what is the best tool (open or commercial) currently available, that lets me send customized requests to a web server and get back a response to check the performance?
i will be sending it a load of more than 20K per second, but i need to get numbers for each call made. also, the numbers might be in some microseconds or nanoseconds. How in this small measurement unit, can i work out a baseline and a benchmark?
If you're using Apache, Apache AB is a benchmarking to test how many requests your serve can serve per second and how well it handles load and concurrency. It's an open-source project - check it out here.
In addition, wikipedia has a nice list of benchmarking software for testing servers.
You can use the Web Application Stress Tool of Microsoft
The Microsoft WAS web stress tool is designed to realistically simulate multiple browsers requesting pages from a web site. You can use this tool to gather performance and stability information about your web application. This tool simulates a large number of requests with a relatively small number of client machines. The goal is to create an environment that is as close to production as possible so that you can find and eliminate problems in the web application prior to deployment.
You can find a list of Open Source software for performance (most of them are for web that send custom request to webserver).
Don't know if either of these have granularity better than milliseconds but check out JMeter (open source) and LoadRunner (Commercial). LoadRunner is not cheap but it allows you to span load generation across multiple machines with aggregated results.