Interconnection Network

The various components of the interconnection network model inside gem5's ruby memory system are described here.

How to invoke the network

Simple Network:

./build/ALPHA/gem5.debug \
                  configs/example/ruby_random_test.py \
                  --num-cpus=16  \
                  --num-dirs=16  \
                  --network=simple
                  --topology=Mesh_XY  \
                  --mesh-rows=4 

The default network is simple, and the default topology is crossbar.

Garnet network:

./build/ALPHA/gem5.debug \
                      configs/example/ruby_random_test.py  \
                      --num-cpus=16 \
                      --num-dirs=16  \
                      --network=garnet2.0
                      --topology=Mesh_XY \
                      --mesh-rows=4  

Topology

The connection between the various controllers are specified via python files. All external links (between the controllers and routers) are bi-directional. All internal links (between routers) are uni-directional -- this allows a per-direction weight on each link to bias routing decisions.

• Related Files:
  • src/mem/ruby/network/topologies/Crossbar.py
  • src/mem/ruby/network/topologies/CrossbarGarnet.py
  • src/mem/ruby/network/topologies/Mesh_XY.py
  • src/mem/ruby/network/topologies/Mesh_westfirst.py
  • src/mem/ruby/network/topologies/MeshDirCorners_XY.py
  • src/mem/ruby/network/topologies/Pt2Pt.py
  • src/mem/ruby/network/Network.py
  • src/mem/ruby/network/BasicLink.py
  • src/mem/ruby/network/BasicRouter.py

• Topology Descriptions:
  • Crossbar: Each controller (L1/L2/Directory) is connected to a simple switch. Each switch is connected to a central switch (modeling the crossbar). This can be invoked from command line by --topology=Crossbar.
  • CrossbarGarnet: Each controller (L1/L2/Directory) is connected to every other controller via one garnet router (which internally models the crossbar and allocator). This can be invoked from command line by --topology=CrossbarGarnet.
  • Mesh_*: This topology requires the number of directories to be equal to the number of cpus. The number of routers/switches is equal to the number of cpus in the system. Each router/switch is connected to one L1, one L2 (if present), and one Directory. The number of rows in the mesh has to be specified by --mesh-rows. This parameter enables the creation of non-symmetrical meshes too.
    • Mesh_XY: Mesh with XY routing. All x-directional links are biased with a weight of 1, while all y-directional links are biased with a weight of 2. This forces all messages to use X-links first, before using Y-links. It can be invoked from command line by --topology=Mesh_XY
    • Mesh_westfirst: Mesh with west-first routing. All west-directional links are biased with a weight of 1, al other links are biased with a weight of 2. This forces all messages to use west-directional links first, before using other links. It can be invoked from command line by --topology=Mesh_westfirst
  • MeshDirCorners_XY: This topology requires the number of directories to be equal to 4. number of routers/switches is equal to the number of cpus in the system. Each router/switch is connected to one L1, one L2 (if present). Each corner router/switch is connected to one Directory. It can be invoked from command line by --topology=MeshDirCorners_XY. The number of rows in the mesh has to be specified by --mesh-rows. The XY routing algorithm is used.
  • Pt2Pt: Each controller (L1/L2/Directory) is connected to every other controller via a direct link. This can be invoked from command line by --topology=Pt2Pt.

In each topology, each link and each router can independently be passed a parameter that overrides the defaults ( in BasicLink.py and BasicRouter.py) :

• Link Parameters:
  • latency: latency of traversal within the link.
  • weight: weight associated with this link. This parameter is used by the routing table while deciding routes, as explained next in Routing.
  • bandwidth_factor: Only used by simple network to specify width of the link in bytes. This translates to a bandwidth multiplier (simple/SimpleLink.cc) and the individual link bandwidth becomes bandwidth multiplier x endpoint_bandwidth (specified in SimpleNetwork.py). In garnet, the bandwidth is specified by ni_flit_size in GarnetNetwork.py)

• Router Parameters:
  • latency: latency of each router. Only supported by garnet2.0.

Routing

Based on the topology, shortest path graph traversals are used to populate routing tables at each router/switch. The default routing algorithm tries to choose the route with minimum number of link traversals. Links can be given weights in the topology files to model different routing algorithms. For example, in Mesh_XY.py and MeshDirCorners_XY.py Y-direction links are given weights of 2, while X-direction links are given weights of 1, resulting in XY traversals. In Mesh_westfirst.py, the west-links are given weights of 1, and all other links are given weights of 2. In garnet2.0, the routing algorithm randomly chooses between links with equal weights. In simple network, it statically chooses between links with equal weights.

Flow-Control and Router Microarchitecture

Ruby supports two network models, Simple and Garnet, which trade-off detailed modeling versus simulation speed respectively.

Simple Network

Details of the Simple Network are here.

Garnet

Details of the original (2009) Garnet network are here. This design is no longer supported in the codebase.

Garnet2.0

Details of the new (2016) Garnet2.0 network are here.

Running the Network with Synthetic Traffic

The interconnection networks can be run in a standalone manner and fed with synthetic traffic. We recommend doing this with garnet2.0.

Running Garnet Standalone