Generic Low Latency NoC Router Architecture for FPGA Computing

Generic Low Latency NoC Router Architecture for FPGA
Computing Systems
A Complete Network on Chip Emulation Framework
 Moore’s law is pushing towards more complex SoCs
 Network on Chip one of the technologies which enable us
to keep up with the law.
 Designing and evaluation of NoC has been a challenge
Complexity of design
 Inaccurate network/component models
 Simulation/CAD tools
 FPGA provides a platform where above concerns can be
addressed and real life data can be measured.
 Present on-chip FPGA resources can be used as
underlying interconnect fabric
NoC Fundamentals
Block diagram of a router.
NoC Fundamentals
Router Architecture
Simplified block diagram of a router. Flits arriving over the input
channels are stored in buffers associated with each input. A set of
allocators assigns buffers on the next node and channel bandwidth
to pending flits. When a flit has been allocated the resources it
needs, it is forwarded by the crossbar switch to an output channel.
Crossbar Example
A 4×5 crossbar switch as implemented with 5 4:1 multiplexers. Each multiplexer
selects the input to be connected to the corresponding output.
Crossbar Symbol
Data Packet Example
Packet format for our simple network. Time, in cycles, is shown in the
vertical direction, while the 18 signals of a channel are shown in the
horizontal direction. The leftmost signals contain the phit type, while the
16 remaining signals contain either a destination address or data, or are
unused in the case of a null phit.
Data Flow Control
Units of resource allocation. Messages are divided into packets for
allocation of control state. Each packet includes routing information
(RI) and a sequence number (SN). Packets are further divided into flits
for allocation of buffer capacity and channel bandwidth. Flits include no
routing or sequencing information beyond that carried in the packet,
but may include a virtual-channel identifier (VCID) to record the
assignment of packets to control state.
Data Flow Algorithms
 Deterministic: Algorithm always chooses the same
path between x and y even if multiple options exist
Simple to implement
 Poor job of balancing load
 Oblivious: Algorithm decides on a path between x
and y without any information on the network’s prior
state. Deterministic is a subset.
 Adaptive: Algorithm adapts to the state of the
network. Information includes node status, channel
load information, queue length etc.
Routing Mechanics
 Mechanism used to implement any routing
 Table based Routing: The set of paths for each pair of
nodes is stored in the table, and the table is indexed
by the source and destination node. Only that
portion of the table that is needed on a particular
node need be stored on that node.
 Algorithmic Routing: Routing relation is computed
using a network specific algorithm. More efficient in
terms of both area and speed.
Flow Control
 Determines how network resources such as channel
bandwidth, buffer capacity and control state are
allocated to the data traversing.
 Bufferless Flow Control
Simple to implement
 Packets are dropped or misrouted if resource is not available
 Buffered Flow Control
Temporarily store packets in a buffer therefore less probability of
dropped packets
 Additional cost of hardware for implementation
Network Topologies
Torus and mesh networks: (a) a torus network (4-ary 2-cube)
includes the connection from node 3 to node 0 in both
dimensions, but (b) a mesh network (4-ary 2-mesh) omits this
FPGA Emulation Framework
 Emulation environment developed to explore,
evaluate and compare a variety of NoC solutions.
Current FPGA implementations are limited in
flexibility and do not allow full test of NoC
Cycle accurate simulations using a combination of
hardware and software modules.
Added flexibility due to modular approach of
16000 times faster than HDL simulator
NoC Emulation Architecture
NoC Emulation Architecture
 Xilinx Virtex 2 Pro V20 with an embedded power PC:
Processor is povides the much needed flexibility for
the emulation proces.
 Monitor Module: Responsible for the interface
between the host PC and FPGA board. Also streams
out data generated from various tests.
 Programmable NoC Platform: Responsible for traffic
generators and receptors. Also keeps the user
defined interconnection set between switches and
Data Flow
Above Traffic Generator (TG) can provide an image of the congestion of
the network at each moment in time of the emulation.
Each time a flit is not acknowledged by its receptor (i.e. switch or TR) and
has to be resent, a readable counter by the processor is incremented.
NoC Emulation Flow
NoC Emulation Flow
 No re synthesizing or re mapping of system due to
HW-SW structure
 Processor is able to initialize parameters in hardware
 Emulation flow is categorized as:
Stochastic Emulation Flow: Implemented at the hardware level
only. Configuration is implemented at the compilation level
 Trace-based Emulation Flow: Entire NoC trace is loaded via
software located on RAM. Processor streams the data into the
emulated NoC and collects information on latency and congestion.
Results – Run Time
The total delivery time with the same amount of packets for the burst-mode is
higher than for the uniform traffic. This is because the probability of collisions
between packets in the burst-mode is significantly higher.
Results – Congestion Rate
Plots indicate that the congestion rate does not increase linearly with the
number of delivered packets in a burst mode.
Results – Avg. Latency
The average latency of packets reaches a limit of congestion, which is the limit of
the NoC in terms of latency
Generic Low Latency Router - Motivation
 New Generation of FPGAs comprise of millions of
LUTs and will contain many parallel soft processor
cores and glue/extra logic.
 Use of traditional interconnect schemes will lead to
under utilization.
 Future designs are perceived to be at a higher level
than traditional gate level. Functionality will be
implemented through programmability of such
 Increased complexity of FPGA will lead to inefficient
RTL based design flow.
Proposed Solution
 Network on Chip can provide a flexible, scalable and
reliable communication solution for such large and
complex solutions.
 NoC provides the ability to change bandwidth and
add processing elements. Cost is linear in this case
whereas, traditional cross bar interconnects scale
 FPGA contains significant global and local routing
resources which can be used to construct the
interconnect fabric and implement routing
Prior Work
 Many routers have been designed for NoC FPGA
Circuit switch router: Head flit charts out the path, body follows. It
has long circuit setup latency and low bandwidth utilization but
once path is setup, Q0S is guaranteed and data latency is less.
 Time multiplexed router: Precomputed communication pattern.
Less flexible. Works well when communication loads are 100% but
performance drops significantly when load < 40%
 Packet switch router: Negotiate network resources dynamically at
run time. Flexible and scalable and low resource utilization but
have high latency (about 8 clock cycles per hop).
 FPGAs primarily used for prototyping and evaluating
latency, throughput, cost and power.
Generic Low Latency NoC Router - Overview
 Reconfigurable wormhole router for packet-switched
NoC designs
Low routing latency, low complexity and high buffer
Designed to be scalable, flexible and reliable for a
variety of FPGA platforms and network
1-D ring, 2-D mesh and 3-D cube network topologies
were used to measure the feasibility of design and
implementation on the FPGA.
2 Cycles per hop latency
Wormhole Router Block Diagram
 Three main components: flow
control, components and
pipeline control
 Wormhole flow control
 Components include input
and output ports, arbiter to
arbitrate between multiple
requests and FSM to maintain
state of output port
 Pipeline control is
instrumental in achieving low
latency per hop and parallel
Wormhole Flow Control
(a) the header arrives at the node, while the
virtual channel is in the idle state (I) and the
desired upper (U) output channel is busy —
allocated to the lower (L) input.
(b) the header is buffered and the virtual
channel is in the waiting state (W), while
the first body flit arrives.
Wormhole Flow Control
(c) the header and first body flit are buffered,
while the virtual channel is still in the waiting
state. In this state, the input channel is blocked.
The second body flit cannot be transmitted, since
it cannot acquire a flit buffer.
(d) the output virtual channel becomes available
and allocated to this packet. The state moves to
active (A) and the head is transmitted to the
next node.
Wormhole Flow Control
(e) The body flits follow
(f) The body flits follow
Wormhole Flow Control
(g) the tail flit is transmitted and frees the
virtual channel, returning it to the idle state.
(h) a time-space diagram showing this
Packet Format
 Packet length is unfixed
 Format is defined by
network topology
 Output Channel (OC)
field stores the output
channel used by packet
Input Port
 Single entry flit buffer
uses dual port memory
 Dimensional Ordering
 Routing computation is
decoupled from
 Head & tail pointers are
used to evaluate whether
a flit is present in the
Output FSM
 Maintains state of
output port
 Active state indicates
that an output port has
been matched with a
downstream input port
 Tail flit departure puts
router in wait state
 Once all flits leave
downstream routers, the
router goes in idle mode.
Router Pipeline Organization
a. Clock 1: Destination address and output channel latched in. Flit is written in
flit buffer. During this period, arbitration result and look ahead routing is
b. Clock 2: Crossbar control signal latched in and flit is read from the granted
Timing of Second Pipeline Stage
T = Tco + Tlut + Trot + Tsu
Tco is clock to register (or
memory) output delay
• Tlut is the delay of LUT cells
(multiplexer logic)
• Trot is the delay due to
programmable wire routing
• Tsu isthe setup time of the
Pipeline Diagram ASIC vs FPGA Router
Network Topologies Used
Credit Based Routing
 Upstream router keeps
count of free buffers of
downstream router.
 The credit count is
decremented every time a
buffer is consumed.
 If count is zero, all
downstream buffers are
 Data is transferred only if
credit count > 0
Results – Resource Utilization
Logic cost vs. Router radix
(32 bits data-path width)
Results – Resource Utilization
logic cost vs. Data-path width
Results - Timing
(a) Maximum clock rate vs. Router radix
(b) two important critical paths within a router
Results - Power
(a) Static and dynamic power (mw) vs. Router radix
(b) Normalized per-packet power consumption vs. radix
Packet Generator and Receiver
Due to pin limitations, packets must be generated using on chip logic
within the FPGA rather than external sources. Each node of the NoC
system is attached with a packet generator and receiver.
Results – Resource Utilization per Configuration
Resource utilization of different network configurations
Results – Resource Utilization per Configuration
 Highly scalable router which is easily used among
different network topologies.
 Low hop by hop propagation delay using a packet
switch NoC router.
 Analysis of router in terms of scalability, hardware
cost, operation speed and power dissipation.
 Real world feasibility of such router architecture has
been demonstrated and its usage within FPGA
platform provides a very robust and cost effective
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