Since I haven't found any documentation of IBM's com.ibm.jscript.std.ObjectObject or com.ibm.jscript.std.ArrayObject I wanted to ask if they are thread-safe and/or synchronized, i.e. if I can safely use them in a multi-thread environment without getting problems with ConcurrentModificationExceptions etc.
PS: I know I could for example replace these objects with Maps and Lists from java.util.concurrent, but this question does not aim at finding a workaround.
Related
I hope that this question is not too broad for Stackoverflow. If it is, I would be grateful to get a suggestion, where this kind of question can be discussed.
Problem:
I'm thinking of creating a multi-threading applications in JRuby, and try to forsee potential pitfalls. My current concept goes like this:
Use the ruby-concurrent library (https://github.com/ruby-concurrency/concurrent-ruby)
Communication between threads uses only Queues and/or Futures from this library
Now I'm wondering what else I would have to observe. For instance, while new code can of course use the classes defined in concurrent-ruby, each thread will also use for its internal work existing Ruby code or Gems, and I don't know in what ways they could jeopardize the parallism.
For instance, the docs of old JRuby versions (<1.7) reportedly had the problem that the implementation of the native Hash and Array classes themselves were not thread-safe, with the effect that even a method using a Hash in a local variable, could comprise another thread. I think this is not present in newer JRuby versions anymore; at least I could not find anything in the current JRuby docs about this.
Then, I see a risk from global variables ($name) and class variables (#name, ##name)? From my understanding, this would also be a possible error sources and I have to check the source code of each gem I am planning to use, whether it perhaps uses such a variable.
Is there anything else I would have to be aware of?
If you only change a MyInt: Integer variable in one or more threads with one of the interlocked functions, lets say InterlockedIncrement, can we guarantee that after the InterlockedIncrement is executed, a plain read of the variable in any thread will return the latest updated value? Yes, no and why?
If not, is it possible to achieve that in Delphi? Note that I'm talking about only one variable, no need to worry about consistency about two or more variables.
The root problems and doubt seems essentially equal to the one in this SO post, but it is targeted at C# there, and I'm using Delphi 2007, so no access to volatile, neither of newer versions of Delphi as well. In that discussion, two major problems that seems to affect Delphi as well were raised:
The cache of the processor reading the variable may not be updated.
The compiler may optimize the code in a way that causes problems to read.
If this is really a problem, I'm very worried to use even a simple counter with InterlockedIncrement, or solutions like the lock-free initialization proposed in here, and would go to just plain Critical Sections of MultiReaderSingleWritter for safety.
Initial analysis
This is what I've found so far, but fell free to address the problems in other ways if appropriate, or even raising other unknown problems so the objective of the question can be achieved:
For the problem 1, I expected that the "full-fence" would also force the cache of other processors to be updated... but reading around it seems to not be the case. It looks that the cache would only be updated if a "read barrier" (or whatever it is called) would be called on the processor what will read the variable. If this is true, is there a way to call such "read barrier" in Delphi, just before reading the variable? Full-fence seems to imply both read and write barriers, so that would also be ok. Since that there is no InterlockedRead function according to the discussion in the first post, could we try (just speculating) to workaround using something like InterlockedCompareExchange (ugh... writing the variable to be able to read it, smells bad), or maybe "lock" low level assembly calls (that could be encapsulated)?
For the problem 2, Delphi optimizations would impact in this matter? Any way to avoid it?
Edit: The solution must work in D2007, but I'd like, preferably, to not make harder a possible future migration to newer Delphi, and use the same piece of code in ARM as well (this became clear to me after David's comments). So, if possible, it would be nice if solution is not coupled with x86/64 memory model. Would be nice if I need only to replace the plain Windows.pas interlocked functions to whatever provides the same interlocked functionality in newer Delphi/ARM, without the need to review the logic for ARM (one less concern).
But, Do the interlocked functions have enough abstraction from CPU architecture in this case? Problem 1 suggests that it doesn't, but I'm not sure if it would affect ARM Delphi. Any way around it, that keeps it simple and still allow relevant better performance over critical sections and similar sync objects?
The goal is to emulate multi-threaded behavior for a .so which is not thread-safe. Memory is plentiful, not a problem. What is important for me is down-calls via JNI. What is not important is up-calls and sharing anything between .so instances (the goal is complete isolation).
I have heard that it is possible to link a shared lib more than once, but I have not seen anyone actually do it.
There is an opinion that doing this is a bad idea, but I am not convinced by the argument.
Is this a good/bad idea, and why?
If this turns out to be a good idea under certain conditions, where can I read more about it? Can anyone share some code that does this?
Let me add that making .so thread-safe is not really an option, and that mutex is the current implementation which I am trying to improve.
The idea of a shared library is just to share a common code segment among multiple applications.
Once you realized this basic fact, you'd realize that what you're trying to do doesn't makes sense. Because the memory allocation will be within your process space.
Question How can I make sure my application is thread-safe? Are their any common practices, testing methods, things to avoid, things to look for?
Background I'm currently developing a server application that performs a number of background tasks in different threads and communicates with clients using Indy (using another bunch of automatically generated threads for the communication). Since the application should be highly availabe, a program crash is a very bad thing and I want to make sure that the application is thread-safe. No matter what, from time to time I discover a piece of code that throws an exception that never occured before and in most cases I realize that it is some kind of synchronization bug, where I forgot to synchronize my objects properly. Hence my question concerning best practices, testing of thread-safety and things like that.
mghie: Thanks for the answer! I should perhaps be a little bit more precise. Just to be clear, I know about the principles of multithreading, I use synchronization (monitors) throughout my program and I know how to differentiate threading problems from other implementation problems. But nevertheless, I keep forgetting to add proper synchronization from time to time. Just to give an example, I used the RTL sort function in my code. Looked something like
FKeyList.Sort (CompareKeysFunc);
Turns out, that I had to synchronize FKeyList while sorting. It just don't came to my mind when initially writing that simple line of code. It's these thins I wanna talk about. What are the places where one easily forgets to add synchronization code? How do YOU make sure that you added sync code in all important places?
You can't really test for thread-safeness. All you can do is show that your code isn't thread-safe, but if you know how to do that you already know what to do in your program to fix that particular bug. It's the bugs you don't know that are the problem, and how would you write tests for those? Apart from that threading problems are much harder to find than other problems, as the act of debugging can already alter the behaviour of the program. Things will differ from one program run to the next, from one machine to the other. Number of CPUs and CPU cores, number and kind of programs running in parallel, exact order and timing of stuff happening in the program - all of this and much more will have influence on the program behaviour. [I actually wanted to add the phase of the moon and stuff like that to this list, but you get my meaning.]
My advice is to stop seeing this as an implementation problem, and start to look at this as a program design problem. You need to learn and read all that you can find about multi-threading, whether it is written for Delphi or not. In the end you need to understand the underlying principles and apply them properly in your programming. Primitives like critical sections, mutexes, conditions and threads are something the OS provides, and most languages only wrap them in their libraries (this ignores things like green threads as provided by for example Erlang, but it's a good point of view to start out from).
I'd say start with the Wikipedia article on threads and work your way through the linked articles. I have started with the book "Win32 Multithreaded Programming" by Aaron Cohen and Mike Woodring - it is out of print, but maybe you can find something similar.
Edit: Let me briefly follow up on your edited question. All access to data that is not read-only needs to be properly synchronized to be thread-safe, and sorting a list is not a read-only operation. So obviously one would need to add synchronization around all accesses to the list.
But with more and more cores in a system constant locking will limit the amount of work that can be done, so it is a good idea to look for a different way to design your program. One idea is to introduce as much read-only data as possible into your program - locking is no longer necessary, as all access is read-only.
I have found interfaces to be a very valuable aid in designing multi-threaded programs. Interfaces can be implemented to have only methods for read-only access to the internal data, and if you stick to them you can be quite sure that a lot of the potential programming errors do not occur. You can freely share them between threads, and the thread-safe reference counting will make sure that the implementing objects are properly freed when the last reference to them goes out of scope or is assigned another value.
What you do is create objects that descend from TInterfacedObject. They implement one or more interfaces which all provide only read-only access to the internals of the object, but they can also provide public methods that mutate the object state. When you create the object you keep both a variable of the object type and a interface pointer variable. That way lifetime management is easy, because the object will be deleted automatically when an exception occurs. You use the variable pointing to the object to call all methods necessary to properly set up the object. This mutates the internal state, but since this happens only in the active thread there is no potential for conflict. Once the object is properly set up you return the interface pointer to the calling code, and since there is no way to access the object afterwards except by going through the interface pointer you can be sure that only read-only access can be performed. By using this technique you can completely remove the locking inside of the object.
What if you need to change the state of the object? You don't, you create a new one by copying the data from the interface, and mutate the internal state of the new objects afterwards. Finally you return the reference pointer to the new object.
By using this you will only need locking where you get or set such interfaces. It can even be done without locking, by using the atomic interchange functions. See this blog post by Primoz Gabrijelcic for a similar use case where an interface pointer is set.
Simple: don't use shared data. Every time you access shared data you risk running into a problem (if you forget to synchronize access). Even worse, each time you access shared data you risk blocking other threads which will hurt your paralelization.
I know this advice is not always applicable. Still, it doesn't hurt if you try to follow it as much as possible.
EDIT: Longer response to Smasher's comment. Would not fit in a comment :(
You are totally correct. That's why I like to keep a shadow copy of the main data in a readonly thread. I add a versioning to the structure (one 4-aligned DWORD) and increment this version in the (lock-protected) data writer. Data reader would compare global and private version (which can be done without locking) and only if they differr it would lock the structure, duplicate it to a local storage, update the local version and unlock. Then it would access the local copy of the structure. Works great if reading is the primary way to access the structure.
I'll second mghie's advice: thread safety is designed in. Read about it anywhere you can.
For a really low level look at how it is implemented, look for a book on the internals of a real time operating system kernel. A good example is MicroC/OS-II: The Real Time Kernel by Jean J. Labrosse, which contains the complete annotated source code to a working kernel along with discussions of why things are done the way they are.
Edit: In light of the improved question focusing on using a RTL function...
Any object that can be seen by more than one thread is a potential synchronization issue. A thread-safe object would follow a consistent pattern in every method's implementation of locking "enough" of the object's state for the duration of the method, or perhaps, narrowed to just "long enough". It is certainly the case that any read-modify-write sequence to any part of an object's state must be done atomically with respect to other threads.
The art lies in figuring out how to get useful work done without either deadlocking or creating an execution bottleneck.
As for finding such problems, testing won't be any guarantee. A problem that shows up in testing can be fixed. But it is extremely difficult to write either unit tests or regression tests for thread safety... so faced with a body of existing code your likely recourse is constant code review until the practice of thread safety becomes second nature.
As folks have mentioned and I think you know, being certain, in general, that your code is thread safe is impossible (I believe provably impossible but I would have to track down the theorem). Naturally, you want to make things easier than that.
What I try to do is:
Use a known pattern of multithreaded design: A thread pool, the actor model paradigm, the command pattern or some such approach. This way, the syncronization process happens in the same way, in a uniform way, throughout the application.
Limit and concentrate the points of synchronization. Write your code so you need synchronization in as few places as possible and the keep the synchronization code in one or few places in the code.
Write the synchronization code so that the logical relation between the values is clear on both on entering and on exiting the guard. I use lots of asserts for this (your environment may limit this).
Don't ever access shared variables without guards/synchronization. Be very clear what your shared data is. (I've heard there are paradigms for guardless multithreaded programming but that would require even more research).
Write your code as cleanly, clearly and DRY-ly as possible.
My simple answer combined with those answer is:
Create your application/program using
thread safety manner
Avoid using public static variable in
all places
Therefore it usually fall into this habit/practice easily but it needs some time to get used to:
program your logic (not the UI) in functional programming language such as F# or even using Scheme or Haskell. Also functional programming promotes thread safety practice while it also warns us to always code towards purity in functional programming.
If you use F#, there's also clear distinction about using mutable or immutable objects such as variables.
Since method (or simply functions) is a first class citizen in F# and Haskell, then the code you write will also have more disciplined toward less mutable state.
Also using the lazy evaluation style that usually can be found in these functional languages, you can be sure that your program is safe fromside effects, and you'll also realize that if your code needs effects, you have to clearly define it. IF side effects are taken into considerations, then your code will be ready to take advantage of composability within components in your codes and the multicore programming.
so that you can make your program concurrent easily in the future.
I focus on making items Immutable. Immutable objects allow you to reason about multi-threaded code a lot easier than "thread safe" objects. The object has one visible state that can be passed between threads without any synchronization. It takes the thought out of multi-threaded programming.
If you're interested, I've published a lot of my work with immutable objects, in particular immutable collections on code gallery. The name of the project is RantPack. In the collection area I have
ImmutableCollection<T>
ImmutableMap<TKey,TValue>
ImmutableAvlTree<T>
ImmutableLinkedList<T>
ImmutableArray<T>
ImmutableStack<T>
ImmutableQueue<T>
There is an additional shim layer which (CollectionUtility) which will produce wrapper objects that implement BCL interfaces such as IList<T> and ICollection<T>. They can't fully implement the interfaces since they are immutable but all possible methods are implemented.
The source code (C#) including the unit testing is also available on the site.
I program mainly in Java. I'm waiting patiently for the day where closures will be added to the language. But as I am still stuck on Java 1.4.2, even if they get added, that's not going to be for me for a long time !
That said, my main "functional" way of programming is making a lot of use of the "final" keyword. I try to have as many classes as possible completely immutable, and for the rest to have a clear distinction between what's transient and what's immutable.
Don't use member variables or global variables. Use the local stack of functions/methods. When a method uses only internally scoped variables and call parameters and returns all information using out/inout/reference parameters or return values, it is functional.
Make everything asynchronic.
Use immutable objects, messages, etc.
Communicate via queues.
Here's a talk on rubyconf 2008 about the subject, it's mostly ruby centered, but several concepts remain valid.
http://rubyconf2008.confreaks.com/better-ruby-through-functional-programming-2.html