System and method for opportunistic request-grant switching
View Patent ↗A method and system are provided for opportunistic request-grant switching. If an ingress has no granted payload segment to send, and a flow exists which requires a request to be sent, an opportunistic payload segment is sent including a request and a payload segment related to the request. If an opportunistic payload segment is sent and the payload is dropped, the request is kept and is then treated as a regular request-grant request. The ingress port consequently only has to transmit the payload a maximum of twice. Ingress ports can thus opportunistically exploit the low latency available when egress ports are not contended for, and yet fall back on the strong fairness and quality of service (QoS) assurances of request-grant semantics. Buffering in the switch core can optionally be implemented to extend performance gains, but fairness and QoS are not dependent on this buffering.
1. A method of opportunistic request-grant switching in a switch having ingress ports, a switch core, and egress ports, the method comprising:
receiving a bid at the switch core front an origin ingress port, the bid having a request and its associated payload;
forwarding the associated payload to a destination egress port as a cut-through in response to a determination of availability of the destination egress port;
sending an accept message to the origin ingress port;
dropping the associated payload in response to a determination of unavailability of the destination egress port; and
retaining the request at the switch core for treatment as a request-grant request.
2. The method of claim 1 , wherein the step of dropping the associated payload includes notifying the origin ingress port of the drop.
3. The method of claim 1 further comprising:
storing the associated payload in a free buffer in response to a determination of availability of the free buffer and in response to a determination of unavailability of the destination egress port.
4. The method of claim 3 , further comprising:
dropping the associated payload in response to a determination of unavailability of the free buffer at the switch core.
5. The method of claim 4 , further comprising:
incrementing a flow count for a flow associate with the request.
6. The method of claim 1 , wherein the treatment as the request-grant request includes:
receiving a payload segment associated with an earlier granted request from the origin ingress port upon granting of the request-grant request; and
forwarding the payload segment associated with an earlier granted request from the switch core to the destination egress port.
7. An ingress device for communicating with a switch core in a data switch, comprising:
determining means for determining that a bid to be sent is to include a request and a its associated payload;
send means for sending the bid to the switch core; and
payload management means for holding a copy of the associated payload for retransmission pending receipt of a grant message.
8. The ingress device of claim 7 wherein the determining means includes means for determining that the bid to be sent is to include the request and its associated payload in response to a determination in an immediately preceding time cycle that no grant has been received which requires sending a payload segment associated with an earlier granted request.
9. The ingress device of claim 7 wherein the payload management means includes means for discarding the copy of the associated payload in response to receipt of an accept message from the switch core.
10. The ingress device of claim 7 wherein the payload management means includes means for sending the copy of the associated payload to the switch core with a sequence number including a unique identifier to establish correspondence between the associated payload and the grant message in response to receipt of the grant message.
11. The ingress device of claim 7 , further comprising a message type identifying means, for identifying a message type associated with a received message.
12. A switch core for opportunistic request-grant switching, the switch core being in communication with ingress ports and egress ports, the switch core comprising:
a receiver for receiving a bid at the switch core from an origin ingress port, the bid having a request and its associated payload;
transmit means for forwarding the associated payload to a destination egress port as a cut-through in response to a determination of availability of the destination egress port and for sending an accept message to the origin ingress port;
payload management means for dropping the associated payload in response to a determination of unavailability of the destination egress port; and
storage means for retaining the request at the switch core for treatment as a request-grant request.
13. The switch core of claim 12 , wherein the payload management means includes means for dropping the associated payload in response to a determination of unavailability of a free buffer at the switch core.
14. The switch core of claim 12 , wherein the payload management means includes notification means for notifying the origin ingress port of the drop.
15. The switch core of claim 12 further comprising:
a buffer for retaining the associated payload; and
a buffer availability indicator for indicating an availability of the buffer.
16. The switch core of claim 15 further comprising:
a request counter for a flow count for a flow associated with the request.
17. The switch core of claim 12 , further comprises:
means for receiving a payload segment associated with an earlier granted request from the origin ingress port upon granting of the request-grant request; and
means for forwarding the payload segment associated with an earlier granted request from the switch core to the destination egress port.
18. A computer-readable medium including statements and instructions which, when executed, cause a computer to perform the steps of:
receiving a bid at the switch core from an origin ingress port, the bid having a request and its associated payload;
forwarding the associated payload to a destination egress port as a cut-through in response to a determination of availability of the destination egress port;
sending an accept message to the origin ingress port;
dropping the associated payload in response to a determination of unavailability of the destination egress port; and
retaining the request at the switch core for treatment as a request-grant request.