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FPGA Applications
IEEE Micro Special Issue on
Reconfigurable Computing
Vol. 34, No.1, Jan.-Feb. 2014
Dr. Philip Brisk
Department of Computer Science and Engineering
University of California, Riverside
CS 223
Guest Editors
Walid Najjar
UCR
Paolo Ienne
EPFL, Lausanne, Switzerland
2
High-Speed Packet Processing
Using Reconfigurable Computing
Gordon Brebner and Weirong Jiang
Xilinx, Inc.
Contributions
• PX: a domain-specific language for packet-processing
• PX-to-FPGA compiler
• Evaluation of PX-designed high-performance reconfigurable
computing architectures
• Dynamic reprogramming of systems during live packet
processing
• Demonstrated implementations running at 100 Gbps and
higher rates
PX Overview
• Object-oriented semantics
– Packet processing described as component objects
– Communication between objects
• Engine
– Core packet processing functions
• Parsing, editing, lookup, encryption, pattern matching, etc.
• System
– Collection of communicating engines and/or sub
• Parsing, editing, lookup, encryption, pattern matching, etc.
Interface Objects
• Packet
– Communication of packets between components
• Tuple
– Communication of non-packet data between
components
OpenFlow Packet Classification in PX
Send packet to
parser engine
OpenFlow Packet Classification in PX
Parser engine
extracts a
tuple from the
packet
Send the tuple
to the lookup
engine for
classification
OpenFlow Packet Classification in PX
Obtain the
classification
response from
the lookup
engine
Forward the
response to the
flowstream
output interface
OpenFlow Packet Classification in PX
Forward the
packet (without
modification) to
the outstream
output interface
PX Compilation Flow
100 Gbps
10 Gbps
:
:
512-bit datapath
64-bit datapath
Faster to reconfigure the generated
architecture than the FPGA itself
(not always applicable)
OpenFlow Packet Parser (4 Stages)
Allowable Packet Structures:
(Ethernet, VLAN, IP, TCP/UDP)
(Ethernet, IP, TCP/UDP)
Stage 1:
Stage 2:
Stage 3:
Stage 4:
Ethernet
VLAN or IP
IP or TCP/UDP
TCP/UDP or bypass
OpenFlow Packet Parser
Max. packet size
Structure
of the tuple
Set relevant
members in
the tuple
Being
populated
Determine
the type of
the next
section of
the packet
Max. number of stacked sections
Ethernet
header
expected first
I/O interface
Determine
how far to go
in the packet
to reach the
next section
OpenFlow Packet Parser
Three-stage Parser Pipeline
• Internal units are customized based on PX requirements
• Units are firmware-controlled
– Specific actions can be altered (within reason) without
reconfiguring the FPGA
– e.g., add or remove section classes handled at that stage
OpenFlow Packet Parser Results
Adjust throughput for wasted bits
at the end of packets
Ternary Content
Addressable Memory (TCAM)
X = Don’t Care
TCAM width and depth are configurable in PX
http://thenetworksherpa.com/wp-content/uploads/2012/07/TCAM_2.png
TCAM Implementation in PX
depth
key length
The parser
(previous
example)
extracted
the tuple
TCAM architecture is generated
automatically as described by one
of the authors’ previous papers
result bitwidth
Set up TCAM
access
Collect the
result
TCAM Architecture
TCAM Parameterization
• PX Description
– Depth (N)
– Width
– Result width
• Operational Properties
– Number of rows (R)
– Units per row (L)
– Internal pipeline stages per unit (H)
• Performance
– Each unit handles N/(LR) TCAM units
– Lookup latency is LH + 2 clock cycles
• LH to process the row
• 1 cycle for priority encoding
• 1 cycle for registered output
Results
Database Analytics:
A Reconfigurable Computing Approach
Bharat Sukhwani, Hong Min, Mathew Thoennes,
Parijat Dube, Bernard Brezzo, Sameh Asaad,
and Donna Eng. Dillenberger
IBM T.J. Watson Research Center
Example: SQL Query
Online Transaction Processing (OLTP)
• Rows are compressed for storage and I/O savings
• Rows are decompressed when issuing queries
• Data pages are cached in a dedicated memory space
called the buffer pool
• I/O operations between buffer pool and disk are
transparent
• Data in the buffer pool is always up-to-date
Table Traversal
• Indexing
– Efficient for locating a small number of records
• Scanning
– Sift through the whole table
– Used when a large number of records match the
search criteria
FPGA-based Analytics Accelerator
Workflow
• DBMS issues a command to the FPGA
– Query specification and pointers to data
• FPGA
– Pulls pages from main memory
– Parses pages to extract rows
– Queries the rows
– Writes qualifying queries back to main memory in
database-formatted pages
FPGA Query Processing
• Join and sort operations are not streaming
– Data re-use is required
– FPGA block RAM storage is limited
– Perform predicate evaluation and projection
before join and sort
• Eliminate disqualified rows
• Eliminate unneeded columns
Where is the Parallelism?
• Multiple tiles process DB pages in parallel
– Concurrently evaluate multiple records from a page
within a tile
• Concurrently evaluate multiple predicates against different
columns within a row
Predicate Evaluation
Stored predicate values
Logical Operations
(Depends on query)
#PEs and reduction network size are configurable at synthesis time
Two-Phase Hash-Join
• Stream the smaller join table
through the FPGA
• Hash the join columns to populate a
bit vector
• Store the full rows in off-chip DRAM
• Join columns and row addresses are
stored in the address table (BRAM)
• Rows that hash to the same position
are chained in the address table
• Stream the second
table through the FPGA
• Hash rows to probe the
bit vector (eliminate
non-matching rows)
• Matches issue reads
from off-chip DRAM
• Reduces off-chip
accesses and stalls
Database Sort
• Support long sort keys (tens of bytes)
• Handle large payloads (rows)
• Generate large sorted batches (millions of records)
• Coloring bins keys into
sorted batches
https://en.wikipedia.org/wiki/Tournament_sort
CPU Savings
Predicate Eval.
Decompression
+ Predicate Eval.
Throughput and FPGA Speedup
Scaling Reverse Time Migration
Performance Through
Reconfigurable Dataflow Engines
Haohan Fu1, Lin Gan1, Robert G Clapp2, Huabin Ruan1,
Oliver Pell3, Oskar Mencer3, Michael Flynn2,
Xiaomeng Huang1, and Guangwen Yang1
1Tsinghua University
2Stanford University
3Maxeler
Technologies
Migration (Geology)
https://upload.wikimedia.org/wikipedia/commons/3/38/GraphicalMigration.jpg
Reverse Time Migration (RTM)
• Imaging algorithm
• Used for oil and gas exploration
• Computationally demanding
RTM Pseudocode
Iterations over shots (sources) are
independent and easy to parallelize
Add the recorded
source signal to
the corresponding
location
Iterate over time-steps, and 3D grids
Propagate source wave
fields from time 0 to
nt - 1
Boundary
conditions
Iterate over time-steps, and 3D grids
Add the recorded
receiver signal to
the corresponding
location
Boundary
conditions
Propagate receiver
wave fields from time
nt - 1 to 0
Cross-correlate the
source and receiver wave
field at the same time
step to accumulate the
result
RTM Computational Challenges
• Cross-correlate source and receiver signals
– Source/receiver wave signals are computed in different
directions in time
– The size of a source wave field for one time-step can be 0.5
to 4 GB
– Checkpointing: store source wave field and certain time
steps and recompute the remaining steps when needed
• Memory access pattern
– Neighboring points may be distant in the memory space
– High cache miss rate (when the domain is large)
Hardware
General Architecture
Java-like HDL / MaxCompiler
Stencil Example
Data type: no reason that all floatingpoint data must be 32- or 64-bit IEEE
compliant (float/double)
Automated construction of a
window buffer that covers
different points needed by the
stencile
Performance Tuning
• Optimization strategies
– Algorithmic requirements
– Hardware resource limits
• Balance resource utilization so that none
becomes a bottleneck
–
–
–
–
LUTs
DSP Blocks
block RAMs
I/O bandwidth
Algorithm Optimization
• Goal:
– Avoid data transfer required to checkpoint source
wave fields
• Strategies:
– Add randomness to the boundary region
– Make computation of source wave fields
reversible
Custom BRAM Buffers
37 pt. Star Stencil
on a MAX3 DFE
• 24 concurrent pipelines
at 125 MHz
• Concurrent access to 37
points per cycle
• Internal memory
bandwidth of 426
Gbytes/sec
More Parallelism
• Process multiple points concurrently
– Demands more I/O
• Cascade multiple time steps in a deep pipeline
– Demands more buffers
Number Representation
• 32-bit floating-point was default
• Convert many variables to 24-bit fixed-point
– Smaller pipelines => MORE pipelines
Floating-point
- 16,943 LUTs
- 23,735 flip-flops
- 24 DSP48Es
Fixed-point
- 3,385 LUTs
- 3,718 flip-flops
- 12 DSP48Es
Hardware Decompression
• I/O is a bottleneck
– Compress data off-chip
– Decompress on the fly
– Higher I/O bandwidth
• Wave field data
– Must be read and written many times
– Lossy compression acceptable
• 16-bit storage of 32-bit data
• Velocity data and read-only Earth model parameters
– Store values in a ROM
– Create a table of indices into the ROM
• Decompression requires ~1300 LUTs and ~1200 flip-flops
Results
Performance Model
• Memory bandwidth constraint
# points processed
in parallel
# bytes
per point
compression
ratio
frequency
memory
bandwidth
• Resource constraint (details omitted)
Performance Model
• Cost (cycles consumed on redundant
streaming of overlapping halos)
• Model
# points processed
in parallel
# time steps cascaded
in one pass
frequency
Model Evaluation
Fast, Flexible High-Level Synthesis
from OpenCL Using
Reconfiguration Contexts
James Coole and Greg Stitt
University of Florida
Compiler Flow
• Intermediate Fabric
– Coarse-grained network of
arithmetic processing units
synthesized on the FPGA
– 1000x faster place-androute than an FPGA directly
– 72-cycle maximum
reconfiguration time
Intermediate Fabric
The intermediate
fabric can
implement each
kernel by
reconfiguring the
fabric routing
network
The core arithmetic
operators are often
shared across
kernels
Multiple datapaths per kernel
• Reconfigure the FPGA to
swap datapaths
Reconfiguration Contexts
• One intermediate
fabric may not be
enough
• Generate an
application-specific
set of intermediate
fabrics
• Reconfigure the FPGA
to switch between
intermediate fabrics
Context Design Heuristic
• Maximize number of
resources reused across
kernels in a context
• Minimize area of individual
contexts
• Use area savings to scale-up
contexts to support kernels
that were not known at
context design-time
• Kernels grouped using Kmeans clustering
Compiler Results
Configuration Bitstream Sizes and
Recompilation Times
ReconOS:
An Operating Systems Approach for
Reconfigurable Computing
Andreas Agne1, Markus Happe2, Ariane Keller2,
Enno Lübbers3, Bernhard Plattner2, Marco Platzner1,
and Christian Plessl1
1University of Paderborn, Germany
2ETH Zürich, Switzerland
3Intel Labs, Europe
Does Reconfigurable Computing Need
an Operating System?
• Application partitioning
– Sequential:
– Parallel/deeply pipelined:
Software/CPU
Hardware/FPGA
• Partitioning requires
– Communication and synchronization
• The OS provides
– Standardization and portability
• The alternative is
–
–
–
–
System- and application-specific services
Error-prone
Limited portability
Reduced designer productivity
ReconOS Benefits
• Application development is structured and starts with software
• Hardware acceleration is achieved by design space exploration
• OS-defined synchronization and communication mechanisms
provide portability
• Hardware and software threads are the same from the application
development perspective
• Implicit support for partial dynamic reconfiguration of the FPGA
Programming Model
• Application partitioned into threads (HW/SW)
• Threads communicate and synchronize using
one of the programming model’s objects
– Communication: Message queues, mailboxes, etc.
– Synchronization: Mutexes
Stream Processing Software
Stream Processing Hardware
OSFSM in VHDL
• VHDL library wraps all OS calls with VHDL procedures
– Transitions are guarded by an OS-controlled signal
• done, Line 47
– Blocking OS calls can pause execution of a HW thread
• e.g., mutex_lock(),
ReconOS System Architecture
Delegate Thread
• Interface between HW
thread and the OS via OSIF
• The OS is oblivious to HW
acceleration
• Imposes non-negligible
overhead on OS calls
OSFSM / OSIF / CPU Interface
• Handshaking provides synchronization
• OS requests are a sequence of words
communicated via FIFOs
HW Thread Interfaces
ReconOS Toolflow
Example: Object Tracking