It blocks the caller attempting to add the task until theres room in queue, applying back pressure. It does not reject it. It mimics the behavior from pre-SEP DebuggableThreadPoolExecutor's RejectionExecutionHandler that the other thread pools use (exception on sampling/trace which just throw away on rejections).
Worth noting this is only really possible in the native transport pool (sep pool) last I checked. Since 2.1 at least, before that there were a few others. That changes version to version. For (basically) all other thread pools the queue is limited by memory. Chris > On Mar 22, 2018, at 10:44 PM, John Sanda <john.sa...@gmail.com> wrote: > > I have been doing some work on a cluster that is impacted by > https://issues.apache.org/jira/browse/CASSANDRA-11363 > <https://issues.apache.org/jira/browse/CASSANDRA-11363>. Reading through the > ticket prompted me to take a closer look at > org.apache.cassandra.concurrent.SEPExecutor. I am looking at the 3.0.14 code. > I am a little confused about the Blocked and All Time Blocked columns > reported in nodetool tpstats and reported by StatusLogger. I understand that > there is a queue for tasks. In the case of RequestThreadPoolExecutor, the > size of that queue can be controlled via the > cassandra.max_queued_native_transport_requests system property. > > I have been looking at SEPExecutor.addTask(FutureTask<?> task), and here is > my question. If the queue is full, as defined by SEPExector.maxTasksQueued, > are tasks rejected? I do not fully grok the code, but it looks like it is > possible for tasks to be rejected here (some code and comments omitted for > brevity): > > public void addTask(FutureTask<?> task) > { > tasks.add(task); > ... > else if (taskPermits >= maxTasksQueued) > { > WaitQueue.Signal s = hasRoom.register(); > > if (taskPermits(permits.get()) > maxTasksQueued) > { > if (takeWorkPermit(true)) > pool.schedule(new Work(this)) > > metrics.totalBlocked.inc(); > metrics.currentBlocked.inc(); > s.awaitUninterruptibly(); > metrics.currentBlocked.dec(); > } > else > s.cancel(); > } > } > > The first thing that happens is that the task is added to the tasks queue. > pool.schedule() only gets called if takeWorkPermit() returns true. I am still > studying the code, but can someone explain what exactly happens when the > queue is full? > > > - John
