1. A computer-implemented method for identifying failure in a tree network of a parallel computer, the parallel computer comprising one or more processing sets including an I/O node and a plurality of compute nodes; and the method comprising for each processing set: selecting a set of test compute nodes, the test compute nodes being a subset of the compute nodes of the processing set; measuring the performance of the I/O node of the processing set; measuring the performance of the selected set of test compute nodes; calculating a current test value in dependence upon the measured performance of the I/O node of the processing set, the measured performance of the set of test compute nodes, and a predetermined value for I/O node performance; comparing the current test value with a predetermined tree performance threshold; and if the current test value is below the predetermined tree performance threshold, selecting another set of test compute nodes; and if the current test value is not below the predetermined tree performance threshold, selecting from the test compute nodes one or more potential problem nodes and testing individually potential problem nodes and links to potential problem nodes.