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