Improving NFS Performance Over Wireless Links
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Improving NFS Performance Over Wireless Links
Rohit Dube, Cynthia D. Rais,and Satish K.Tripathi.Fellow,IEEE
報告人 : 蔡昭明 學號 :492515265
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Introduction (1) 1.Now:Mobile computing→The performance
currently limited by low bandwidth ◎ ◎high error rates
◎temporary disconnect
◎high latencies
◎limited disk space
◎ fetch files from severs on the wired network via wireless links
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Introduction (2)
2.Testbed Conditions:
◎ Limited wireless LAN networks
(bandwidths on the order of 1 Mbps ). ◎ NFS is designed (on the order of 10 Mbps) 3.This Paper:improve response time by up to
62%,which brings the performance to whithin 5% of that obtained in zero error conditions
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Solution Approach(1) 1. Modifications to the mobile NFS client 2. Modifications the wireless device drivers.
3.Test Method:
◎ NFS uses block sizes
( From 8.192 to→1024bytes)
◎ Physiacl link→Maximum Transmission Unit (1500bytes)
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Solution Approach(2)
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Solution Approach(3)
1.Poor performance on the NFS client:
◎ Few losses will be retransmitted
(Add the timeout period)
◎ Exponentially increasing timeout values.
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Solution Approach(4) 1.Solution: ◎ Use simpler mechanisms with modificatio
ns only to the mobile NFS client and the wireless device drivers.
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Markov error model (1)
Mobile host Base-station
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Markov error model (2)
Results:Implement a temporal model by using a distribution for the period spent in the good and bad states:
─Distribution with a good period mean of 1 sec.
─Distribution with a bad period means between 0 and 160 ms.(1 MB files and 10 times)
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Effect of Block Size (1)
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Effect of Block Size (2)
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Effect of Block Size (3) NFS use block size. The best performace is expected between t
he two extremes of 1,024 and 8,192 bytes ─ A large block sizes increases the prob
ability of loss.(fragment number ↑) ─ A small block sizes increases in request
and acknowledgment messages. ─ 4,096 bytes is the best on the testbed
evironment .
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Effect of Hybrid Linear Back-off Algorithm (1)
R is the number of retransmissions of the current packet.
a equals the number of times linear back-off
c and Initial are small constants
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Effect of Hybrid Linear Back-off Algorithm (2)
Modified NFS client uses a hybrid of linear and exponential algorithms.
─The first few timeouts value are calcaulated by hybrid of linear algorithms
─The next few timeouts value are calcaulated by exponential algorithms
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Effect of Hybrid Linear Back-off Algorithm (3)
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Effect of Hybrid Linear Back-off Algorithm (4)
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Performance with Multiple Mobile Hosts(1)Modified system
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Performance with Multiple Mobile Hosts(2)Modified system
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Performance with Multiple Mobile Hosts(3)Modified system
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Conclusion
1.Experiment results:improve response time by up to 62%(corresponding to a 160% increase in throughput)
Brings the performance to whithin 5% of the”zero error on the wireless link” case