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New Statistical Algorithms for the Analysis of Mass - FU Berlin, FB MI ...

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120 CHAPTER 5. COMPUTER SCIENCE GRID STRATEGIES<br />

Figure 5.3.7: Per<strong>for</strong>mance <strong>of</strong> GridFTP with OS-default buffer size (64KB, left)<br />

versus Per<strong>for</strong>mance <strong>of</strong> GridFTP with dynamically adapted buffer sizes (right). Shown<br />

are <strong>the</strong> average bandwidth values depending on <strong>the</strong> size <strong>of</strong> <strong>the</strong> transferred file (16, 32,<br />

64, 128, 256, 512 MB).<br />

BSopt = 0.08sec · 100 Mbit<br />

sec<br />

= 8Mbit<br />

sec<br />

Since with a transmission rate <strong>of</strong> 1 Mbit/Sec one can transfer 0.125 MByte/Sec<br />

we get:<br />

BSopt = 8 Mbit<br />

/8 = 1MByte<br />

sec<br />

There<strong>for</strong>e, <strong>the</strong> TCP buffer size <strong>for</strong> this connection should be set to 1 MByte.<br />

Now, in reality <strong>the</strong> assumption made at <strong>the</strong> beginning are not very likely<br />

to hold. First, if packet loss starts to occur (Mathis et al., 1997) have shown<br />

that throughput is <strong>the</strong>n bounded by <strong>the</strong> Maximum Segment size (MSS), which<br />

is Maximum Transmission Unit12 (MTU) minus TCP/IP headers (in practice<br />

default values are: MTU: 1500bytes, TCP/IP Header: 40bytes and <strong>the</strong>re<strong>for</strong>e<br />

MSS=1460bytes).<br />

T hroughput ∝∼ 0.7 ·<br />

MSS<br />

RT T · √ packet loss<br />

This problem can be tackled by increasing <strong>the</strong> frame size to about 8KByte<br />

(Chase et al., 2001), that is using so-called Jumbo Frames. Since <strong>the</strong> maximum<br />

frame size is set by switches in <strong>the</strong> Internet on <strong>the</strong> path from sender to receiver<br />

we cannot do anything about this. The second problem we face is that <strong>the</strong><br />

bandwidth delay (RTT values) can fluctuate quite wildly over lifetime as shown<br />

in Figure 5.3.6. Following (Thulasidasan et al., 2003) <strong>for</strong> <strong>the</strong> transmission part<br />

we have implemented dynamically adaptive TCP buffer sizes to allow TCP<br />

flow control to adapt to changing high-speed WAN environments, especially<br />

when transmitting large files. Figure 5.3.7 shows <strong>the</strong> effect <strong>of</strong> this tuning in a<br />

concrete example where this technique was used in GridFTP. It can be easily<br />

seen that per<strong>for</strong>mance approximately quadruplicates.<br />

User Management<br />

In <strong>the</strong> QAD Grid a user is modeled as a database object with <strong>the</strong> following<br />

properties:<br />

Account in<strong>for</strong>mation: This can be a local account or a network account.<br />

In both cases a user name is stored. In <strong>the</strong> local case an additional<br />

12 Size <strong>of</strong> <strong>the</strong> largest packet that a network protocol can transmit.

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