apt-get 安裝出現 fork failed: Cannot allocate memory錯誤

在安裝git-ftp時出現下面這個錯誤

# apt-get -f install git-ftp
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
  git-ftp
0 upgraded, 1 newly installed, 0 to remove and 42 not upgraded.
Need to get 0 B/14.6 kB of archives.
After this operation, 77.8 kB of additional disk space will be used.
dpkg: unrecoverable fatal error, aborting:
 fork failed: Cannot allocate memory
E: Sub-process /usr/bin/dpkg returned an error code (2)

原因：
用 free -m 或 top 看，發現所剩的記憶體太少了

解法：
把apache或gitlab關掉 再裝
# service apache2 stop
or
# gitlab-ctl stop

# apt-get -f install git-ftp
... 安裝成功