The Intel Iris graphics chip is an integrated graphics system, not a discrete graphics module as is the NVIDIA on the 15" MBPr.
It is dedicated, and in use all of the time...but not the discrete module that you may be thinking about.
Seehttp://www.apple.com/macbook-pro/specs-retina/ for how the two models are setup.
does it mean that if I have 8gb ram,1gb of those will be constantly used by the graphics chip?
No, the Apple graphics article indicates that the Iris will use UP TO 1.5 GB of the ram...the amount taken depends on the graphics needs and what has been allocated by the cpu already.
The current versions of Mac OS X show in the Activity Monitor almost all memory as being used, but that really means the operating system is taking credit for the memory even when it has not been allocated. So, if the system allocates only a small fraction of the memory, the gpu can grab what it needs to run easiest from the unallocated memory...as long as that does not exceed 1.5 GB.
You can see how your system is loading the memory by looking at the Activity Monitor, Finder > Applications > Utilities > Activity Monitor, and then click the Memory button, and look at the Memory Pressure...green is good, yellow you are pushing the machine, red and you are overloading and it will slow down.