I'm very new to Python and uWSGI. I'm trying to understand how uWSGI and threads work. I got confused by some of the statements in uWSGI documents. Example:
By default the Python plugin does not initialize the GIL. This means your app-generated threads will not run. If you need threads, remember to enable them with enable-threads. Running uWSGI in multithreading mode (with the threads options) will automatically enable threading support. This “strange” default behavior is for performance reasons, no shame in that.
I created a test project to see this in action, a very simple app that uses a ThreadExecutor pool where threads are not allowed by uWSGI ini file.
When you run the test, uWSGI logs that it starts with multiple processes and one core (wth?, I assume this is thread in their lingo?), it seems like threads are executing just fine.
My question is why is this working even when threads are NOT allowed explicitly in the uWSGI configuration? What's the downside of using threads in such context where threads are not allowed?
Related
Let's assume i have a nodejs serverProgram with one api and it does some manipulations on the video file, sent via the http request.
const saveVideoFile=(req,res)=>{
processAndSaveVideoFile(); // can run for minimum of 10 minutes
res.send({status: "video is being processed"})
}
i decided to to make use of a workerThread to do this processing as my machine has 3 cores (core1,core2,core3) and there is no hyperthreading enabled here
Assume that my nodejs program is running on core1. When i fire up a single workerThread, will the workerThread run on core2/core3 or core1?
i read that workerThread is not the same as childProcess. ChildProcess will fork a new process which will facilitate the childProcess to choose from available free cores (core2 or core3).
i read that workerThread shares memory with the mainThread. Let's assume that i create 2 workerThreads (wt1,wt2). Will my nodejs program, wt1, wt2 run on the same core i.e core1 ?
Also, in nodejs we have eventloop (mainthread) and otherThreads doing the background operations i.e I/O. is it correct to assume that all of these are utilizing the resources available in a single core (core1). if this is the case, is creating and using additional workerThread's an overkill on the nodejs server?
Below is an excerpt from this blog
We can run things in parallel in Node.js. However, we need not to
create threads. The operating system and the virtual machine
collectively run the I/O in parallel and the JS code then runs in a
single thread when it is time to send the data back to the JavaScript
code.
i keep reading this same information about nodejs in many articles and video presentations. But what i do not understand is this,
The operating system and the virtual machine collectively run the I/O in parallel
How can the operating system run the I/O requests from nodejs program in parallel without using any of the childProcess or threads spawned from nodejs? if those I/O requests from nodejs program is running in parallel, does it mean that all 3 cores (core1,core2,core3) will be utilized?
There are lot of contents on nodejs, but it doesn't clear doubts related to my above questions. if you have idea on how these things actually work, please share the detail.
A worker thread in node.js is an actual OS thread running in a different instance of V8. As such, it's totally up to the operating system to decide how to allocate it among available CPU cores. If there are cores with available time, then it will not generally be run on the same core as the main nodejs thread when that thread is busy because the OS will allocate busy threads across the various cores.
But, again this is entirely up to the OS and is not something that nodejs controls and the exact strategy for which cores are used will vary by OS. But, in all modern operating systems, the design goal is that available cores are used for threads that are currently executing. Now, if there are more threads active at once than there are cores, the threads will be time-sliced and all the cores will be active.
Also, in nodejs we have eventloop (mainthread) and otherThreads doing the background operations i.e I/O. is it correct to assume that all of these are utilizing the resources available in a single core (core1). if this is the case, is creating and using additional workerThread's an overkill on the nodejs server?
No, it is not correct to assume those threads all use the same core.
A workerThread in nodejs has its own event loop. For the most part, it does not share memory. In fact, if you want to share memory, you have to very specifically allocated SharedMemory and pass that to the workerThread.
Is it overkill? Well, it depends upon what you're doing. There are very useful things to do with workerThreads and there are things that they would not be necessary for.
The operating system and the virtual machine collectively run the I/O in parallel
I/O in node.js is either asynchronous at the OS level (such as networking) or run in separate threads (such as disk I/O). That means it runs separately from the main thread in node.js that runs your Javascript and can run in parallel with it, synchronizing only at the completion of an event. "Parallel" in this case means that both make progress at the same time. If there are multiple cores, then they can truly be running at exactly the same time. If there was only one core, then the OS will timeslice between the various threads and they will be both make progress (in an interleaved fashion that will seem to be parallel, but really they are taking turns).
How can the operating system run the I/O requests from nodejs program in parallel without using any of the childProcess or threads spawned from nodejs? if those I/O requests from nodejs program is running in parallel, does it mean that all 3 cores (core1,core2,core3) will be utilized?
The OS has its own threads for managing things like a network interface or a disk interface. The job of those threads is to interface with the hardware and bring data to an appropriate application or take data from the application and send it to the hardware. These are OS-level threads that exists independent of node.js. Yes, other cores can be used by those OS-level threads. It is important to realize that many operations such as networking are inherently non-blocking. Thus, if you're waiting for some data to arrive on a network interface, you don't need to have a thread doing something the whole time.
I want to add that it appears in your questions that you've combined questions about a several different things. Mentioned in your questions are:
Worker Threads
Internal node.js threads
Operating system threads
These are all different things.
A worker thread is a new thread you can start to run specific pieces of Javascript in another thread so you can have more than one Javascript thread running at the same time. In node.js, this is done by creating a whole new instance of V8, setting up a whole new global environment and loaded modules environment and using almost entirely separate memory.
Internal node.js threads are used by node.js as part of implementing its event loop and its standard library. Specifically, disk I/O and some crypto operations are run in internal native threads and they communicate with your Javascript via events/callbacks through the event loop.
Operating system threads are threads that the OS uses to implement it's own system APIs. Since the OS is responsible for lots of things, these threads ca have many different uses. Depending upon native implementations, they may be used to facilitate things like disk I/O or networking I/O. These threads are the responsibility of the OS to create and use and are not directly controlled by node.js.
Some additional questions asked in comments:
what is the difference b/w workerThread & childProcess concept in nodejs? is childProcess = workerThread without sharedMemory ?
A child process can be any type of program - it does not have to be a node.js program. A worker thread is node.js code.
A worker thread can share memory if sharedMemory is specifically allocated and shared with the worker thread and if it is carefully managed for concurrency issues.
It is more efficient to copy memory back and forth between worker thread and main thread than with child process.
If main program exits, worker threads will exit. If main program exits, child process can be configured to exit or to continue.
If worker thread calls process.exit(), the main thread will exit too. If child program exits, it cannot cause main program to exit without main program's cooperation.
how nodejs is able to magically interact with the os level thread without nodejs itself creating any threads?, i need additional details on this, your explanation is the common one present in most places including the blog i shared?
nodejs just calls an OS API. It's the OS API that manages communicating with its own threads (if threads are needed for that specific OS API). How it does that communication internally is implementation dependent and will vary by OS. It will even vary by OS which OS APIs use threads and which don't.
I have been reading about multi-processing on NodeJS to get the best understanding and try to get a good performance in heavy environments with my code.
Although I understand the basic purpose and concept for the different ways to take profit of the resources to handle the load, some questions arise as I go deeper and it seems I can't find the particular answers in the documentation.
NodeJS in a single thread:
NodeJS runs a single thread that we call event loop, despite in background OS and Libuv are handling the default worker pool for I/O asynchronous tasks.
We are supossed to use a single core for the event-loop, despite the workers might be using different cores. I guess they are sorted in the end by OS scheduler.
NodeJS as multi-threaded:
When using "worker_threads" library, in the same single process, different instances of v8/Libuv are running for each thread. Thus, they share the same context and communicate among threads with "message port" and the rest of the API.
Each worker thread runs its Event loop thread. Threads are supposed to be wisely balanced among CPU cores, improving the performance. I guess they are sorted in the end by OS scheduler.
Question 1: When a worker uses I/O default worker pool, are the very same
threads as other workers' pool being shared somehow? or each worker has its
own default worker pool?
NodeJS in multi-processing:
When using "cluster" library, we are splitting the work among different processes. Each process is set on a different core to balance the load... well, the main event loop is what in the end is set in a different core, so it doesn't share core with another heavy event loop. Sounds smart to do it that way.
Here I would communicate with some IPC tactic.
Question 2: And the default worker pool for this NodeJS process? where
are they? balanced among the rest of cores as expected in the first
case? Then they might be on the same cores as the other worker pools
of the cluster I guess. Shouldn't it be better to say that we are balancing main threads (event loops) rather than "the process"?
Being all this said, the main question:
Question 3: Whether is better using clustering or worker_threads? If both are being used in the same code, how can both libraries agree the best performance? or they
just can simply get in conflict? or at the end is the OS who takes
control?
Each worker thread has its own main loop (libuv etc). So does each cloned Node.js process when you use clustering.
Clustering is a way to load-balance incoming requests to your Node.js server over several copies of that server.
Worker threads are a way for a single Node.js process to offload long-running functions to a separate thread, to avoid blocking its own main loop.
Which is better? It depends on the problem you're solving. Worker threads are for long-running functions. Clustering makes a server able to handle more requests, by handling them in parallel. You can use both if you need to: have each Node.js cluster process use a worker thread for long-running functions.
As a first approximation for your decision-making: only use worker threads when you know you have long-running functions.
The node processes (whether from clustering or worker threads) don't get tied to specific cores (or Intel processor threads) on the host machine; the host's OS scheduling assigns cores as needed. The host OS scheduler minimize context-switch overhead when assigning cores to runnable processes. If you have too many active Javascript instances (cluster instances + worker threads) the host OS will give them timeslices according to its scheduling algorithms. Other than avoiding too many Javascript instances, there's very little point in trying second-guess the OS scheduler.
Edit Each Node.js instance, with any worker threads, uses a single libuv thread pool. A main Node.js process shares a single libuv thread pool with all its worker threads. If your Node.js program uses many worker threads, you may, or may not, need to set the UV_THREADPOOL_SIZE environment variable to a value greater than the default 4.
Node.js's cluster functionality uses the underlying OS's fork/exec scheme to create a new OS process for each cluster instance. So, each cluster instance has its own libuv pool.
If you're running stuff at scale, lets say with more than ten host machines running your Node.js server, then you can spend time optimizing Javascript instances.
Don't forget nginx if you use it as a reverse proxy to handle your https work. It needs some processor time too, but it uses fine-grain multithreading so you won't have to worry about it unless you have huge traffic.
I am aware that gunicorn allows for multiple worker processes and multiple threads within a process, in order to handle multiple requests simultaneously. That’s great.
What I’m trying to do, however, is within the context of handling one request. I want to spawn my own threads to allow for some parallel processing of different aspects of that request. This all works just fine if I run my program on its own – or in a unit test using Flask’s test client.
But when I try running under gunicorn (eventlet), I can still spawn those threads just fine, but they end up running in serial: the first one runs to conclusion, then the second one runs. Any idea of what might be going on?
I have client and server threads in my applications. When I run these apps as standalone apps, these threads communicate properly.
But when I run client as JUnit and server as standalone, client thread dies within few seconds.
I couldn't get, why such different behavior.
When the JUnit runner terminates, all spawned threads etc. are killed too (as it is most likely run in a separate JVM instance).
Here is a (rather old) article describing the problem you experienced (the GroboUtils library it is recommending seems to have been abandoned long time ago though). And another, recent one, with a more modern solution using the new Java concurrency framework.
The gist of the latter solution is that it runs the threads via an executor, which publishes the results of the runs via Futures. And Future.get is blocking until the thread finishes with the task, automatically keeping the JUnit tests alive. You may be able to adapt this trick to your case.
Is it possible that a program which does not kill its threads properly before exiting still be running some piece of code somewhere even though it has been killed in system monitor? I am running ubuntu in a non virtual environment. My application is made with QT, it contains QThreads, a main thread and concurent functions.
If you kill the process then you kill all its threads. The only cause for concern would be if your application had spawned multiple processes - if that is the case then you may still have code executing on the machine.
This is all very speculative though as I don't know what operating system you code is running on, whether or not your application runs in a virtual environment, etc. Environment-specific factors are very relevant to the discussion, can you share a bit more about your application?
It is not possible, all modern heaviliy used operating systems manage these resources quite tightly. Threads cannot run without a process... They are all brantches from the original thread.
I don't know of any OS that doesn't fully terminate all it's threads when you kill the processes, it's possible to spawn child processes that live on after the main process has exited but in the case of threads i'd say it's not possible.