Interconnection Networks: Deadlock and Livelock Feb. 26 th, 2007 Prof. Chung-Kuan Cheng Transcribed by: Mohammad Al-Fares

Definitions Deadlock: A group of agents (packets) are unable to make progress because they’re waiting on one another to release resources (buffers, channels, etc.) Livelock: Packets continue to move through the network, but without making progress towards their destination

Deadlock Cycle of deadlock: 1. Construct edges hold(a, b): Agent a holds resource b 2. Construct edges waitfor(a, b): Agent a waits for resource b 3. Reverse waitfor edges 4. Find Cycles of the graph: The cycles cause deadlock

Example

Example: 1D Torus

Example: 1D Mesh

Example: 2D Mesh

Deadlock Avoidance 1. Restricted Physical Route 1. Dimension Order (K-ary n-mesh) i.e. x → y → z +x packet waitfor +x, -y, +y, -z, +z -x packet waitfor -x, -y, +y, -z, +z +y packet waitfor +y, -z, +z -y packet waitfor -y, -z, +z +z packet waitfor +z -z packet waitfor -z

Deadlock Avoidance 1. Restricted Physical Route 1. Dimension Order (K-ary n-mesh)

Deadlock Avoidance 2. Turn Model Routing

Deadlock Avoidance Eliminate +y -x turns: 1) Eliminate -y -x turns: west-first route 2) Eliminate +y +x turns: north-last route 3) Eliminate +x -y turns: negative-first route 4) Eliminate -y -x turns: possible cycle, not allowed

Deadlock Avoidance For restricted physical routes, we can enforce the rule by numbering the channels, and we go only from a channel to a channel with a higher number Reduces path diversity → (performance, fault- tolerance)