After adding some simple logging in TestLauncher I observed that sometime it may run more jobs in parallel than the value specified with --test-launcher-jobs. The --test-launcher-jobs value is passed to TaskScheduler as max_tasks for number of blocking tasks in TaskScheduler, but max_tasks doesn't really limit number of tasks executed in parallel. It limits number of tasks "that aren't blocked", see https://codesearch.chromium.org/chromium/src/base/task/task_scheduler/scheduler_worker_pool_params.h?type=cs&sq=package:chromium&g=0&l=16 

This issue caused  bug 913156 .