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Scaling Intel Many-to-Many Connections with RDMA

Learn how to optimize scalability with considerations for addressing, path resolution, and connection management in Intel Many-to-Many connections. Explore solutions for ARP storms, SA caching, QoS challenges, and more.

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Scaling Intel Many-to-Many Connections with RDMA

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  1. IB Scalability Sean Hefty – Intel

  2. Many-to-Many Connections • Obtain GID of remote endpoint • RDMA CM uses ARP for address mapping • ARP message sent over IPoIB broadcast multicast group • IPoIB obtains path record to DGID • Obtain path record (PR) to endpoint • Exchange QP information • 3-way CM protocol

  3. Address Resolution • ARP messages are sent over IPoIB’s broadcast multicast group • Creates ARP storm • IPoIB starts dropping packets • Longer time-outs and deeper queues help, some… • E.g. 1000 nodes  1 million ARP entries fabric wide • 15 minute timeout  1000 entries timeout / second • 24 hour timeout  12 entries timeout / second

  4. Route Resolution • Obtain path record to endpoint • Wait, didn’t IPoIB just do that? • Yes, it did • And the path records were cached locally • Queries take minutes to complete • 1000 nodes hit SA with 500,000 queries • PR caching is provided by QLogic and Cisco stacks – Voltaire? Path record caching is critical for scalability

  5. Connection CM protocol time is relatively small unless using ARP and PR caching • CM message exchanges occur within seconds • Apps may be slow to respond to CM messages during processing • MRA patch added to address this

  6. Other Issues • ARP only works within a single IP subnet • IB routers separating IP subnets will call for a different mapping method • SA query retries use different TIDs • Each request - response pair looks unique Is connection scaling across subnets a requirement?

  7. Other Issues • QoS makes distributing PR data more difficult (but not impossible) • But also increases the burden on a centralized SA • Need CM testing on larger clusters • Discourage non-CM solutions OFA should examine vendor/application solutions

  8. Options Assuming most users prefer RDMA CM over IB CM • Merge OFED local SA solution upstream • Path record caching only • Does not support QoS • IPoIB still caches (but uses local SA) • Allow manual redirection of SM LID • Moves local SA solution to userspace • Enables non-local caching

  9. Options • Define new group connection capability • Leave existing connection model alone • Combine address and route resolution into a single, more efficient step • Avoid ARP storm, support multiple IP subnets? • Need new APIs, protocols • Interaction with job schedulers is unknown • E.g. connect to 192.168.10.0 / 24 port 7174 Speaker is just winging it now

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