About fastpath mode in screen latency test

There is fastpath mode in screen latency test, In the code it invoke "setSharedBufferMode" method which means use single Buffer to render and display. But from the systrace of fastpath test, the graphicBuffer was dequeued and queued normally, and there is 2 graphicBuffer:SurfaceView - org.chromium.latency.walt/org.chromium.latency.walt.MainActivity#0:0 and SurfaceView - org.chromium.latency.walt/org.chromium.latency.walt.MainActivity#0: 1。

So the queston is:
1. Does fastpath mode use the single Buffer mode?
2. If so, why the fastpath mode use 2 graphic Buffer, does it need hardware support?
Thanks!