Cisco Selective Packet Discard (SPD) and Input Queue mismatch

From what I can tell the SPD is supposed to be the lowest input queue size found on any interface on the router. So does that mean that the packet goes through the interface's input queue before the final route processor input queue? Or is the SPD input queue added onto the interface's input queue?

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I ask because If I log into a router I see a default input queue MAX of 375, but if I run "show ip spd" I see the max is 75. So how do they work together?

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EDIT:

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here are some facts I've gathered

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The SPD headroom is shared by all interfaces

SPD devides the queue to the route processor into a general packet queue and priority queue. The general packet queue is a global one. Both of these facts from "BGP design and implementation".

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This to me suggests the SPD queue comes AFTER the interface input hold queue.

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