i can change the I/O scheduler at run time by following command
echo scheduler_name > /sys/block//queue/scheduler
after this command  brackets are found around I/O scheduler name in scheduler file
"But how do know which I/O scheduler is being used at particular moment."
beside this in linux 2.6.13(which i use) it registers all the I/O scheduler at boot time.
so if i want to add my i/o scheduler where should i make changes so that my scheduler get
registered and identified.