I have a KeyedProcessFunction implementation that override "processElement()" and "onTimer()" methods.
Are they threads safe to each other?
Seems that the onTimer() method is called by a TimerService thead pool. So it looks like to me they could be executed in the same time.
processElement and onTimer are never executed in parallel.
Prior to Flink 1.10, both methods would be invoked after acquiring the checkpoint lock.
Since Flink 1.10, they are only invoked through the same task thread, so it's even safe to access fields in a non-synchronized fashion (even if using non-managed fields is discouraged to begin with).
Related
The project I am working for is using Spring WebFlux. I came across a very odd issue.
The detail is that some of pieces of code are purely wrote in Reactor style (couples of Flux/Mono pipelines), however, in a inner publishers, I have to call a method where there is "Mono.block()" inside.
The weird thing I aware is that the whole service would become totally stuck, and when I captured a thread dump, I saw all those "nioEventLoopGroup-*" threads were hung.
A fun fact is that if I leverage a "simple" thread (new Thread(..)) to call the method (there is .block inside), everything works fine.
So my question is that, are those "nioEventLoopGroup-*" threads not allowed to call any blocking code.
Sorry for asking a dumb question, but it's blocking issue for now, so I am looking forward your insight.
Reactor, by default, uses a fixed size thread pool. When you use block(), the actual work needs to be done in some thread or another, which depends on the nature of the subscription and the Mono/Flux. Most likely a set of new tasks will be scheduled on the same scheduler, but block() will suspend its thread, waiting for those tasks to complete, so there is one fewer thread for those other tasks to be scheduled on. Evidently you have enough of these calls to exhauast the entire thread pool. All your block() calls are waiting for other tasks to complete, but there are no threads available for them to be run on.
There's no reason to call block() inside a mapping in a reactive stream. There are always other ways of achieving the same goal without blocking - flatMap(), zip() etc etc.
Nothing seems to prevent a perform method to use the Sidekiq API. It should be safe in read-only mode.
What if it calls a "write" methods ? Especially when this method acts on the current job itself.
We would like to reschedule a job without creating a new job because we need to track the job completion with the sidekiq-status gem from another worker.
Using MyWorker.perform_in or MyWorker.perform_at to reschedule the job from inside the worker creates a new job, making it difficult to track the total completion. We're thinking of using Sidekiq::ScheduledSet.new.find and the reschedule method but it seems awkward and potentially dangerous to reschedule a job that is about to complete.
Does Sidekiq and its API support this use case ?
You might be able to hack something together but it'll be really slow if you try to modify the Sets and Lists in Redis directly. They aren't designed to be used that way.
The official Sidekiq solution to this problem is a Batch.
https://github.com/mperham/sidekiq/wiki/Batches#status
You create a one-job batch. If the job needs to be rescheduled, it adds a new job to the Batch to be executed later. Your other worker just checks the status of the overall Batch and if it is 100% complete.
I've noticed that if I create an NSURLConnection and fire the request, all is well. My delegate methods get called and the last delegate method get's called well after the code block invoking the connection completes. Great.
That leads me to believe the connections are asynchronous which implies that they're multi-threaded. Is that correct? Could they be asynchronous but in the same thread? No, that's crazy - right?
But, in every example I've seen using an NSOperation, NSURLConnections are always scheduledInRunLoop after which [runLoop runMode ...] is invoked in a while loop.
Can someone explain exactly what is happening here? It seems to me that the first case requires spawning secondary threads but no manual invocation of the run loop (on those threads) while NSOperation (in a new thread) does require manual invocation of the run loop.
Why is no manual invocation required for the first case?
NSURLConnection does spawn a single background thread to manage all instances of itself, but this is generally irrelevant to the caller, since the delegate calls are made on whatever thread owns the runloop the connection was scheduled in. (This fact turned out to be very relevant to me recently, but these things really only come up when dealing with insane crashers in multi-threaded apps.)
For more caller-relevant details, you should look at the docs for -[NSURLConnection scheduleInRunLoop:forMode:]. It explains how to manually handle scheduling and unscheduling, and how NSURLConnections behave in a multi-threaded environment.
If you are unclear on how run loops work and how they perform asynchronous actions without requiring additional threads, you should read Run Loops in the Threading Programming Guide. This is a very important topic for moving to more advanced Cocoa development.
Because the main thread already has a run loop, I'd imagine.
If you want to run NSURLConnection in another thread, you should create a run loop like this in your thread's main method:
while (!finished)
{
[[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:1]];
}
I'm trying to learn about threading and I'm thoroughly confused. I'm sure all the answers are there in the apple docs but I just found it really hard to breakdown and digest. Maybe somebody could clear a thing or 2 up for me.
1)performSelectorOnMainThread
Does the above simply register an event in the main run loop or is it somehow a new thread even though the method says "mainThread"? If the purpose of threads is to relieve processing on the main thread how does this help?
2) RunLoops
Is it true that if I want to create a completely seperate thread I use
"detachNewThreadSelector"? Does calling start on this initiate a default run loop for the thread that has been created? If so where do run loops come into it?
3) And Finally , I've seen examples using NSOperationQueue. Is it true to say that If you use performSelectorOnMainThread the threads are in a queue anyway so NSOperation is not needed?
4) Should I forget about all of this and just use the Grand Central Dispatch instead?
Run Loops
You can think of a Run Loop to be an event processing for-loop associated to a thread. This is provided by the system for every thread, but it's only run automatically for the main thread.
Note that running run loops and executing a thread are two distinct concepts. You can execute a thread without running a run loop, when you're just performing long calculations and you don't have to respond to various events.
If you want to respond to various events from a secondary thread, you retrieve the run loop associated to the thread by
[NSRunLoop currentRunLoop]
and run it. The events run loops can handle is called input sources. You can add input sources to a run-loop.
PerformSelector
performSelectorOnMainThread: adds the target and the selector to a special input source called performSelector input source. The run loop of the main thread dequeues that input source and handles the method call one by one, as part of its event processing loop.
NSOperation/NSOperationQueue
I think of NSOperation as a way to explicitly declare various tasks inside an app which takes some time but can be run mostly independently. It's easier to use than to detach the new thread yourself and maintain various things yourself, too. The main NSOperationQueue automatically maintains a set of background threads which it reuses, and run NSOperations in parallel.
So yes, if you just need to queue up operations in the main thread, you can do away with NSOperationQueue and just use performSelectorOnMainThread:, but that's not the main point of NSOperation.
GCD
GCD is a new infrastructure introduced in Snow Leopard. NSOperationQueue is now implemented on top of it.
It works at the level of functions / blocks. Feeding blocks to dispatch_async is extremely handy, but for a larger chunk of operations I prefer to use NSOperation, especially when that chunk is used from various places in an app.
Summary
You need to read Official Apple Doc! There are many informative blog posts on this point, too.
1)performSelectorOnMainThread
Does the above simply register an event in the main run loop …
You're asking about implementation details. Don't worry about how it works.
What it does is perform that selector on the main thread.
… or is it somehow a new thread even though the method says "mainThread"?
No.
If the purpose of threads is to relieve processing on the main thread how does this help?
It helps you when you need to do something on the main thread. A common example is updating your UI, which you should always do on the main thread.
There are other methods for doing things on new secondary threads, although NSOperationQueue and GCD are generally easier ways to do it.
2) RunLoops
Is it true that if I want to create a completely seperate thread I use "detachNewThreadSelector"?
That has nothing to do with run loops.
Yes, that is one way to start a new thread.
Does calling start on this initiate a default run loop for the thread that has been created?
No.
I don't know what you're “calling start on” here, anyway. detachNewThreadSelector: doesn't return anything, and it starts the thread immediately. I think you mixed this up with NSOperations (which you also don't start yourself—that's the queue's job).
If so where do run loops come into it?
Run loops just exist, one per thread. On the implementation side, they're probably lazily created upon demand.
3) And Finally , I've seen examples using NSOperationQueue. Is it true to say that If you use performSelectorOnMainThread the threads are in a queue anyway so NSOperation is not needed?
These two things are unrelated.
performSelectorOnMainThread: does exactly that: Performs the selector on the main thread.
NSOperations run on secondary threads, one per operation.
An operation queue determines the order in which the operations (and their threads) are started.
Threads themselves are not queued (except maybe by the scheduler, but that's part of the kernel, not your application). The operations are queued, and they are started in that order. Once started, their threads run in parallel.
4) Should I forget about all of this and just use the Grand Central Dispatch instead?
GCD is more or less the same set of concepts as operation queues. You won't understand one as long as you don't understand the other.
So what are all these things good for?
Run loops
Within a thread, a way to schedule things to happen. Some may be scheduled at a specific date (timers), others simply “whenever you get around to it” (sources). Most of these are zero-cost when idle, only consuming any CPU time when the thing happens (timer fires or source is signaled), which makes run loops a very efficient way to have several things going on at once without any threads.
You generally don't handle a run loop yourself when you create a scheduled timer; the timer adds itself to the run loop for you.
Threads
Threads enable multiple things to happen at the exact same time on different processors. Thing 1 can happen on thread A (on processor 1) while thing 2 happens on thread B (on processor 0).
This can be a problem. Multithreaded programming is a dance, and when two threads try to step in the same place, pain ensues. This is called contention, and most discussion of threaded programming is on the topic of how to avoid it.
NSOperationQueue and GCD
You have a thing you need done. That's an operation. You can't have it done on the main thread, or you'd simply send a message like normal; you need to run it in the background, on a secondary thread.
To achieve this, express it as either an NSOperation object (you create a subclass of NSOperation and instantiate it) or a block (or both), then add it to either an NSOperationQueue (NSOperations, including NSBlockOperation) or a dispatch queue (bare block).
GCD can be used to make things happen on the main thread, as well; you can create serial queues and add blocks to them. A serial queue, as its name suggests, will run exactly one block at a time, rather than running a bunch of them in parallel.
So what should I do?
I would not recommend creating threads directly. Use NSOperationQueue or GCD instead; they force you into better thinking habits that will reduce the risk of your threaded code inducing headaches.
For things that run periodically, not fitting into the “thing I need done” model of NSOperations and GCD blocks, consider just using the run loop on the main thread. Chances are, you don't need to put it on a thread after all. A rendering loop in a 3D game, for example, can be a simple timer.
I've read the documentation for ReadDirectoryChangesW() and also seen the CDirectoryChangeWatcher project, but neither say why one would want to call it asynchronously. I understand that the current thread will not block, but, at least for the CDirectoryChangeWatcher code that uses a completion port, when it calls GetQueuedCompletionStatus(), that thread blocks anyway (if there are no changes).
So if I call ReadDirectoryChangesW() synchronously in a separate thread in the first place that I don't care if it blocks, why would I ever want to call ReadDirectoryChangesW() asynchronously?
When you call it asynchronously, you have more control over which thread does the waiting. It also allows you to have a single thread wait for multiple things, such as a directory change, an event, and a message. Finally, even if you're doing the waiting in the same thread that set up the watch in the first place, it gives you control over how long you're willing to wait. GetQueuedCompletionStatus has a timeout parameter that ReadDirectoryChangesW doesn't offer by itself.
You would call ReadDirectoryChangesW such that it returns its results asynchronously if you ever needed the calling thread to not block. A tautology, but the truth.
Candidates for such threads: the UI thread & any thread that is solely responsible for servicing a number of resources (Sockets, any sort of IPC, independent files, etc.).
Not being familiar with the project, I'd guess the CDirectoryChangeWatcher doesn't care if its worker thread blocks. Generally, that's the nature of worker threads.
I tried using ReadDirectoryChanges in a worker thread synchronously, and guess what, it blocked so that the thread wouldn't exit by itself at the program exit.
So if you don't want to use evil things like TerminateThread, you should use asynchronous calls.