the
        packages for the previous distribution release still work).


     3. Disable the repository, so yum won't use it by default. Yum will then
        just ignore the repository until you permanently enable it again or use
        --enablerepo for temporary usage:


            yum-config-manager --disable <repoid>


     4. Configure the failing repository to be skipped, if it is unavailable.
        Note that yum will try to contact the repo. when it runs most commands,
        so will have to try and fail each time (and thus. yum will be be much
        slower). If it is a very temporary problem though, this is often a nice
        compromise:


            yum-config-manager --save --setopt=<repoid>.skip_if_unavailable=true


Cannot find a valid baseurl for repo: poptop-stable/7