Tweaking TCPs Timers Kieran Mansley Laboratory for Communication - - PowerPoint PPT Presentation

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Tweaking TCPs Timers Kieran Mansley Laboratory for Communication - - PowerPoint PPT Presentation

Dec 26, 2022 114 likes •285 views

Tweaking TCPs Timers Kieran Mansley Laboratory for Communication Engineering Context Researching user-level TCP for my PhD. Focusing on how to implement it efficiently at user level, particularly in a server room. Timers are a

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Tweaking TCP’s Timers

Kieran Mansley Laboratory for Communication Engineering

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Context

  • Researching user-level TCP for my PhD.
  • Focusing on how to implement it efficiently

at user level, particularly in a server room.

  • Timers are a small but interesting part of

that.

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Historical Context

  • TCP was first specified in the early 1980s
  • OS support for time was poor and costly
  • Networks were slower, so time intervals longer.
  • Portability very important, all leads to...

TCP has weak requirements of the OS

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What are TCP Timers?

  • Not measuring time (usually).
  • Enable an action to be performed later.
  • Mostly used to deal with inactivity:
  • Timer set when activity is expected
  • Timer cancelled when activity occurs
  • If timer expires, recovery action is executed.
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Delayed Acknowledgments

  • TCP sends acks for reliability and flow

control.

  • Can either be a separate packet or

piggyback on a data packet.

  • Acks are delayed to encourage piggybacking.
  • Timer used to ensure delay is limited.
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Delayed Ack Illustration

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Timer Ticks Illustrated

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Timer Techniques

  • How to implement timers?
  • Trad. scheme based on 100ms clock ticks
  • Maintain flags in per-connection state.
  • Each tick, check list of connections for timers.
  • Modern scheme based on hashed

hierarchical timing wheel.

  • Ordered list of timers, use hardware clock to

trigger the check and schedule operation.

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Problems

  • Inaccurate delay of acks: from 0ms to 200ms
  • List of connections must be searched each

clock tick.

  • A busy connection will still regularly send

separate ack packets.

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Potential Solution (i)

  • Change profile of delay:
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Potential Solution (ii)

  • Use two timer buckets to achieve delay

limits:

Target Bucket A Target Bucket B Empty Bucket B Empty Bucket A x x+100 x+200 x+300 x+400 x x+100 x+200 x+300 x+400

Min Max

time (ms) time (ms)

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Potential Solution (iii)

  • Implementation of buckets:
  • Lazy switch avoids need for scheduling.
  • Timer execution when blocking op encountered.
  • Data thread used for active connections.
  • Time checking done using “rtdsc” counter.
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Potential Solution (iv)

  • Handle timers for active connections from

the data thread.

  • Removes need for locking, other than for

handing connections between threads.

  • No list searching, but...
  • Increased set/clear timer complexity.
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CPU Usage Tradeoff

0.0001 0.001 0.01 0.1 1 10 10 100 1000 10000 100000 CPU usage (%) Connection list size CPU to iterate list (%) measured inline 0.0000754x + 0.0001191 CPU to iterate list (%) measured with ’time’ 0.0000692x + 0.001060 CPU for bucket scheme (%)

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Summary

  • Timers at user level can benefit from a

different solution.

  • Change the way timers are implemented to:
  • Give guaranteed lower, reduced upper bound;
  • Avoid locking by checking timers in data thread.
  • Minor performance issue for current TCPs
  • May be more important in future.
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Questions/Comments?

Kieran Mansley kjm25@cam.ac.uk http://www-lce.eng.cam.ac.uk/~kjm25 Available as technical report CUED/F-INFENG/TR.487