I need to run around 20 scheduled task on coldfusion server every minute. what shall be the performance impact?

In general, 20 requests a minute is nothing for CF.

I have worked on servers getting 100s requests a second and CF performs fine.

However, if you are sending out 1000 emails in a request, or processing results sets in the 1000s then that is more batch work.  I would run that on it's own CF instance(s) as well. 

Mike

If you have one scheduled task that runs every minute and is performing 20+ jobs, then making 20+ scheduled tasks that run every minute and perform only one task shouldn't make much (if any) difference on performance.***

I'm just curious as to _why_ you're running tasks every minute.  That, alone, can make a difference.  Is there no way to detect when something needs to be run, and run it only then?

V/r,

^ _ ^

*** This is going on the assumption that you do not schedule any of them to run at the same time.

Depends on what the scheduler does

If it queries the database, it might slow down the complete system.