We train deep neural networks to solve the acoustic modeling problem for large-vocabulary continuous speech recognition. We employ distributed processing using a cluster of GPUs. On modern GPUs, the sequential implementation takes over a day to train, and efficient parallelization without losing accuracy is notoriously hard. We show that ASGD methods for parallelization are not efficient for this application. Even with 4 GPUs, the overhead is significant, and the accuracies achieved are poor. We adapt a P-learner K-step model averaging algorithm that with 4 GPUs achieves accuracies comparable to that achieved by the sequential implementation. We further introduce adaptive measures that make our parallel implementation scale to the full cluster of 20 GPUs. Ultimately our parallel implementation achieves better accuracies than the sequential implementation with a 6.1 times speedup.