Impact of Supporting Out-of-order Communication on In-order Performance

This study delves into the impact of out-of-order communication on in-order performance in high-end computing systems, specifically investigating the potential solutions, challenges, and network handling strategies associated with this issue. The research focuses on the implications of supporting out-of-order communication using iWARP over Ethernet, highlighting the complexities of packet segmentation, reordering, and network buffering. The discussion also explores the relevance of multi-pathing in scaling network subsystems to meet the growing demands of large-scale computing environments.

• Computing Systems
• High Performance Computing
• Network Subsystems
• iWARP
• Packet Segmentation

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1. Analyzing the Impact of Supporting Out-of-order Communication on In-order Performance with iWARP P. Balaji, W. Feng, S. Bhagvat, D. K. Panda, R. Thakur and W. Gropp Mathematics and Computer Science, Argonne National Laboratory Department of Computer Science, Virginia Tech Scalable Systems Group, Dell Inc. Computer Science and Engineering, Ohio State University Computer Science, University of Illinois at Urbana Champagne

2. Motivation High-end computing systems growing rapidly in scale 128K processor system at LLNL (HPC CPU growth of 50%) 1M processor systems as soon as next year Network subsystem has to scale accordingly Fault-tolerance and hot-spot avoidance important Possible Solution: Multi-pathing Supported by many networks InfiniBand uses subnet management to discover paths 10-Gigabit Ethernet uses VLAN based multi-pathing Disadvantage: Out-of-order Communication!

3. Out-of-order Communication 2 1 3 2 1 4 4 3 Different packets taking different paths mean that later injected packets might arrive earlier Physical networks only deal with sending packets out-of-order Protocols on top of networks (either in hardware or software) have to deal with reordering packets Networks such as IB handle this by dropping out-of-order packets FECN, BECN and throttling on congestion Network buffering (with FECN/BECN) helps, but not perfect

4. Overview of iWARP over Ethernet Relatively new initiative by IETF and RDMAC Application Sockets SDP, MPI etc. Backward compatibility with TCP/IP/Ethernet RDMAP Verbs Software TCP/IP RDDP Sender stuffs iWARP packets within TCP/IP packets MPA When sent, one TCP packet contains one iWARP packet Offloaded TCP/IP 10-Gigabit Ethernet What about on receive?

5. Ethernet Packet Segmentation Packet Header Packet Header Packet Header Data Payload Data Payload iWARP HeaderData Payload iWARP Header iWARP Header Intermediate Switch Segmentation Partial Payload Packet Header Packet Header Packet Header Packet Header Partial Payload Data Payload Data Payload iWARP Header iWARP Header iWARP Header Delayed Packet Out-Of-Order Packets (Cannot identify iWARP header) Intermediate switch segmentation Packets split or coalesced Current iWARP implementations do not handle out-of-order packets Follow approaches used by IB

6. Problem Statement How do we design a feature-complete iWARP stack? Provide support for out-of-order arriving packets Maintaining performance of in-order communication What are the tradeoffs in designing iWARP? Host-based iWARP Host-offloaded iWARP Host-assisted iWARP

7. Presentation Layout Introduction and Motivation Details of the iWARP Standard Design Choices for iWARP Experimental Evaluation Concluding Remarks and Future Work

8. Dealing with Out-of-order packets in iWARP iWARP specifies intelligent approaches to deal with out-of-order packets Out-of-order data placement and In-order data delivery If packets arrive out-of-order, they are directly placed in the appropriate location in memory Application notified about the arrival of the message only when: All packets of the message have arrived All previous messages have arrived It is necessary that iWARP recognize all packets !

9. MPA Protocol Frame DDP Header Payload (IF ANY) Pad CRC DDP Header Payload (IF ANY) Segment Length Marker Deterministic approach to identify packet header Can distinguish in-order packets from out-of-order packets

10. Presentation Layout Introduction and Motivation Details of the iWARP Standard Design Choices for iWARP Experimental Evaluation Concluding Remarks and Future Work

11. iWARP components iWARP consists of three layers RDMAP: Thin layer that deals with interfacing upper layers with iWARP RDDP: Core of the iWARP stack Component 1: Deals with connection management issues and packet de-multiplexing between connections MPA: Glue layer to deal with backward compatibility with TCP/IP Component 2: Performs CRC Component 3: Adds marker strips of data to point to the packet header

12. Component Onload vs. Offload Connection Management and Packet Demultiplexing Connection lookup and book-keeping --> CPU intensive Can be done efficiently on hardware Data Integrity: CRC-32 CPU intensive Can be done efficiently on hardware Marker Strips: Tricky as they need to be inserted in between the data Software implementation requires an extra copy Hardware implementation might require multiple DMAs

13. Task distribution for different iWARP designs RDMAP RDDP HOST RDMAP RDMAP Markers CRC Markers RDDP CRC NIC RDDP CRC TCP/IP TCP/IP Markers TCP/IP Host-based Host-offloaded Host-assisted

14. Host-based and -offloaded Designs Host-based iWARP: Completely in software Deals with overheads for all components Host-offloaded iWARP: Completely in hardware Good for packet demultiplexing and CRC Is it good for inserting marker strips? Ideal: True Scatter/Gather DMA engine. Not available. Contiguous DMA and Decoupled Marker Insertion Large chunks DMAed and moved on the NIC to insert markers A lot of NIC memory transactions Scatter/Gather DMA with Coupled Marker Insertion Small chunks DMAed and non-contiguously A lot of DMA operations

15. Hybrid Host-assisted Implementation Performs tasks such as: packet demultiplexing and CRC in hardware marker insertion in software (requires an extra-copy) Fully utilizes both the host and the NIC Summary: Host-based design suffers from software overheads for all tasks Host-offloaded design suffers from the overhead of multiple DMA operations Host-based design suffers from the extra memory copy to add the markers but benefits from less DMAs

16. Presentation Layout Introduction and Motivation Details of the iWARP Standard Design Choices for iWARP Experimental Evaluation Concluding Remarks

17. Experimental Test bed 4-node cluster 2 Intel Xeon 3.0GHz processors with 533MHz FSB, 2GB 266-MHz DDR SDRAM and 133 MHx PCI-X slots Chelsio T110 10GE TCP Offload Engines 12-port Fujitsu XG800 switch Red Hat Operating system (2.4.22smp)

18. iWARP Microbenchmarks iWARP Latency iWARP Bandwidth 7000 100 60 Host-offloaded iWARP CPU 90 6000 Host-based iWARP CPU 50 80 Host-offloaded iWARP Host-assisted iWARP CPU 5000 Host-offloaded iWARP 70 Bandwidth (Mbps) Host-based iWARP 40 Host-based iWARP Latency (us) 60 Host-assisted iWARP 4000 Host-assisted iWARP CPU(%) 50 30 3000 40 20 30 2000 20 10 1000 10 0 0 0 1 2 4 8 16 32 64 128 256 512 1K 2K 16 64 1K 4K 1 4 256 16K 64K 256K Message Size (Bytes) Message Size (bytes)

19. Out-of-cache Communication iWARP Bandwidth 4000 Host-offloaded iWARP Host-based iWARP Host-assisted iWARP 3500 3000 Bandwidth (Mbps) 2500 2000 1500 1000 500 0 1 1K 4K 16 64 256 Message Size (Bytes) 16K 64K 4 256K

20. Computation Communication Overlap Message Size 4KB Message Size 128KB 6000 7000 6000 5000 5000 Bandwidth (Mbps) Bandwidth (Mbps) 4000 4000 3000 3000 2000 2000 1000 1000 0 1 0 1 2 4 8 2 4 8 16 32 64 256 2K 2K 16 32 64 256 128 512 1024 128 512 1024 Computation Time (us) Computation Time (us) Host-offloaded iWARP Host-based iWARP Host-assisted iWARP

21. Iso-surface Visual rendering application Data Distribution Size : 8KB Data Distribution Size : 1MB 450 600 400 500 350 Execution Time (secs) Execution Time (secs) 300 400 250 300 200 150 200 100 100 50 0 0 1024x1024 2048x2048 4096x4096 8192x8192 1024x1024 2048x2048 4096x4096 8192x8192 Dataset Dimensions Dataset Dimensions Host-offloaded iWARP Host-based iWARP Host-assisted iWARP

22. Presentation Layout Introduction and Motivation Details of the iWARP Standard Design Choices for iWARP Experimental Evaluation Concluding Remarks

23. Concluding Remarks With growing scales of high-end computing systems, network infrastructure has to scale as well Issues such as fault tolerance and hot-spot avoidance play an important role While multi-path communication can help with these problems, it introduces Out-of-order communication We presented three designs of iWARP that deal with out-of-order communication Each design has its pros and cons No single design could achieve the best performance in all cases

24. Thank You Email Contacts: P. Balaji: balaji@mcs.anl.gov W. Feng: feng@cs.vt.edu S. Bhagvat: sitha_bhagvat@dell.com D. K. Panda: panda@cse.ohio-state.edu R. Thakur: thakur@mcs.anl.gov W. Gropp: wgropp@uiuc.edu

25. Backup Slides

26. Segment Not Complete IDLE Send Request READY READY Segment Complete Host DMA Free Host DMA Free Host DMA Busy Host DMA Busy Host DMA Free DMA BUSY DMA BUSY Host DMA Free SDMA SDMA Marker Inserted Integrated

27. IDLE IDLE Segment Available Segment Complete Segment Available Send Request Calculate CRC IDLE Segment Complete READY READY Host DMA Free CRC SDMA Done Host DMA In Use COPY PARTIAL SEGMENT DMA BUSY Segment Available Host DMA Free Marker Inserted Segment Not Complete IDLE SEND SDMA INSERT MARKERS Segment Complete SEND Processing SDMA

28. iWARP Out-of-Cache Communication Bandwidth Cache Traffic (Transmit Side) Cache Traffic (Receive Side) 6 6 Ratio of Cache to Network Traffic Ratio of Cache to Network Traffic 5 5 4 4 3 3 2 2 1 1 0 0 1 16 256 4K 64K 256K 1 16 256 4K 64K 256K Message Size (Bytes) Message Size (Bytes) Host-offloaded iWARP Host-based iWARP Host-assisted iWARP

29. Impact of marker separation on iWARP performance Host-offloaded iWARP Latency NIC-offloaded iWARP Bandwidth 35 8000 7000 30 6000 25 Bandwidth (Mbps) Latency (us) 5000 20 4000 15 3000 10 2000 5 1000 0 0 1 1 2 4 8 16 32 64 128 256 512 1K 2K 4 16 64 256 1K 4K 64K 16K 256K Message Size (Bytes) Message Size (Bytes) iWARP (original) iWARP (1KB marker separation) iWARP (2KB marker separation) iWARP (no markers)