Performance analysis of a parallel downloading scheme from mirror sites throughout the internet
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Performance Analysis of a Parallel Downloading Scheme from Mirror Sites Throughout the Internet. Allen Miu, Eugene Shih 6.892 Class Project December 3, 1999. Overview. Problem Statement Advantages/Disadvantages Operation of Paraloading Goals of Experiment Setup of Experiment

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Performance Analysis of a Parallel Downloading Scheme from Mirror Sites Throughout the Internet

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Performance analysis of a parallel downloading scheme from mirror sites throughout the internet

Performance Analysis of a Parallel Downloading Scheme from Mirror Sites Throughout the Internet

Allen Miu, Eugene Shih

6.892 Class Project

December 3, 1999


Overview

Overview

  • Problem Statement

  • Advantages/Disadvantages

  • Operation of Paraloading

  • Goals of Experiment

  • Setup of Experiment

  • Current Results

  • Summary

  • Questions


Problem statement is paraloading good

Problem Statement: Is “Paraloading” Good?

Paraloading is the downloading from multiple

mirror sites in parallel.

Mirror C

Paraloader

Mirror A

Mirror B


Advantages of paraloading

Advantages of Paraloading

  • Performance is proportional to the realized aggregate bandwidth of the parallel connections

  • Less prone to complete download failures compared to the single connection download

  • Facilitates dynamic load balancing among parallel connections

  • Facilitates reliable, out-of-order delivery (similar to Netscape)


Disadvantages of paraloading

Disadvantages of Paraloading

  • Can be overly aggressive

  • Consumes more server resources

  • Overhead costs for scheduling, maintaining buffers, and sending block request messages

  • Only effective when mirror servers are available


Step 1 obtain mirror list

Step 1: Obtain Mirror List

  • Hard-coded

  • DNS?

Mirror

List

Mirror C

Paraloader

Mirror B

Mirror A


Step 2 obtain file length

Step 2: Obtain File Length

Mirror C

Paraloader

Mirror B

Mirror A


Step 3 send block requests

Step 3: Send Block Requests

Mirror C

Paraloader

Mirror B

Mirror A


Step 4 re order

Step 4: Re-order

Mirror C

Paraloader

Mirror B

Mirror A


Step 5 send next request

Step 5: Send Next Request

Mirror C

Paraloader

Mirror B

Mirror A


Goals of experiment

Goals of Experiment

  • Main goal: To compare the performance of serial and parallel downloading

  • To verify the results of Rodriguez et al.

  • To examine whether varying the degree of parallelism, the number of mirror servers used, affects performance

  • To gain experience with paraloading and to find out what issues are involved in designing efficient paraloading systems


Experiment setup

Experiment Setup

  • Implemented a paraloader application in Java, using HTTP1.1 (range-requests and persistent connections)

  • Files are downloaded at MIT from 3 different sets (kernel, mars, tucows) of 7 mirror servers

  • Degree of parallelism examined: M = 1, 3, 5, 7

  • Downloaded a 1MB and a 300KB file (S = 1MB, 300KB) in 1 hour intervals for 7 days

  • Block Size = 32KB


Results

Results

  • Paraloading decreases download time over the average single connection case

  • Speedup is far from optimal case (aggregate bandwidth)

    • Block request gaps result in wasted bandwidth

      • Gaps are proportional to RTT

    • Congestion at client? Possible but unlikely.


Performance analysis of a parallel downloading scheme from mirror sites throughout the internet

S = 1MB


Performance analysis of a parallel downloading scheme from mirror sites throughout the internet

S = 1MB


Performance analysis of a parallel downloading scheme from mirror sites throughout the internet

S = 763KB, B = 30, M = 4

S - 763K


Acknowledgements

Acknowledgements

  • Dave Anderson

  • Dorothy Curtis

  • Wendi Heinzelmann

  • WIND Group


Questions

Questions


Summary of contributions

Summary of Contributions

  • Implemented a paraloader

  • Verified that paraloading indeed provides performance gain… sometimes

    • Increasing degree of parallelism improves overall performance

  • Performance gains are not as good as those reported by Rodriguez et al.


Future work

Future Work

  • Examine how block size affects performance gain

  • Examine cost of paraloading

  • Implement and test various optimization techniques

  • Perform measurements at different client sites


Paraloading will not be effective in all situations

Paraloading Will Not Be Effective In All Situations

  • Clients should have enough “slack” bandwidth capacity to open more than one connection

  • Parallel connections are bottleneck disjoint

  • Target data on mirror servers is consistent and static

  • Security and authentication services are installed where appropriate

  • Data transport is reliable

  • Mirror locations are quickly and easily obtained


Step by step process of the block scheduling paraloading scheme

Step-by-step Process of the Block Scheduling Paraloading Scheme

1. Obtain a list of mirror sites

2. Open a connection to a mirror server and obtain file length

3. Divide file length into blocks

4. Send a block request to each open connection

5. Wait for a response

6. Send a new block request to the first connection that finished downloading a block

7. Loop back to 5 until all blocks are retrieved


Paraloading is not a well studied concept

Paraloading is Not a Well-studied Concept

  • Byers et al. proposed using Tornado codes to facilitate paraloading.

  • Rodriguez et al. proposed the block scheduling paraloading scheme that is used in our project


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