Reducing.. Kernel Page cache, Inode and dentry caches.

Kernel 2.6 never provide a mechanism to have the kernel drop the page cache and/or inode and dentry caches on command, which can help free up a lot of memory.

Writing to this will cause the kernel to drop clean caches, dentries and inodes from memory, causing that memory to become free.

To free pagecache:
#  echo 1 > /proc/sys/vm/drop_caches

To free dentries and inodes:
#  echo 2 > /proc/sys/vm/drop_caches

To free pagecache, dentries and inodes:
#  echo 3 > /proc/sys/vm/drop_caches

As this is a non-destructive operation, and dirty objects are not free-able, the user should run "sync" first in order to make sure all cached objects are freed.

How to speed up SSH login?

Whenever I login to my Centos Server using SSH, it takes about 6 seconds for me to get the prompt for password, however when I login to my web hosting server it takes about 1 second. What can I do to speed this up?

If you run a lot of terminal tabs or scripts that all need to make OpenSSH connections to the same server, you can speed them all up with multiplexing: making the first one act as the master and letting the others share its TCP connection to the server.

If you don't already have a config file in the .ssh directory in your home directory, create it with permissions 600: readable and writeable only by you.
Speed up SSH 

Step 1 -Add below to /etc/ssh/sshd_config

The number one reason I've seen for this is a configuration option in SSHD UseDNS this option (enabled by default) causes the server to perform DNS resolution on the incoming requests. A time consuming operation. I've seen logins go from one minute plus waiting for password 

#  vi /etc/ssh/sshd_config

UseDNS no

Step 2- Setting up ssh client with compression 

#  vi /etc/ssh/ssh_config

Host *
Ciphers arcfour,blowfish-cbc
Compression yes
AddressFamily inet
ControlMaster auto
ControlPath ~/.ssh/socket-%r@%h:%p

Save and exit !
Step 3- Restart the SSH daemon with service sshd restart

You should see an improvement next time you connect.