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DAPSYS Conference
Innsbruck, 21-23 September 2006


        The wandering token
   Congestion Avoidance of a Shared
              Resource
Augusto Ciuffoletti
  INFN-CNAF - Bologna
  CoreGRID Institute of Grid Information,
     Resource and Workflow Monitoring Services
Problem and solution

●   Problem: design a scalable (thousands of agents),
    resource sharing solution
●   Requirements:
    –   access granted regularly, not strict real time;
    –   overload gradually degrades service, with limited
        impact on the resource;
    –   algorithm runs on clients; resources and infrastructure
        are legacy
A case study: Video on Demand
●   A provider offers a VoD service to a number of
    susbscribers (news on a 150*120 screen)
●   Video is trasferred using a 200 Mbps
    infrastructure
●   Each transmission uses a 650 Kbps bandwidth
●   Each subscriber downloads distinct chunks of
    video
●   The server uploads chunks of video on demand
●   The infrastructure allows short bursts of transfer
    rate up to two times the agreed 200Mbps
Other applications

●   Intrusive network monitoring applications
    –   only one test per run to avoid interference and overload
●   Expensive queries to a centralized database
    –   only one query at a time allowed
The token wandering idea
●   A unique token circulates in the system, enabling
    members to access the resource only when holding
    the token.
●   Token circulation does not follow a predetermined
    path: next hop destination is selected randomly
    among all members.
●   According to many theoretical results, the
    interarrival time is quite regular, although
    randomly variable.
●   Like any token circulation algorithm, it suffers
    from token loss and duplication.
Token (re)generation rule
●   The agent sets up a timer: when such timer
    expires, a new token is generated, bearing a unique
    identifier.
●   The timer value has the following properties:
    –   it is randomized, to avoid synchronization effects;
    –   minimum value considers application requirements,
        and is well below the required access rate;
    –   expected value is such that the overall distribution of
        timeouts on the time line has an average interarrival
        time below the required access rate;
●   The token regeneration rule can introduce
    spurious tokens (with different id).
Token removal rule
●   This rule aims at removing tokens unnecessarily
    generated by the token regeneration rule
●   Duplicate tokens are prevented by the 3-way token
    passing protocol
●   The token removal rule is applied whenever a
    token is received.
●   Such token is silently dropped if:
    –   the same token was already received and
    –   from that time another token was received with a
        timestamp lower than that of the resident token
●   Removal occurs with a latency, and during that
    time the token can be effectively lost.
Simulation setup
●   The simulator is an ad-hoc finite state machine
    that simulates a variable number of agents:
    –   synchronous token passing operations, and
    –   asynchronous alarms.
●   The case study (650 Kbps VoD on a 200Mbps
    backbone) accommodates up to 300 subscribers.
●   The access rate should be on the average one
    every 1200 seconds.
●   Each subscriber downloads a chunk of 100 Mbytes
    during a slot of time of 4 seconds
Simulation results: n. of tokens




●   The requirement of a unique token is enforced
●   Token loss events (injected one every 10000 time
    units) are recovered: largest gap is 1700 time
    units.
●   No transients after token loss recovery.
●   Limited presence of spurious tokens
Simulation results: concurrency




●   Resource is protected from overload
●   Even under 20% overload:
    –   80% of time there is just one protected event running
    –   during less than 10% of the time there are two
        concurrent downloads (expensive rate applied)
    –   during 1% of the time there are three or more
        concurrent downloads (shaping may occur)
Simulation results: interarrival time




●   Observed performance depends on load.
●   Most events occur before 1200 time units, even on
    full or overload conditions.
●   Longer tail in case of full and overload conditions
    (buffering needed)
Conclusions
●   A Proof of Concept: a random walk approach can
    provide a robust, scalable and efficient solution to
    a distributed coordination problem.
●   A solution to a class of problems: we provide a
    solution to the Soft Mutual Exclusion problem,
    based on a wandering token.
●   A real world case study: the simulation in a Video
    on Demand environment demonstrates
    applicability.

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The wandering token

  • 1. DAPSYS Conference Innsbruck, 21-23 September 2006 The wandering token Congestion Avoidance of a Shared Resource Augusto Ciuffoletti INFN-CNAF - Bologna CoreGRID Institute of Grid Information, Resource and Workflow Monitoring Services
  • 2. Problem and solution ● Problem: design a scalable (thousands of agents), resource sharing solution ● Requirements: – access granted regularly, not strict real time; – overload gradually degrades service, with limited impact on the resource; – algorithm runs on clients; resources and infrastructure are legacy
  • 3. A case study: Video on Demand ● A provider offers a VoD service to a number of susbscribers (news on a 150*120 screen) ● Video is trasferred using a 200 Mbps infrastructure ● Each transmission uses a 650 Kbps bandwidth ● Each subscriber downloads distinct chunks of video ● The server uploads chunks of video on demand ● The infrastructure allows short bursts of transfer rate up to two times the agreed 200Mbps
  • 4. Other applications ● Intrusive network monitoring applications – only one test per run to avoid interference and overload ● Expensive queries to a centralized database – only one query at a time allowed
  • 5. The token wandering idea ● A unique token circulates in the system, enabling members to access the resource only when holding the token. ● Token circulation does not follow a predetermined path: next hop destination is selected randomly among all members. ● According to many theoretical results, the interarrival time is quite regular, although randomly variable. ● Like any token circulation algorithm, it suffers from token loss and duplication.
  • 6. Token (re)generation rule ● The agent sets up a timer: when such timer expires, a new token is generated, bearing a unique identifier. ● The timer value has the following properties: – it is randomized, to avoid synchronization effects; – minimum value considers application requirements, and is well below the required access rate; – expected value is such that the overall distribution of timeouts on the time line has an average interarrival time below the required access rate; ● The token regeneration rule can introduce spurious tokens (with different id).
  • 7. Token removal rule ● This rule aims at removing tokens unnecessarily generated by the token regeneration rule ● Duplicate tokens are prevented by the 3-way token passing protocol ● The token removal rule is applied whenever a token is received. ● Such token is silently dropped if: – the same token was already received and – from that time another token was received with a timestamp lower than that of the resident token ● Removal occurs with a latency, and during that time the token can be effectively lost.
  • 8. Simulation setup ● The simulator is an ad-hoc finite state machine that simulates a variable number of agents: – synchronous token passing operations, and – asynchronous alarms. ● The case study (650 Kbps VoD on a 200Mbps backbone) accommodates up to 300 subscribers. ● The access rate should be on the average one every 1200 seconds. ● Each subscriber downloads a chunk of 100 Mbytes during a slot of time of 4 seconds
  • 9. Simulation results: n. of tokens ● The requirement of a unique token is enforced ● Token loss events (injected one every 10000 time units) are recovered: largest gap is 1700 time units. ● No transients after token loss recovery. ● Limited presence of spurious tokens
  • 10. Simulation results: concurrency ● Resource is protected from overload ● Even under 20% overload: – 80% of time there is just one protected event running – during less than 10% of the time there are two concurrent downloads (expensive rate applied) – during 1% of the time there are three or more concurrent downloads (shaping may occur)
  • 11. Simulation results: interarrival time ● Observed performance depends on load. ● Most events occur before 1200 time units, even on full or overload conditions. ● Longer tail in case of full and overload conditions (buffering needed)
  • 12. Conclusions ● A Proof of Concept: a random walk approach can provide a robust, scalable and efficient solution to a distributed coordination problem. ● A solution to a class of problems: we provide a solution to the Soft Mutual Exclusion problem, based on a wandering token. ● A real world case study: the simulation in a Video on Demand environment demonstrates applicability.