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The following page has been changed by SriranjanManjunath: http://wiki.apache.org/pig/PigSampler ------------------------------------------------------------------------------ Since the frequency distribution of keys in the input is highly skewed, the underlying data can be modeled using a poisson distribution. The skewed join sampler tries to identify the keys that are too big to fit in memory and allocates reducers to those skewed keys. Given an input file of size N, we need to estimate the number of samples which represents this input. The main purpose of a skewed join sampler is to come up with a reducer allocation map of the skewed keys. A custom slicer is used to estimate the number of maps that are required to run the sampler job. Although, the number of partitions provides us a base number of samples for the input, for an uniformly distributed random samples, we may be under sampling the data. Hence, we use a Poisson cumulative distribution function to estimate the total number of samples that are required to represent the underlying data. - - For an 1TB file running on nodes which have 512 MB of memory, assuming a conversion factor of 2, the number of base samples turn out to be 4000. To compute the base number of samples, we calculate the number of partitions of the file based on the amount of memory available and file size. For a 1TB file running on nodes having 512 mb of memory each:
