Multi-tier switch interface unit arbiter

A method comprising:

• sending packets from one or more coherence control units to a switch interface unit (SIU);
• arbitrating among packets at a credit group level to select packets with available credits, wherein one or more credits are needed for forwarding a packet from a switch fabric on a given downstream channel;
• routing selected packets to corresponding packet-type arbiters;
• arbitrating among long packets at a first packet-type arbiter to select a long packet for forwarding to a final arbiter;
• arbitrating among short packets at a second packet-type arbiter to select a short packet for forwarding to the final arbiter;
• and
• arbitrating among the selected short packet and the selected long packet at the final arbiter to select a packet for forwarding to the switch fabric.

A method comprising:

• routing packets to a first level of arbiters based on a credit group and a sub-channel of the packets;
• arbitrating among the packets at the first level of arbiters to select packets to be forwarded to a second level of arbiters;
• arbitrating among the selected packets at the second level of arbiters to select a long packet and a short packet to be forwarded to a third level arbiter;
• and
• arbitrating among the selected long packet and the selected short packet at the third level arbiter to select a packet to be forwarded to a switch fabric, wherein the third level arbiter is configured to alternate between long and short packets.

An apparatus comprising:

• one or more coherence control units (CCUs);
• a switch interface unit (SIU) coupled to the one or more CCUs;
• and
• a switch fabric coupled to the SIU and the one or more CCUs;
• wherein the SIU is configured to:
• receive a plurality of packets from the one or more CCUs;
• arbitrate among the plurality of packets at a first level of arbiters to select packets to be forwarded to a second level of arbiters, wherein the first level of arbiters selects packets that are eligible to be sent out of the switch fabric on one or more downstream channels;
• arbitrate among the selected packets at the second level of arbiters, wherein the second level of arbiters are configured to select a long packet and a short packet to be forwarded to a third level arbiter;
• and
• arbitrate among the selected long packet and the selected short packet at the third level arbiter to select a final packet to be forwarded to the switch fabric.