Hi,
I was working on it for more than 6 months, this is not a memory cache issue and you CANNOT fix it by “sync; echo 3 > /proc/sys/vm/drop_caches”. It’s only the kernel issue that fixed in kernel 4.x lt. So you can use this instruction for upgrade your kernel: https://www.tecmint.com/install-upgrade-kernel-version-in-centos-7/
I hope it will be helpful everyone has this issue.