Ratpack creates unlimited(*) thread-pool for blocking operations. It gets created in DefaultExecController:
public DefaultExecController(int numThreads) {
this.numThreads = numThreads;
this.eventLoopGroup = ChannelImplDetector.eventLoopGroup(numThreads, new ExecControllerBindingThreadFactory(true, "ratpack-compute", Thread.MAX_PRIORITY));
this.blockingExecutor = Executors.newCachedThreadPool(new ExecControllerBindingThreadFactory(false, "ratpack-blocking", Thread.NORM_PRIORITY));
}
Threads that are created in this pool don't get killed right after blocking operation is done - they are idling in the pool and waiting for the next job to do. The main reason behind it is that keeping thread in idle state is cheaper than spawning new threads when they are needed. That's why when you simulate 2500 concurrent users calling and endpoint which executes blocking operation, you will see 2500 threads in this pool. Cached thread-pool that gets created uses following ThreadPoolExecutor object:
public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
return new ThreadPoolExecutor(0, 2147483647, 60L, TimeUnit.SECONDS, new SynchronousQueue(), threadFactory);
}
where 2147483647 is maximum pool size, 60L is TTL expressed in seconds. It means that executor service will keep those threads for 60 seconds and when they don't get re-used after 60 seconds, it will clean them up.
High CPU in this case is actually expected. 2500 threads are utilizing a few cores of the CPU. It's also important - where your SQL database is running? If you run it on the same machine then your CPU has even harder job to do. If the operations you run on blocking thread-pool are consuming significant CPU time, then you have to optimize those blocking operations. Ratpack's power comes with async and non-blocking architecture - handlers use ratpack-compute thread-pool and delegate all blocking operations to ratpack-blocking so your application is not blocked and can handle tons of requests.
(*) unlimited in this case means limited by available memory or if you have enough memory it is limited by 2147483647 threads (this value is used in ExecutorService.newCachedThreadPool(factory)).
