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Dr. Multicast for Data Center Communication Scalability
Ymir Vigfusson Hussam Abu-Libdeh Mahesh Balakrishnan Ken BirmanCornell University
Yoav TockIBM Research Haifa
LADIS, September 15, 2008
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IP Multicast in Data Centers• IPMC is not used in data centers
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IP Multicast in Data Centers• Why is IP multicast rarely used?
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IP Multicast in Data Centers• Why is IP multicast rarely used?
o Limited IPMC scalability on switches/routers and NICs
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IP Multicast in Data Centers• Why is IP multicast rarely used?
o Limited IPMC scalability on switches/routers and NICs
o Broadcast storms: Loss triggers a horde of NACKs, which triggers more loss, etc.
o Disruptive even to non-IPMC applications.
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IP Multicast in Data Centers• IP multicast has a bad reputation
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IP Multicast in Data Centers• IP multicast has a bad reputation
o Works great up to a point, after which it breaks catastrophically
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IP Multicast in Data Centers• Bottom line:
o Administrators have no control over multicast use ...
o Without control, they opt for never.
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Dr. Multicast
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Dr. Multicast (MCMD)
• Policy: Permits data center operators to selectively enable and control IPMC
• Transparency: Standard IPMC interface, system
calls are overloaded. • Performance: Uses IPMC when possible,
otherwise point-to-point UDP • Robustness: Distributed, fault-tolerant service
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Terminology
• Process: Application that joins logical IPMC groups
• Logical IPMC group: A virtualized abstraction• Physical IPMC group: As usual• UDP multi-send: New kernel-level system-call
• Collection: Set of logical IPMC groups with
identical membership
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Acceptable Use Policy
• Assume a higher-level network management tool compiles policy into primitives
• Explicitly allow a process to use IPMC groupso allow-join(process,logical IPMC)o allow-send(process,logical IPMC)
• UDP multi-send always permitted • Additional restraints
o max-groups(process,limit)o force-udp(process,logical IPMC)
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Overview
• Library module• Mapping module• Gossip layer
• Optimization
questions • Results
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• Transparent. Overloads the IPMC functions o setsockopt(), send(), etc.
• Translation. Logical IPMC map to a
set of P-IPMC/unicast addresses.o Two extremes
MCMD Library Module
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• MCMD Agent runs on each machineo Contacted by the library modules o Provides a mapping
• One agent elected to be a leader:
o Allocates IPMC resources according to the current policy
MCMD Mapping Role
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• Allocating IPMC resources: An optimization problem
Procs
L-IPMC
MCMD Mapping Role
This box intentionally left
BLACKProcs
Collections
L-IPMC
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• Runs system-wide • Automatic failure detection
• Group membership fully replicated via gossip
o Node reports its own stateo Future: Replicate more selectively o Leader runs optimization algorithm on data and
reports the mapping
MCMD Gossip Layer
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• But gossip is slow... • Implications:
o Slow propagation of group membershipo Slow propagation of new mapso We assume a low rate of membership churn
• Remedy: Broadcast module
o Leader broadcasts urgent messages o Bounded bandwidth of urgent channelo Trade-off between latency and scalability
MCMD Gossip Layer
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Overview
• Library module• Mapping module• Gossip layer
• Optimization
questions • Results
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Optimization Questions
Procs L-IPMC
BLACK
Collections
Procs L-IPMC
• First step: compress logical IPMC groups
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klk;l Optimization Questions
• How compressible are subscriptions?o Multi-objective optimization:
Minimize number of collectionsMinimize bandwidth overhead on network
• Ties in with social preferences
o How do people's subscriptions overlap?
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klk;l Optimization Questions
• How compressible are subscriptions?o Multi-objective optimization:
Minimize number of groupsMinimize bandwidth overhead on network
o Thm: The general problem is NP-completeo Thm: In uniform random allocation, "little"
compression opportunity. o Replication (e.g. for load balancing) can
generate duplicates (easy case).
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klk;l Optimization Questions
• Which collections get an IPMC address?o Thm: Ordered by decreasing traffic*size,
assign P-IPMC addresses greedily, we minimize bandwidth.
• Tiling heuristic:o Sort L-IPMC by traffic*sizeo Greedily collapse identical groupso Assign IPMC to collections in reverse order of
traffic*size, UDP-multisend to the rest• Building tilings incrementally
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• Insignificant overhead when mapping L-IPMC to P-IPMC.
klk;l Overhead
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• Linux kernel module increases UDP-multisend throughput by 17% (compared to user-space UDP-multisend)
klk;l Overhead
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• A malfunctioning node bombards an existing IPMC group.• MCMD policy prevents ill-effects
klk;l Policy control
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• A malfunctioning node bombards an existing IPMC group.• MCMD policy prevents ill-effects
klk;l Policy control
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klk;l Network Overhead
• MCMD Gossip Layer uses constant background bandwidth
• Latency of leaves/joins/new tilings bounded by
gossip dissemination latency
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Conclusion
• IPMC has been a bad citizen...
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Conclusion
• IPMC has been a bad citizen...
• Dr. Multicast has the cure!
• Opportunity for big performance enhancements and policy control.
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Thank you!