This is related to the previous question WebAssembly in async code
Basically, that question is about the problem of the WebAssembly blocking the main thread, and the answer to the question is to move the WebAssembly code to a web worker. That works.
The problem now is that the WebAssembly blocks the onmessage() on the worker.
My long running WebAssembly code has functions like play(), pause(), stop(), etc. The play() checks a pause flag and a stop flag periodically to determine if the play() should return. The pause() and the stop() are used to set those flags.
The JavaScript main thread calls postMessage() to send a message to the worker, which further calls the play().
Since the onmessage() is blocked, the worker will have no chance to receive further messages to do pause() or stop() until the play() is completed. That will defeat the very purposes of the pause/stop.
It seems the simple use case of play/pause/stop cannot be supported by the WebAssembly.
Any comments or suggestions?
By the way, that use case is well supported by the defunct Google PNaCl.
Thanks.
In short: Web worksers do not ignore messages even if the web worker thread is blocked.
All browsers events, including web worker postMessage()/onmessage() events are queued. This is the fundamental philosophy of JavaScript (onmessage() is done in JS even if you use WebAssembly). Have a look at "Concurrency model and Event Loop" from MDN for further detail.
So what going to happen in your case is, while onmessage() is blocked, the events from main thread postMessage() are queued automatically. When a single onmessage() job is finished in the worker thread, from the worker event queue, will check if postMessage() is called before it finishes and catch the message if there is. So you don't need to worry about that case as long as the onmessage() job takes like 10 seconds and the you get hundreds of events in the queue.
This is how asynchronous execution is done everywhere in the browser.
Considering you are targeting recent browsers (WebAssembly), you can most likely rely on SharedArrayBuffer and Atomics. Have a look at these solutions Is it possible to pause/resume a web worker externally? , which in your case will need to be handled inside WebAssembly (Atomics.wait part)
Related
I was reading the documentation of Microsoft specifically the Async programming article and I didn't understand this section while he is explaining the work of the server's threads when using Async code.
because it(The server) uses async and await, each of its threads is freed up when the I/O-bound work starts, rather than when it finishes.
Could anyone help what does it mean by the threads r freed up when the I/O starts??
Here is the article : https://learn.microsoft.com/en-us/dotnet/standard/async-in-depth
When ASP.NET gets an HTTP request, it takes a thread from the thread pool and uses that to execute the handler for that request (e.g., a specific controller action).
For synchronous actions, the thread stays assigned to that HTTP request until the action completes. For asynchronous actions, the await in the action method may cause the thread to return an incomplete task to the ASP.NET runtime. In this case, ASP.NET will free up the thread to handle other requests while the I/O is in flight.
Further reading about the difference between synchronous and asynchronous request handling and how asynchronous work doesn't require a thread at all times.
When your application makes a call to an external resource like Database or HttpClient thread, that initiated connection needs to wait.
Until it gets a response, it waits idly.
In the asynchronous approach, the thread gets released as soon as the app makes an external call.
Here is an article about how it happens:
https://medium.com/#karol.rossa/asynchronous-programming-73b4f1988cc6
And performance comparison between async and sync apporach
https://medium.com/#karol.rossa/asynchronous-performance-1be01a71925d
Here's an analogy for you: have you ever ordered at a restaurant with a large group and had someone not be ready to order when the waiter came to them? Did they bring in a different waiter to wait for him or did the waiter just come back to him after he took other people's orders?
The fact that the waiter is allowed to come back to him later means that he's freed up immediately after calling on him rather than having to wait around until he's ready.
Asynchronous I/O works the same way. When you do a web service call, for example, the slowest part (from the perspective of the client at least) is waiting for the result to come back: most of the delay is introduced by the network (and the other server), during which time the client thread would otherwise have nothing to do but wait. Async allows the client to do other things in the background.
I'd like to start using asynchronous processing in IIS. Edit: I'm talking about using the task parallel library.
For example, on certain page loads I want to log a bunch of crap, send an email, update some tables, etc. But I don't want to make the user wait for me to log all that crap.
So normally what I do is I have a static Queue that I push the log info onto, and then I have a cron job that calls a special page every 10 minutes whose OnLoad flushes out the queue. This works, but it's kind of clunky to setup, especially when you want to log 50 things. I'd rather do this:
Task.CreateNew(() => Log(theStuff));
However I'm terrified of running tasks in IIS because one slip up and your entire website goes down.
So now I have
SafeTask.FireAndForget(() => Log(theStuff));
This wraps the delegate in some try/catch and passes it into Task.CreateNew. So if someone changes something that affects something else that generates an exception somewhere else that accidentally gets thrown on the task thread, we get a notification instead of a crashed website. Also, the error notification inside the catch is also inside its own try/catch, and the catch for that also has a try/catch that tries to log in a different way.
Now that I can safely run stuff asynchronously in IIS, what other things do I need to worry about before I can start using my SafeTask class?
Every request in IIS and .net is processed in one thread by default. This thread comes from a thread pool called the "Application Pool". Existing threads are reused so you can't really use them for thread state unless you clear or set it every time. You define the size of this thread pool using a formula from MSDN in the machine.config or even your web.config.
Now, every async function call is put on a different thread. This includes async web service calls, async page functions, async delegates, etc. This thread comes from the "application pool" thus reducing the number of thread available for IIS to service new requests.
Most likely, your application will work just fine while using async function calls. In case you are worried or you have a lot of async tasks then you may want to create your own thread pool or look at SmartThreadPool on codeplex.
Hope this helps.
Consider using the page's OnUnload event. Read about it here: http://msdn.microsoft.com/en-us/library/ms178472.aspx
This event fires after the content is sent to the user (so the user isn't blocked while you do work), and should completely satisfy your requirement without introducing additional threads.
Specific to your question, you should be concerned about thread pool exhaustion only if your load and performance testing suggests you're running up against thread limits. If you're not then what you propose is certainly reasonable.
I'm not talking about asynchronous pages or asynchronous handlers, I just want to know if I should be afraid of any side effect when I invoke an asynchronous method that will end after the page is finished rendering.
Example given: Each time that a user do login, I have to launch a heavy and time consuming SQL operation, but the user doesn't need to know the result of that operation, so I can execute the query using BeginExecuteNonQuery without pass any callback, and finish rendering the page.
My concern is, what happen if the HTTP call ends (because the page is served) and whatever I've executed asynchronously is already running? is ASP.NET or IIS going to cut, destroy, void anything?
Cheers.
That operation will run, even when the request has finished. However, please note that the ASP.NET host aggressively kills threads. When IIS has any reason for unloading or recycling the AppDomain, your background thread will be killed. Unloading happens in several situations. For instance when no new requests have come in for a certain period of time. Or when too many exceptions are fired from the application within a certain period of time. Or when the memory pressure gets too high.
If you need the guarantee, that the operation will finish, I think there are three things you can do:
Speed up the operation so that it can run synchronously, or
Move that that heavy operation to a Windows Service and let that execute it, or
You can hook onto the HostingEnvironment.RegisterObject method (as Phill Haack explains here) (demands full trust) to prevent the AppDomain to go down while that thread is running.
If you have a callback registered, the process will comeback to notify the callback otherwise it will still complete the job. AFAIK - neither ASP.NET or IIS will cut/destroy or void anything as the execution was already ordered and it has to complete.
What is the difference between Web Services Asynchronous Call and Asynchronous Task's.
We are working an a ASP.NET application that requires to make a call to a Web Service Method that will process thousand rows of data. This process usually takes between 2 to 3 minutes (maybe more maybe less it depends of the amount of Data). So we run all the time in Timeout's on that specific page.
So we decided to go in rout of calling this Web Service Method Asynchronously, but we had a conflict caused by HTTP handler of one of the UI component's that we are using. Well lucky on that case we could remove the page from the httphandler directives.
So far no issues, but here it comes the question, a coworker find out that we can use instead of Asynchronous Webs Services Call, wrap a Synchronous call in a Asynchronous Task in the ASP.NET page and be able to keep the directives to the component, and execute the Web Service Method with out getting a Timeout.
So now my concern is what kind of issues we can find using Asynchronous Task's instead of an Asynchronous Call.
Thank you in advance.
Web services should not be used in this manner by the way. There's a reason HTTP timeouts are so low. You should have the Web service trigger the task either by setting a flag in the DB that an actual service picks up on or the web service should spawn a process.
If I understand your scenario, there should be no issues. In both cases, your page is asynchronous. In both cases, you don't wait for the service to complete - you give up the request thread while the service is running. In both cases, your page takes the same amount of time to execute as it would if you had called the service synchronously.
Is it possible to use BackGroundWorker thread in ASP.NET 2.0 for the following scenario, so that the user at the browser's end does not have to wait for long time?
Scenario
The browser requests a page, say SendEmails.aspx
SendEmails.aspx page creates a BackgroundWorker thread, and supplies the thread with enough context to create and send emails.
The browser receives the response from the ComposeAndSendEmails.aspx, saying that emails are being sent.
Meanwhile, the background thread is engaged in a process of creating and sending emails which could take some considerable time to complete.
My main concern is about keeping the BackgroundWorker thread running, trying to send, say 50 emails while the ASP.NET workerprocess threadpool thread is long gone.
If you don't want to use the AJAX libraries, or the e-mail processing is REALLY long and would timeout a standard AJAX request, you can use an AsynchronousPostBack method that was the "old hack" in the .net 1.1 days.
Essentially what you do is have your submit button begin the e-mail processing in an asynchronous state, while the user is taken to an intermediate page. The benefit to this is that you can have your intermediate page refresh as much as needed, without worrying about hitting the standard timeouts.
When your background process is complete, it will put a little "done" flag in the database/application variable/whatever. When your intermediate page does a refresh of itself, it detects this flag and automatically redirects the user to the "done" page.
Again, AJAX makes all of this moot, but if for some reason you have a very intensive or timely process that has to be done over the web, this solution will work for you. I found a nice tutorial on it here and there are plenty more out there.
I had to use a process like this when we were working on a "web check-in" type application that was interfacing with a third party application and their import API was hideously slow.
EDIT: GAH! Curse you Guzlar and your god-like typing abilities 8^D.
You shouldn't do any threading from ASP.NET pages. Any thread that is long running is in danger of being killed when the worker process recycles. You can't predict when this will happen. Any long-running processes need to be handled by a windows service. You can kick off these processes by dropping a message in MSMQ, for example.
ThreadPool.QueueUserWorkItem(delegateThatSendsEmails)
or on System.Net.Mail.SmtpServer use the SendAsync method.
You want to put the email sending code on another thread, because then it will return the the user immediately, and will just process, no matter how long it takes.
It is possible. Once you start a new thread asynchronously from page, page request will proceed and send the page back to the user. The async thread will continue to run on the server but will no longer have access to the session.
If you have to show task progress, consider some Ajax techniques.
What you need to use for this scenario is Asynchronous Pages, a feature that was added in ASP.NET 2.0
Asynchronous pages offer a neat
solution to the problems caused by
I/O-bound requests. Page processing
begins on a thread-pool thread, but
that thread is returned to the thread
pool once an asynchronous I/O
operation begins in response to a
signal from ASP.NET. When the
operation completes, ASP.NET grabs
another thread from the thread pool
and finishes processing the request.
Scalability increases because
thread-pool threads are used more
efficiently. Threads that would
otherwise be stuck waiting for I/O to
complete can now be used to service
other requests. The direct
beneficiaries are requests that don't
perform lengthy I/O operations and can
therefore get in and out of the
pipeline quickly. Long waits to get
into the pipeline have a
disproportionately negative impact on
the performance of such requests.
http://msdn.microsoft.com/en-us/magazine/cc163725.aspx
If you want using multitheading in your ASP page, you might using simple threading model like this:
{
System.Threading.Thread _thread = new Thread(new ThreadStart(Activity_DoWork));
_thred.Start();
}
Activity_DoWork()
{
/*Do some things...
}
This method is correct working with ASP pages. The ASP page with BackgroundWorker will not start while BackgroundWorker will finish.
5 years later, but problems the same… If you want to perform fire-and-forget operations from your application and forget about all difficulties related to background job processing in ASP.NET applications, you can use http://hangfire.io.
It does not loose your jobs on recycling process, because it uses persistent storage to keep information about background jobs.
It automatically retries your background jobs that were aborted or failed due to transient exception (SMTP Server connectivity errors).
It allows you to easily debug background jobs through the integrated web interface.
It is very easy to install/configure/use HangFire.
There is also tutorial Sending Mail in Background with ASP.NET MVC for using HangFire with Postal.