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difx:difx2.0baselbased [2009/10/29 07:37]
adamdeller created
difx:difx2.0baselbased [2015/10/21 10:08] (current)
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 ====== Baseline-based efficiency improvements in DiFX2.0 ====== ====== Baseline-based efficiency improvements in DiFX2.0 ======
  
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 The following diagram illustrates what I mean: The following diagram illustrates what I mean:
  
-{{baselinebased_after.png}}+{{baselinebased_after.png?800}}
  
 Say you buffer 10 FFTs at each station. ​ That means you only get a cache miss for 1 of every 10 cycles, rather than every single cycle. ​ This is little enough to make the cache missing a relatively small addition to the total processing increase. ​ (There are unavoidable non-linear increases in processing cost going to either more stations or more channels. ​ With more stations, you have more cross-multiplications to do, since there are more baselines. ​ This starts to get significant at around 10-15 stations. ​ With more channels, the cost of the FFT is going up with log N.  Since the FFT is now dominating in DiFX2.0, that means going from 128 channels to 2048 [x16, = x4 cost to FFT] is likely to hit you pretty substantially - a factor of 2-3 at least in station-based cost.) Say you buffer 10 FFTs at each station. ​ That means you only get a cache miss for 1 of every 10 cycles, rather than every single cycle. ​ This is little enough to make the cache missing a relatively small addition to the total processing increase. ​ (There are unavoidable non-linear increases in processing cost going to either more stations or more channels. ​ With more stations, you have more cross-multiplications to do, since there are more baselines. ​ This starts to get significant at around 10-15 stations. ​ With more channels, the cost of the FFT is going up with log N.  Since the FFT is now dominating in DiFX2.0, that means going from 128 channels to 2048 [x16, = x4 cost to FFT] is likely to hit you pretty substantially - a factor of 2-3 at least in station-based cost.)
difx/difx2.0baselbased.1256762277.txt.gz · Last modified: 2009/10/29 07:37 by adamdeller