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Assessment of Data Path Implementations for Download and Streaming

International conference on distributed multimedia systems (DMS’05), Banff, Canada, September 2005. Assessment of Data Path Implementations for Download and Streaming. Pål Halvorsen 1,2 , Tom Anders Dalseng 1 and Carsten Griwodz 1,2 1 Department of Informatics, University of Oslo, Norway

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Assessment of Data Path Implementations for Download and Streaming

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  1. International conference on distributed multimedia systems (DMS’05), Banff, Canada, September 2005 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

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

  3. bus(es) Delivery Systems Network

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

  5. 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

  6. 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 makes a lot of switches between user and kernel space

  7. application user space kernel space file system communication system Basic Idea of Zero–Copy Data Paths data_pointer data_pointer bus(es)

  8. 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 makes 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?

  9. Existing Linux Data Paths

  10. application user space kernel space file system communication system Content Download bus(es)

  11. 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

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

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

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

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

  16. 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

  17. 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  Previous solution three less calls per packet

  18. 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

  19. 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)

  20. Enhanced Streaming Data Paths

  21. 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  Previous solution one more copy per packet

  22. 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  previous solution require three more calls per packet

  23. 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  previous solution require one more copy per packet  previous solution require one more call per packet

  24. 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 require three more calls per packet

  25. 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  previous solution require one more copy per packet  previous solution require one more call per packet

  26. 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)

  27. 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

  28. Questions??

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