The implementations of Poission regression in mllib/regression. It includes 1)the gradient of the negative log-likelihood, 2)the implementation of PoissonRegressionModel, the generalized linear algorithm class which uses L-BFGS and SGD for parameter estimation respectively, 3) the test suites for the gradient/loss computation, the regression method on generated and real-world data set, and 4) a Poisson regression data generator in mllib/util for producing the test data.