Transcript Using Packet Information for Efficient Communication in NoCs

Using Packet Information for
Efficient Communication in
NoCs
Prasanna Venkatesh R, Madhu Mutyam
PACE Lab
IIT Madras
21-07-2015
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Agenda
• Motivation
• Existing techniques to handle multicasts at NoC
• Dynamic Multicast Tree
• VC as Cache
• Packet Concatenation
• IPC Results
• Energy Analysis
• Conclusion
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Motivation
Max Sharers Per Multicast
Mean Sharers Per Multicast
30
28
Num Sharers
25
20
20
15
20
18
16.71
11
11
9
10
5
2.2
2.6
2
2.5
Fmm
Ocean
2.1
3.4
1.9
2.39
0
Barnes
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Cholesky
Radiosity
Radix
Swaptions
SPLASH and PARSEC benchmarks have upto 87% of nodes participating in a
multicast. But the average is 7.5% only.
Mean
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Motivation
% Sharers
% Multicasts Above 0.5 * Max Sharers
9
8
7
6
5
4
3
2
1
0
Barnes
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Cholesky
Fmm
Ocean
Radiosity
Radix
Swaptions
Mean
SPLASH and PARSEC benchmarks have upto 87% of nodes participating in a
multicast. But the maximum communication exists for < 4% of the time.
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Multicasts: Solutions in the literature
• Separate injections flood the
network with redundant copies
• Multicasts: Single copy till a
common path and forks to
multiple copies
• Simplifies routing logic
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Dynamic Multicast routing can make use of idle paths to avoid congestion.
But is it possible to meet timing constraints?
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Our Proposals to achieve multicast efficiency
• Dynamic multicast tree construction using redundant route
computation units
• Will penalize unicasts and create starvation?
• Three optimizations on unicasts to enhance dynamic multicasting
• VC as cache
• Packet Concatenation
• Critical word first
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Critical Word First
• Borrowed from Cache data transfer optimization technique
• Make efficient use of the flit level split of a packet containing cache
block
• Send the requested word with the header flit
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Dynamic Multicast Tree
Method
• Compute Odd-Even route at each router for all multicast destinations
• Takes one RC cycle per destination
• Add a redundant RC unit to speed this process
• No extra chip area because of the simplicity
Caveats
• Bottlenecks unicasts
• Slow when there is no congestion
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VC as Cache: Scenario
• A shared cache block is requested by more than one node at a given
time frame
• The owner sends a multicast of the block to all the requestors
• A request arrives after this multicast
• The owner resends the block after processing this request
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Solution – Add the new requestor to the
processed multicast midway!
• Compare up to five multicast
packets with an incoming request
packet at the router
• If matched,
• Forward the request to the owner
for coherence and book keeping
with a time stamp of the previous
message
• Add this requestor to the multicast
destinations
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Packet Concatenation
• A request is a single flit packet
• When RC units are busy, we can
club single flit packets to the same
destination to form a “superpacket”
• This means it is going to take one
RC cycle to compute multiple
packet routes from there on.
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Configuration for simulations
• Simulators Multi2sim 4.0.1, Booksim 2.0, Orion 2.0
• Real time simulation
• 64 Nodes with 32 cores + L1 nodes and 32 shared distributed L2
cache banks
• 1 Flit for request and coherence packets, 5 flits for cache block
• Benchmarks:
• SPLASH2 and PARSEC workloads with 32 threads
• All high injection workloads are picked after an initial study on their injection
rates
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IPC Results
% Improvement over Odd Even
(Base)
Radix
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90
80
70
60
50
40
30
20
10
0
Abbreviations:
C – Critical Word first
V – VC as cache
D – Dynamic Multicast Tree
P – Packet Concatenation
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IPC Results
% Improvement over Odd Even
(Base)
Geomean
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15
10
Abbreviations:
C – Critical Word first
V – VC as cache
D – Dynamic Multicast Tree
P – Packet Concatenation
5
0
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Scaling to 512 Nodes: IPC Results
% Improvement over Odd Even
(Base)
MultiPrograms
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8
6
4
Abbreviations:
C – Critical Word first
V – VC as cache
D – Dynamic Multicast Tree
P – Packet Concatenation
2
0
15
% Energy Improvement over
Base
Fine Grained Energy Footprint of Barnes
25
20
Abbreviations:
C – Critical Word first
V – VC as cache
D – Dynamic Multicast Tree
P – Packet Concatenation
15
10
5
0
Clock Cycles
Base+D
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Base+CD
Base+CVD
Base+CVDP
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Conclusion and future extensions
• Scalable solution for multicasts
• Can fit with existing techniques
• Easy to implement
• Energy Efficient
• Packet Concatenation can be switched on selectively depending on
the load requirements
• Other architecture level inputs can also be used for further
performance.
• Example: #Instructions waiting, memory level parallelism
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Thank you
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