What should I do to increase bitcoind sync speed? My VPS has 2gb RAM and CPU core is 1 and 1 TB HDD storage. The sync speed is very slow. Currently it has about 700 MB free RAM. bitcoind is taking about 66% of the 2GB ram.
Should I add 1GB more RAM or is there any way to get similar improvement using any configuration?
First, make sure you have listen=1 in your bitcoin.conf file, and that the P2P port is open on your VPS so that it can make more than 8 connections.

Then you'll want to make sure you have a high disk speed or IO specified (definitely not HDD - I have a feeling you meant SSD). The faster the SSD the better. 2GB RAM should be fine, but you may want to up it to 4GB and 4 CPUs, at least while syncing. bitcoind will use the most resources while syncing, then you can downgrade once it's synced.