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

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

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

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)

Solution Approach(2)

Solution Approach(3)

1.Poor performance on the NFS client:

◎ Few losses will be retransmitted

(Add the timeout period)

◎ Exponentially increasing timeout values.

Solution Approach(4) 1.Solution: ◎ Use simpler mechanisms with modificatio

ns only to the mobile NFS client and the wireless device drivers.

Markov error model (1)

Mobile host Base-station

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)

Effect of Block Size (1)

Effect of Block Size (2)

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 .

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

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

Effect of Hybrid Linear Back-off Algorithm (3)

Effect of Hybrid Linear Back-off Algorithm (4)

Performance with Multiple Mobile Hosts(1)Modified system

Performance with Multiple Mobile Hosts(2)Modified system

Performance with Multiple Mobile Hosts(3)Modified system

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