ND The research group on Networks & Distributed systems

1. NDThe research group onNetworks & Distributed systems

2. ND activities • ICON – Interconnection Networks • Interconnection networks are tightly coupled/short distance networks with extreme demands on bandwidth, latency, and delivery • Problem areas: Effective routing/topologies, fault-tolerance/dynamic reconfiguration, and Quality of Service • VINNER – End-to-end Internet communications • Problem area: Network resilience – as a set of methods and techniques that improve the user perception of network robustness and reliability.

3. ND activities • QuA - Support of Quality of Service in component architectures • Problem area: How to develop applications that are sensitive to QoS on a component architecture platform and how dynamic QoS management and adaptation can be supported • Relay – Resource utilization in time-dependent distributed systems • Problem area: Reduce the effects of resource limitations and geographical distances in interactive distributed applications – through a toolkit of kernel extensions, programmable subsystems, protocols and decision methods

4. Assessment of Data Path Implementations for Download and Streaming Pål Halvorsen1,2, Tom Anders Dalseng1 and Carsten Griwodz1,2 1Department of Informatics, University of Oslo, Norway 2Simula Research Laboratory, Norway

5. Overview • Motivation • Existing mechanisms in Linux • Possible enhancements • Summary and Conclusions

6. bus(es) Delivery Systems Network

7. application user space kernel space file system communication system Delivery Systems bus(es)

8. application file system communication system disk network card I/O controller hub memory controller hub file system communication system application network card disk Intel Hub Architecture • several in-memory data movements and context switches Pentium 4 Processor registers cache(s) RDRAM RDRAM RDRAM RDRAM PCI slots PCI slots PCI slots

9. Motivation • Data copy operations are expensive • consume CPU, memory, hub, bus and interface resources (proportional to data size) • profiling shows that ~40% of CPU time is consumed by copying data between user and kernel space • gap between memory and CPU speeds increase • different access times to different banks • System calls make a lot of switches between user and kernel space

10. application user space kernel space file system communication system Zero–CopyData Paths data_pointer data_pointer bus(es)

11. Motivation • Data copy operations are expensive • consume CPU, memory, hub, bus and interface resources (proportional to data size) • profiling shows that ~40% of CPU time is consumed by copying data between user and kernel • gap between memory and CPU speeds increase • different access times to different banks • System calls make a lot of switches between user and kernel space • A lot of research has been performed in this area • BUT, what is the status today of commodity operating systems?

12. Existing Linux Data Paths

13. application user space kernel space file system communication system ContentDownload bus(es)

14. read send Content Download: read / send application application buffer kernel copy copy page cache socket buffer DMA transfer DMA transfer • 2n copy operations • 2n system calls

15. mmap send Content Download: mmap / send application kernel page cache socket buffer copy DMA transfer DMA transfer • n copy operations • 1 + n system calls

16. sendfile Content Download: sendfile application kernel gather DMA transfer page cache socket buffer append descriptor DMA transfer • 0 copy operations • 1 system calls

17. Content Download: Results • Tested transfer of 1 GB file on Linux 2.6 • Both UDP (with enhancements) and TCP UDP TCP

18. application user space kernel space file system communication system Streaming bus(es)

19. read send Streaming: read / send application application buffer kernel copy copy page cache socket buffer DMA transfer DMA transfer • 2n copy operations • 2n system calls

20. writev read Streaming: read / writev application application buffer kernel copy copy copy page cache socket buffer DMA transfer DMA transfer • 3n copy operations • 2n system calls  One copy more than previous solution

21. uncork mmap send send cork Streaming: mmap / send application application buffer kernel copy page cache socket buffer copy DMA transfer DMA transfer • 2n copy operations • 1 + 4n system calls

22. writev mmap Streaming: mmap / writev application application buffer kernel copy page cache socket buffer copy DMA transfer DMA transfer • 2n copy operations • 1 + n system calls  Three calls less than previous solution

23. sendfile uncork send cork Streaming: sendfile application application buffer copy kernel gather DMA transfer page cache socket buffer append descriptor DMA transfer • n copy operations • 4n system calls

24. Streaming: Results • Tested streaming of 1 GB file on Linux 2.6 • RTP over UDP Compared to not sending an RTP header over UDP, we get an increase of 29% (additional send call) More copy operations and system calls required  potential for improvements TCP sendfile (content download)

25. Enhanced Streaming Data Paths

26. uncork msend cork send send mmap Enhanced Streaming: mmap / msend application application buffer msend allows to send data from an mmap’ed file without copy copy kernel gather DMA transfer page cache socket buffer append descriptor copy DMA transfer DMA transfer • n copy operations • 1 + 4n system calls  One copy less than previous solution

27. rtpmsend uncork cork mmap send send Enhanced Streaming: mmap / rtpmsend application application buffer RTP header copy integrated into msend system call copy kernel gather DMA transfer page cache socket buffer append descriptor DMA transfer • n copy operations • 1 + n system calls  Three calls less than previous solution

28. krtpmsend rtpmsend Enhanced Streaming: mmap/krtpmsend application application buffer An RTP engine in the kernel adds RTP headers copy kernel gather DMA transfer RTP engine page cache socket buffer append descriptor DMA transfer • 0 copy operations • 1 system call  One copy less than previous solution  One call less than previous solution

29. rtpsendfile sendfile uncork send cork Enhanced Streaming: rtpsendfile application application buffer RTP header copy integrated into sendfile system call copy kernel gather DMA transfer page cache socket buffer append descriptor DMA transfer • n copy operations • n system calls  existing solution requires three more calls per packet

30. krtpsendfile rtpsendfile Enhanced Streaming: krtpsendfile application application buffer An RTP engine in the kernel adds RTP headers copy kernel gather DMA transfer RTP engine page cache socket buffer append descriptor DMA transfer • 0 copy operations • 1 system call  One copy less than previous solution  One call less than previous solution

31. Enhanced Streaming: Results • Tested streaming of 1 GB file on Linux 2.6 • RTP over UDP mmap based mechanisms sendfile based mechanisms Existing mechanism (streaming) ~25% improvement ~27% improvement TCP sendfile (content download)

32. Conclusions • Current commodity operating systems still pay a high price for streaming services • However, small changes in the system call layer might be sufficient to remove most of the overhead • Conclusively, commodity operating systems still have potential for improvement with respect to streaming support • What can we hope to be supported? • Road ahead: optimize the code, make patch and submit to kernel.org

33. Questions??