Straggler tasks continue to be a major hurdle in achieving faster completion of data intensive applications running on modern data-processing frameworks. Existing straggler mitigation techniques are inefficient due to their reactive and replicative nature – they rely on a wait-speculate-reexecute mechanism, thus leading to delayed straggler detection and inefficient resource utilization. Existing proactive techniques also over-utilize resources due to replication. Existing modeling-based approaches are hard to rely on for production-level adoption due to modeling errors. We present Wrangler, a system that proactively avoids situations that cause stragglers. Wrangler automatically learns to predict such situations using a statistical learning technique based on cluster resource utilization counters. Furthermore, Wrangler introduces a notion of a confidence measure with these predictions to overcome the modeling error problems; this confidence measure is then exploited to achieve a reliable task scheduling. In particular, by using these predictions to balance delay in task scheduling against the potential for idling of resources, Wrangler achieves a speed up in the overall job completion time. For production-level workloads from Facebook and Cloudera’s customers, Wrangler improves the 99th percentile job completion time by up to 61% as compared to speculative execution, a widely used straggler mitigation technique. Moreover, Wrangler achieves this speed-up while significantly improving the resource consumption (by up to 55%).