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thnx but now i am facing another problem that is when i am posting a new post then the server load is increase a lot.

I am using a vps of 2gb ram 4 core processor and centos 06

what is my solution now.

If you want to limit the allocated memory for the whole WP, do this: Controlling allocated memory for PHP

If you want to limit memory allocated for a particular file (particular process), use the following lines (ex. for edit.php file) and put it into your wp-config.php after wp-settings.php line:

if( $_SERVER['SCRIPT_FILENAME'] == './wp-admin/edit.php' )
	ini_set('memory_limit','48M');

It might also be the caching modules you are running. List which ones you are running, this is commonly an issue with object or db caching.

i am using latest wordpress 4.2.2 and vps is 4 core 2 gb ram but my vps cannot load the site when there is 100+ visitors on real time. I am using w3 total cache.

what is the base requirements of hardware to run 200+ visitors on wordpress using w3 total cache.
Another server i am using w3 total cache where the server load is 0 when the visitor is also 2000+ in real time. In this case the hardware is very high such as 24 core 64 gb ram. so i need a specific hardware configuration of a server to run a 500 real time visitors.

There is no quick answer to your question. The utilization of a server depends on many things. I use shared hosting on one of my client’s site and it is able to serve 200+ users in real time.
Your examples provide different results with W3TC. I guess that one or more plugins in your WP installations consume much memory than usual. It has made on-the-fly processing on uncached pages in your server to use excessive amount of memory to do its on-the-fly job. You may monitor the memory usage for each plugin and discover which plugin is causing the memory overload. You can use plugin such as Dev Monitor or Server IP & Memory Usage Display. Then, limit the memory usage of the plugin, or find another plugin with similar functionality to substitute it.

It could also be a poor implementation of the plugin. What caching modules are you running?