Garnet1.0

Garnet Network Model

Note: This model of garnet is no longer supported in gem5. The updated model is garnet2.0.

Garnet is a detailed interconnection network model inside gem5. The details can be found in the Garnet ISPASS 2009 Paper: [1]. If your use of Garnet contributes to a published paper, please cite the following paper:

@inproceedings{garnet,
  title={GARNET: A detailed on-chip network model inside a full-system simulator},
  author={Agarwal, Niket and Krishna, Tushar and Peh, Li-Shiuan and Jha, Niraj K},
  booktitle={Performance Analysis of Systems and Software, 2009. ISPASS 2009. IEEE International Symposium on},
  pages={33--42},
  year={2009},
  organization={IEEE}
}

Garnet consists of 2 pipeline models: a detailed fixed-pipeline model, and an approximate flexible-pipeline model.

The fixed-pipeline model is intended for low-level interconnection network evaluations and models the detailed micro-architectural features of a 5-stage Virtual Channel router with credit-based flow-control. Researchers interested in investigating different network microarchitectures can readily modify the modeled microarchitecture and pipeline. Also, for system level evaluations that are not concerned with the detailed network characteristics, this model provides an accurate network model and should be used as the default model.

The flexible-pipeline model is intended to provide a reasonable abstraction of all interconnection network models, while allowing the router pipeline depth to be flexibly adjusted. A router pipeline might range from a single cycle to several cycles. For evaluations that wish to easily change the router pipeline depth, the flexible-pipeline model provides a neat abstraction that can be used.

• Configuration

Garnet uses the generic network parameters in Network.py. Additional parameters are specified in garnet/BaseGarnetNetwork.py:

• • ni_flit_size: flit size in bytes. Flits are the granularity at which information is sent from one router to the other. Default is 16 (=> 128 bits). [This default value of 16 results in control messages fitting within 1 flit, and data messages fitting within 5 flits]. Garnet requires the ni_flit_size to be the same as the bandwidth_factor (in network/BasicLink.py) as it does not model variable bandwidth within the network.
  • vcs_per_vnet: number of virtual channels (VC) per virtual network. Default is 4.

The following parameters in garnet/fixed-pipeline/GarnetNetwork_d.py are only valid for fixed-pipeline:

• • buffers_per_data_vc: number of flit-buffers per VC in the data message class. Since data messages occupy 5 flits, this value can lie between 1-5. Default is 4.
  • buffers_per_ctrl_vc: number of flit-buffers per VC in the control message class. Since control messages occupy 1 flit, and a VC can only hold one message at a time, this value has to be 1. Default is 1.

The following parameters in garnet/flexible-pipeline/GarnetNetwork.py are only valid for flexible-pipeline:

• • buffer_size: Size of buffers per VC. A value of 0 implies infinite buffering.
  • number_of_pipe_stages: number of pipeline stages in each router in the flexible-pipeline model. Default is 4.

• Additional features
  • Routing: Currently, garnet only models deterministic routing using the routing tables described earlier.
  • Modeling variable link bandwidth: The bandwidth_factor specifies the link bandwidth as the number of bytes per cycle per network link. ni_flit_size has to be same as this value. Links which have low bandwidth can be modeled by specifying a longer latency across them in the topology file (as explained earlier).
  • Multicast messages: The network modeled does not have hardware multi-cast support within the network. A multi-cast message gets broken into multiple uni-cast messages at the interface to the network.

• Garnet fixed-pipeline network

The garnet fixed-pipeline models a classic 5-stage Virtual Channel router. The 5-stages are:

1. Buffer Write (BW) + Route Compute (RC): The incoming flit gets buffered and computes its output port.
2. VC Allocation (VA): All buffered flits allocate for VCs at the next routers. [The allocation occurs in a separable manner: First, each input VC chooses one output VC, choosing input arbiters, and places a request for it. Then, each output VC breaks conflicts via output arbiters]. All arbiters in ordered virtual networks are queueing to maintain point-to-point ordering. All other arbiters are round-robin.
3. Switch Allocation (SA): All buffered flits try to reserve the switch ports for the next cycle. [The allocation occurs in a separable manner: First, each input chooses one input VC, using input arbiters, which places a switch request. Then, each output port breaks conflicts via output arbiters]. All arbiters in ordered virtual networks are queueing to maintain point-to-point ordering. All other arbiters are round-robin.
4. Switch Traversal (ST): Flits that won SA traverse the crossbar switch.
5. Link Traversal (LT): Flits from the crossbar traverse links to reach the next routers.

The flow-control implemented is credit-based.

• Garnet flexible-pipeline network

The garnet flexible-pipeline model should be used when one desires a router pipeline different than 5 stages (the 5 stages include the link traversal stage). All the components of a router (buffers, VC and switch allocators, switch etc) are modeled similar to the fixed-pipeline design, but the pipeline depth is not modeled, and comes as an input parameter number_of_pipe_stages. The flow-control is implemented by monitoring the availability of buffers at each output port before sending.