Good morning,
I would like to use a TThread object to process numeric values.
On a recurring basis (through the TTimer object), different/updated values are always presented in processing.
Basically:
The first time I create and launch the TThread which is actually executed but not deleted.
Subsequently (by TTimer), I get new values for TThread to process.
Request:
Is there a way to "restart" the TThread with the new values, without creating a new TThread object every time? (TThread already exists)
This would save time, since the values would always use the space allocated in the first TThread creation.
A TThread (or any other kind of thread wrapper, for that matter) cannot be "restarted" once it has finished running. All you can do is free it.
So, to do what you are asking for, you would need to make the thread's Execute() method run a loop that processes your numeric values on each iteration as needed, until you are ready to signal the thread to terminate itself.
You will have to implement your own thread-safe system to push new numerics into the thread when needed, and to make the thread wait for new numerics to arrive between each loop iteration. For instance, you could push values into a TThreadList or TThreadedQueue, signaling a waitable TEvent or TMonitor on each push, and then the thread loop can wait on that signal before pulling values on each iteration.
Otherwise, consider using TTask instead of TThread. Tasks can utilize thread pooling internally, so you can just create a new TTask each time you have a new numeric to process, and let it pull out an available thread from the pool, and put the thread back in the pool when finished.
Related
I'm using IOmniParallelJoin to compute the several tasks in parallel with NoWait function because I want the GUI to stay responsive. But I also need to know when the computation is finished. Is there any event which is triggered in such case?
You can either use the OnStop function to inject some code or use a Task Configuration via TaskConfig and assign the code via OnTerminated. The difference is that OnStop is called inside one of the worker threads while OnTerminated is called inside the main thread.
I have recently played around with one demo opensource project for the basic functionality of the INDY10 TCP/IP server and stumbled upon the problem of internal multitasking implementation of INDY and its interaction with VCL components. Since there are many different topics in SO on the subject, I decided to make a simple client-server application and test some of the solutions and approaches suggested, at least the ones that I understood correctly. Below I would like to summarize and review an approach that was previously suggested on SO, and if possible listen to your expert opinion on the subject.
Problem: Encapsulation the VCL for thread-safe usage inside an indy10-based client/server application.
Description of the Development Env.:
Delphi Version: Delphi® XE2 Version 16.0
INDY Version 10.5.8.0
O.S. Windows 7 (32Bit)
As mentioned in the article ([ Is the VCL Thread-safe?]) (sorry I do not have enough reputation to post the link) special care should be taken when one wishes to use any kind of VCL components inside a multithreaded (multitasking) application. VCL is not thread safe, but can be used in a thread safe way!
The how and the why usually depend on the application at hand but one can attempt to generalize a bit and suggest some kind of general approach to this problem. First of all, as in the case of INDY10, one does not need to be explicitly parallelizing his code, i.e. create and execute multiple threads, in order to expose VCL to deadlocks and data inter dependencies.
In every sclient-server application, the server has to be able to handle multiple requests simultaneously, so naturally, INDY10 internally implements this functionality. This would mean that the INDY10 set of classes are responsible to manage the program's thread creation, execution and destruction procedures internally.
The most obvious place where our code is exposed to the inner workings of INDY10 and hence possible thread conflicts, is the IdTCPServerExecute (TIdTCPServer onExecute event) method.
Naturally, INDY10 provides classes (wrappers) that ensure thread-safe program flow, but since I did not manage to get enough explanation on their application and usage, I prefer a custom made approach.
Below I summarize a method ( the suggested technique is based on a previous comment I found in SO How to use TIdThreadSafe class from Indy10 ) that attempts (and presumably succeeds) in dealing with this problem:
The question I tackle below is: How to make a specific class "MyClass" ThreadSafe?
The main idea is to create kind of a wrapper class that encapsulates "MyClass" and queues the threads that try to access it in First-In-First-Out principle. The underlying objects that are used for synchronization are [Windows's Critical Section Objects.].
In the context of a client-server application, "MyClass" will contain all thread unsafe functionality of our server, so we will try to ensure that those procedures and functions are not executed by more than one working thread simultaneously. This naturally means loss of parallelism of our code, but since the approach is simple and seems to be , in some cases this maybe a useful approach.
Wrapper class Implementation:
constructor TThreadSafeObject<T>.Create(originalObject: T);
begin
tsObject := originalObject; // pass it already instantiated instance of MyClass
tsCriticalSection:= TCriticalSection.Create; // Critical section Object
end;
destructor TThreadSafeObject<T>.Destroy();
begin
FreeAndNil(tsObject);
FreeAndNil(tsCriticalSection);
inherited Destroy;
end;
function TThreadSafeObject<T>.Lock(): T;
begin
tsCriticalSection.Enter;
result:=tsObject;
end;
procedure TThreadSafeObject<T>.Unlock();
begin
tsCriticalSection.Leave;
end;
procedure TThreadSafeObject<T>.FreeOwnership();
begin
FreeAndNil(tsObject);
FreeAndNil(tsCriticalSection);
end;
MyClass Definition:
MyClass = class
public
procedure drawRandomBitmap(abitmap: TBitmap); //Draw Random Lines on TCanvas
function decToBin(i: LongInt): String; //convert decimal number to Bin.
procedure addLineToMemo(aLine: String; MemoFld: TMemo); // output message to TMemo
function randomColor(): TColor;
end;
Usage:
Since threads execute in order and wait for the thread which has the current ownership of the critical section to finish (tsCriticalSection.Enter; and tsCriticalSection.Leave;) it is logical that if you want to manage that ownership relay, you need one unique instance TThreadSafeObject (you can consider using the singleton pattern). so include:
tsMyclass:= TThreadSafeObject<MyClass>.Create(MyClass.Create);
in Form.Create and
tsMyclass.Destroy;
in Form.Close; Here tsMyclass is a global variable of type MyClass.
Usage:
Regarding the usage of MyClass try the following:
with tsMyclass.Lock do
try
addLineToMemo('MemoLine1', Memo1);
addLineToMemo('MemoLine2', Memo1);
addLineToMemo('MemoLine3', Memo1);
finally
// release ownership
tsMyclass.unlock;
end;
, where Memo1 is an instance of a TMemo component on the form.
With this, we are supposed to ensure that anything that happens when tsMyClass is locked
will be executed by only one thread at a time. An obvious drawback of this approach, however, is that since I have only one instance of tsMyclass, even if one thread is trying to draw for e.g. on the Canvas, while another is writing on the Memo, the first thread will have to wait for the second to finish and only then it will be able to carry out its job.
My questions here are:
Is the above suggested method correct? Am I still free of race
conditions or do I have some "loopholes" in the code, from where
data conflicts could occur?
How can one, in general, test for thread
unsafety of his/her applicaiton?
I would like to stress that the above approach is in no way my own doing. It is basically a summary of the solution found in 2. Nevertheless, I have decided to post again in an attempt to get some kind of closure on the topic or a kind of proof of validity for the suggested solution. Besides, repetition is mother of all knowledge, as they say.
With this, we are supposed to ensure that anything that happens when
tsMyClass is locked will be executed by only one thread at a time. An
obvious drawback of this approach, however, is that since I have only
one instance of tsMyclass, even if one thread is trying to draw for
e.g. on the Canvas, while another is writing on the Memo, the first
thread will have to wait for the second to finish and only then it
will be able to carry out its job.
I see one big problem here: the VCL (forms, drawing, etc...) lives on the main thread. Even if you block concurrent thread access, the updates need to be done in the context of the main thread. This is the part where you need to use Synhronize(), the big difference with a lock (Criticalsection) is that synchronized code is ran in the context of the main thread. The end result is basically the same, your threaded code is serialized and you lose the advantage of using threads in the first place.
Locking on the whole object can be much too coarse.
Imagine cases where some properties or methods are independent of others. If the lock works on a "global" level, many operations will be blocked needlessly.
From Reduce lock granularity – Concurrency optimization
So, how can we reduce lock granularity? With a short answer, by asking
for locks as less as possible. The basic idea is to use separate locks
to guard multiple independent state variables of a class, instead of
having only one lock in class scope.
First things first: You don't need to implement a LOCK for each of your objects, Delphi's done that for you with the TMonitor class:
TMonitor.Enter(WhateverObject);
try
// Your code goes here.
finally TMonitor.Leave(WhateverObject);
end;
just make sure you free the WhateverObject when your application shuts down, or else you'll run into a bug that I've opened on QC: http://qc.embarcadero.com/wc/qcmain.aspx?d=111795
Secondly, making an application multi-threading is a bit more involved. You can't just wrapp each call between Enter/Leave calls: your "locking" needs to take into account what the object does and what the access pattern is. Wrapping calls within Enter/Leave simply make sure that only one thread runs that method at any time, but race conditions are much more complex, and might arise from successive calls to your locked methods. Even those each method is locked, and only one thread ever called those methods at any given time, the state of the locked object might change between as a consequence of other thread's activity.
This kind of code would be just fine in a single-threaded application, but locking at method level is not enough when switching to multi-threaded:
if List.IndexOf(Something) = -1 then
List.Add(Something);
I've got a sort of spell checker written in Delphi. It analyzes the text sentence by sentence.
It encolors wrong items according to some rules after parsing each sentence. The user is able to interrupt this process, which is important.
How can I parallelize this process in general using some 3rd party Delphi libraries?
In the current state I've got on the fly sentence coloration after check. Thus the user sees the progress.
The algorithm would be as such:
Create multiple workers.
Create a spell-checker in each worker.
Grab the text and split it into work units (word or sentences). Each work unit must be accompanied with the location in original text.
Send work units to workers. Good approach is to send data into common queue from which workers are taking work units. Queue must either support multiple readers or you must use locking to access it.
Each worker takes a work unit, runs a spell-check and returns the result (together with the location in the original text) to the owner.
The simplest way to return a result is to send a message to the main thread.
Alternatively, you can write results into a result queue (which must either use locking or support multiple writers) and application can then poll those results (either from a timer or from the OnIdle handler).
How the multiple spell-checkers will access the dictionary is another problem. You can load a copy of the dictionary in each worker or you can protect access to the dictionary with a lock (but that would slow things down). If you are lucky, dictionary is thread-safe for reading and you can do simultaneous queries without locking.
Appropriate OmniThreadLibrary abstraction for the problem would be either a ParallelTask or a BackgroundWorker.
To parallelize, just create a new class descendent from TThread, create an object from it, give part of the job to the new thread, run Execute, and collect the results in the main thread.
Like this:
TMySpellChecker = class(TThread)
protected
FText: String;
FResult: String;
public
procedure Execute; override;
property Text: String read FText write FText;
property Result: String read FResult write FResult;
end;
TMySpellChecker.Execute;
begin
// Analyze the text, and compute the result
end;
In the main thread:
NewThread := TMySpellChecker.Create(True); // Create suspended
NewThread.Text := TextSegment;
NewThread.Execute;
The thread object will then do the analyzing in the background, while the main thread continues to run.
To handle the results, you need to assign a handler to the OnTerminate event of the thread object:
NewThread.OnTerminate := HandleMySpellCheckerTerminate;
This must be done before you run Execute on the thread object.
To allow for interruptions, one possibility is to break the main text up into segments, place the segments in a list in the main thread, and then analyze the segments one by one using the thread object. You can then allow for interruptions between each run.
I'm allocating my pthread thread-specific data from a fixed-size global pool that's controlled by a mutex. (The code in question is not permitted to allocate memory dynamically; all the memory it's allowed to use is provided by the caller as a single buffer. pthreads might allocate memory, I couldn't say, but this doesn't mean that my code is allowed to.)
This is easy to handle when creating the data, because the function can check the result of pthread_getspecific: if it returns NULL, the global pool's mutex can be taken there and then, the pool entry acquired, and the value set using pthread_setspecific.
When the thread is destroyed, the destructor function (as per pthread_key_create) is called, but the pthreads manual is a bit vague about any restrictions that might be in place.
(I can't impose any requirements on the thread code, such as needing it to call a destructor manually before it exits. So, I could leave the data allocated, and maybe treat the pool as some kind of cache, reusing entries on an LRU basis once it becomes full -- and this is probably the approach I'd take on Windows when using the native API -- but it would be neatest to have the per-thread data correctly freed when each thread is destroyed.)
Can I just take the mutex in the destructor? There's no problem with thread destruction being delayed a bit, should some other thread have the mutex taken at that point. But is this guaranteed to work? My worry is that the thread may "no longer exist" at that point. I use quotes, because of course it certainly exists if it's still running code! -- but will it exist enough to permit a mutex to be acquired? Is this documented anywhere?
The pthread_key_create() rationale seems to justify doing whatever you want from a destructor, provided you keep signal handlers from calling pthread_exit():
There is no notion of a destructor-safe function. If an application does not call pthread_exit() from a signal handler, or if it blocks any signal whose handler may call pthread_exit() while calling async-unsafe functions, all functions may be safely called from destructors.
Do note, however, that this section is informative, not normative.
The thread's existence or non-existence will most likely not affect the mutex in the least, unless the mutex is error-checking. Even then, the kernel is still scheduling whatever thread your destructor is being run on, so there should definitely be enough thread to go around.
The documentation of delphi says that the WaitFor function for TMutex and others sychronization objects wait until a handle of object is signaled.But this function also guarantee the ownership of the object for the caller?
Yes, the calling thread of a TMutex owns the mutex; the class is just a wrapper for the OS mutex object. See for yourself by inspecting SyncObjs.pas.
The same is not true for other synchronization objects, such as TCriticalSection. Any thread my call the Release method on such an object, not just the thread that called Acquire.
TMutex.Acquire is a wrapper around THandleObjects.WaitFor, which will call WaitForSingleObject OR CoWaitForMultipleHandles depending on the UseCOMWait contructor argument.
This may be very important, if you use STA COM objects in your application (you may do so without knowing, dbGO/ADO is COM, for instance) and you don't want to deadlock.
It's still a dangerous idea to enter a long/infinite wait in the main thread, 'cause the only method which correctly handles calls made via TThread.Synchronize is TThread.WaitFor and you may stall (or deadlock) your worker threads if you use the SyncObjs objects or WinAPI wait functions.
In commercial projects, I use a custom wait method, built upon the ideas from both THandleObjects.WaitFor AND TThread.WaitFor with optional alertable waiting (good for asynchronous IO but irreplaceable for the possibility to abort long waits).
Edit: further clarification regarding COM/OLE:
COM/OLE model (e.g. ADO) can use different threading models: STA (single-threaded) and MTA (multi or free-threaded).
By definition, the main GUI thread is initialized as STA, which means, the COM objects can use window messages for their asynchronous messaging (particulary when invoked from other threads, to safely synchronize). AFAIK, they may also use APC procedures.
There is a good reason for the CoWaitForMultipleHandles function to exist - see its' use in SyncObjs.pas THandleObject.WaitFor - depending on the threading model, it can process internal COM messages, while blocking on the wait handle.