Your SlideShare is downloading. ×
0
The wandering token
The wandering token
The wandering token
The wandering token
The wandering token
The wandering token
The wandering token
The wandering token
The wandering token
The wandering token
The wandering token
The wandering token
Upcoming SlideShare
Loading in...5
×

Thanks for flagging this SlideShare!

Oops! An error has occurred.

×
Saving this for later? Get the SlideShare app to save on your phone or tablet. Read anywhere, anytime – even offline.
Text the download link to your phone
Standard text messaging rates apply

The wandering token

825

Published on

More on the topic at "http://www.di.unipi.it/~augusto/WanderingToken/"

More on the topic at "http://www.di.unipi.it/~augusto/WanderingToken/"

Published in: Technology, Education
0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total Views
825
On Slideshare
0
From Embeds
0
Number of Embeds
2
Actions
Shares
0
Downloads
1
Comments
0
Likes
0
Embeds 0
No embeds

Report content
Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
No notes for slide

Transcript

  • 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.

×