Try to do a sudo yum updates command this morning on one of my DigitalOcean droplets today and encounter weird error messages. Everything was running fine until it suddenly stops and vomit out the word Killed. Further attempts of sudo yum updates and yum-complete-transaction also did not end well. Only after doing a ps -ef | grep defunct command that I realized what is the problem. There are couple of zombie Apache processes that sucking up the memory and thus causing yum updates to fail. To fix the problem : shutdown Apache with sudo /etc/init.d/httpd stop kill -9 all the zombie process and this time sudo yum updates works! NOTE : this situation is unique to me and as for yours...you need to find out which process is taking up the memory. It could be something other that Apache.
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