Assessment%20of%20Data%20Path%20Implementations%20for%20Download%20and%20Streaming

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Assessment of Data Path Implementations for
Download and Streaming
Pål Halvorsen
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Overview

• RELAY overview???
• Existing mechanisms in Linux
• Tested enhancements
• Ongoing
• Summary and Conclusions

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RELAYResource Utilization in Large-Scale
Time-Dependent Systems
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RELAY people

• Internal
• Wladimir Palant
• Knut-Helge Vik
• Andreas Petlund
• Håkon Stensland
• Carsten Griwodz
• Pål Halvorsen

• External
• Svetlana Boudko
• Haakon Riiser

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Picture Today
network
network
network
network
P2P
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RELAY

• System support for improved resource
  utilization QoS
• Multimedia (game and video) servers

• Some current areas
• protocols for interactive applications
• multicast group maintenance
• latency hiding
• resource availability adaptation
• hybrid P2P streaming / streaming to mobile
  devices
• asymmetric multiprocessor scheduling

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Linux Data Path Implementations
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Delivery Systems
Network
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Delivery Systems
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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
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Cost of Data Transfers

• Data copy operations are expensive
• consume CPU, memory, hub, bus and interface
  resources (proportional to size)
• profiling shows that 40 of CPU time is
  consumed by copying data in a disk-network
  scenario
• speed-gap between memory and CPU increase
• different access times to different banks
• System calls makes a lot of switches between user
  and kernel space
• 450 ns on 933MHz PentiumIII
• 920 ns on 1.7GHz PentiumIV

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Observation and Question
A lot of research has been performed in this
area!!!!
BUT, what is the status today of commodity OSes?
IO-Lite
splice
MMBUF
stream
sendfile
.
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Content Download
bus(es)
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Content Download read / send
application
application buffer
kernel
copy
copy
page cache
socket buffer
DMA transfer
DMA transfer

• 2n copy operations
• 2n system calls

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Content Download mmap / send
application
kernel
page cache
socket buffer
copy
DMA transfer
DMA transfer

• n copy operations
• 1 n system calls

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Content Download sendfile
application
kernel
gather DMA transfer
page cache
socket buffer
append descriptor
DMA transfer

• 0 copy operations
• 1 system calls

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Content Download Results

• Tested transfer of 1 GB file on Linux 2.6
• Both UDP (with enhancements) and TCP

UDP
TCP
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Streaming
bus(es)
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Streaming read / send
application
application buffer
kernel
copy
copy
page cache
socket buffer
DMA transfer
DMA transfer

• 2n (3n) copy operations
• 2n system calls

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Streaming read / writev
application
application buffer
kernel
copy
copy
copy
page cache
socket buffer
DMA transfer
DMA transfer

• 3n copy operations
• 2n system calls

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Streaming mmap / send
application
application buffer
kernel
copy
page cache
socket buffer
copy
DMA transfer
DMA transfer

• 2n copy operations
• 1 4n system calls

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Streaming mmap / writev
application
application buffer
kernel
copy
page cache
socket buffer
copy
DMA transfer
DMA transfer

• 2n copy operations
• 1 n system calls

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Streaming sendfile
application
application buffer
copy
kernel
gather DMA transfer
page cache
socket buffer
append descriptor
DMA transfer

• n copy operations
• 4n system calls

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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)
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Enhanced Streaming Data Paths
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Enhanced Streaming mmap / msend
application
application buffer
msend allows to send data from an mmaped 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

27
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

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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

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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

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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

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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)
27 improvement
25 improvement
TCP sendfile (content download)
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Ongoing Work
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Enhanced Streaming rtpsendfile
application
application buffer
copy
kernel
gather DMA transfer
page cache
socket buffer
append descriptor
DMA transfer

• n copy operations
• n system calls

? Calls like writev, sendfilev, exist
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Enhanced Streaming sendfilew
len, off, src_fd, flags
application
application buffer
copy
kernel
gather DMA transfer
page cache
socket buffer
append descriptor
DMA transfer

• Batched system call enabling an arbitrary
  interleaving of blocks from files and user-space
  buffers to be sent as one or more packets

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Conclusions

• sendfile works nice for download scenarios
• 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?

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Questions??