Interprocess communication: Shared memory vs thread object access - c++

I always learnt, that shared memory is the fastest way to share data between two threads (like e.g. http://www.boost.org/doc/libs/1_55_0/doc/html/interprocess.html). However, today I discovered that using boost::ref(X) it is possible to give boost a reference to X enabling access to X from outside the thread. Therefore the following pseudocode should work:
MyObjext X(para1,para2); // MyObject has a () operator
boost::thread thr(boost::ref(X));
X.setSomeMember(1);
This got me thinking: Assuming setSomeMember is thread safe, then - for most applications - this approach seems much easier, since most applications spawn their threads as they need and thus can always save and access the object X. So, why would I use shared memory or message queues anyway, if I have access to the thread object directly? Is it maybe faster? Or am I missing something here?

They're just different features - you happen to highlight the similarities.
Yes, threads are more lightweight than processes.
What you lose is isolation (processes can only share what's explicitely exposed, and only given the right permissions). There is no such control for inter-thread sharing.
If one thread messes up the shared state, all threads die, the same goes for shared memory. However, if one thread dies, the whole process dies, which doesn't happen for separate processes.
All in all, it's different. Inter-process synchronization/sharing is more heavy weight but has more features (how will you run a separate thread on a different host :)).

Related

Simulating Thread with fork()

What's your idea about simulating thread with "fork() function" and a "shared memory" block ...
Is it possible ?
How much is it reasonable to do this for a program ? ( I mean , Will it work well..?)
For starters, don't mix a thread and fork().
A fork gives you a brand new process, which is a copy of the current process, with the same code segments. As the memory image changes (typically this is due to different behavior of the two processes) you get a separation of the memory images, however the executable code remains the same. Tasks do not share memory unless they use some Inter Process Communication (IPC) primitive.
In contrast a thread is another execution thread of the same task. One task can have multiple threads, and the task memory object are shared among threads, therefore shared data must be accessed through some primitive and synchronization objects that allow you to avoid data corruption.
Yes, it is possible, but I cannot imagine it being a good idea, and it would be a real pain to test.
If you have a shared heap, and you make sure all semaphores etc. are allocated in the heap, and not the stack, then there's no inherent reason you couldn't do something like it. There would be some tricky differences though.
For example, anything you do in an interrupt handler in a multi-threaded program can change data used by all the threads, while in a forked program, you would have to send multiple interrupts, which would be caught at different times, and might lead to unintended effects.
If you want threading behavior, just use a thread.
AFAIK, fork will create a separate process with its own context, stack and so on. Depends what you mean by "simulating"...
You might want to check this out : http://www.linuxprogrammingblog.com/threads-and-fork-think-twice-before-using-them
A few of the answers here focus on "don't mix fork and threads". But the way I read your question is: "can you use two different processes, and still communicate quickly and conveniently with shared memory between them, just like how threads have access to each others' memory?"
And the answer is, yes you can, but you have to remember to explicitly mark which memory areas you want shared. You can not just share your variables between the processes. Also, you can communicate this way between processes not related to each other at all. It is not limited to processes forked from each other.
Have a look at shared memory or "shm".

How do I protect a character string in shared memory between two processes?

I have a piece of shared memory that contains a char string and an integer between two processes.
Process A writes to it and Process B reads it (and not vice versa)
What is the most efficient and effective way to make sure that Process A doesn't happen to update (write to it) that same time Process B is reading it? (Should I just use flags in the shared memory, use semaphores, critical section....)
If you could point me in the right direction, I would appreciate it.
Thanks.
Windows, C++
You cannot use a Critical Section because these can only be used for synchronization between threads within the same process. For inter process synchronization you need to use a Mutex or a Semaphore. The difference between these two is that the former allows only a single thread to own a resource, while the latter can allow up to a maximum number (specified during creation) to own the resource simultaneously.
In your case a Mutex seems appropriate.
Since you have two processes you need a cross-process synchronisation object. I think this means that you need to use a mutex.
A mutex object facilitates protection against data races and allows
thread-safe synchronization of data between threads. A thread obtains
ownership of a mutex object by calling one of the lock functions and
relinquishes ownership by calling the corresponding unlock function.
If you are using boost thread, you can use it's mutex and locking, more to read see the link below:
http://www.boost.org/doc/libs/1_47_0/doc/html/thread/synchronization.html#thread.synchronization.mutex_types
Since you're talking about two processes, system-wide mutexes will work, and Windows has those. However, they aren't necessarily the most efficient way.
If you can put more things in shared memory, then passing data via atomic operations on flags in that memory should be the most efficient thing to do. For instance, you might use the Interlocked functions to implement Dekker's Algorithm (you'll probably want to use something like YieldProcessor() to avoid busy waiting).

Design and Technical issue in Multi Threaded Application

I wanted to Discuss the Design and technical issue/challenges related with multi threaded application.
Issue I faced
1.I came across the situation where there is multiple thread is using the shared function/variable crash the application, so proper guard is required on that occasion.
2. State Machine and Multi thread-
There are several point one should remember before delve in to the multi thread application.
There can issue related to 1. Memory 2. Handle 3. Socket etc.
please share your experience on the following point
what are the common mistake one do in the multi threaded application
Any specific issue related to multi threaded.
Should we pass data by value or by referen in the thread function.
Well, there are so many...
1) Shared functions/procedures - they are just code and, unless the code modifies itself, there can be no problem. Local variables are no problem because each thread calls on a separate stack, (amost by definition:). Any other data can an issue and may need protection. 99.99% of all household API calls on multiTasking OS are thread-safe, again, almost by definition. Another poster has already warned about thread-local storage...
2) State machines. Can be a little awkward. You can easly lock all the events firing into the SM, so ensuring the integrity of the state, but you must not make blocking calls from inside the SM while it is locked, (might seem obvious, but I have done this.. once :).
I occasionally run state-machines from one thread only, queueing event objects to it. This moves the locking to the input queue and means that the SM is somewhat easier to debug. It also means that the thread running the SM can implement timeouts on an internal delta queue and so itself fire timeout calls to the objects on the delta queue, (classic example: TCP server sockets with connection timeouts - thousands of socket objects that each need an independent timeout).
3) 'Should we pass data by value or by referen in the thread function.'. Not sure what you mean, here. Most OS allow one pointer to be passed on thread creation - do with it what you will. You could pass it an event it should signal on work completion or a queue object upon which it is to wait for work requests. After creation, you need some form of inter-thread comms to send requests and get results, (unless you are going to use the direct 'read/write/waitForExit' mechanism - AV/deadlock/noClose generator).
I usually use a simple semaphore/CS producer-consumer queue to send/receive comms objects between worker threads, and the PostMessage API to send them to a UI thread. Apart from the locking in the queue, I don't often need any more locking. You have to try quite hard to deadlock a threaded system based on message-passing and things like thread pools become trivial - just make [no. of CPU] threads and pass each one the same queue to wait on.
Common mistakes. See the other posters for many, to which I would add:
a) Reading/writing directly to thread fields to pass parameters and return results, (esp. between UI threads and 'worker' threads), ie 'Create thread suspended, load parameters into thread fields, resume thread, wait on thread handle for exit, read results from thread fields, free thread object'. This causes performance hit from continually creating/terminating/destroying threads and often forces the developer to ensure that thread are terminated when exiting an app to prevent AV/216/217 exceptions on close. This can be very tricky, in some cases impossible because a few API's block with no way of unblocking them. If developers would stop this nasty practice, there would be far fewer app close problems.
b) Trying to build multiThreaded apps in a procedural fashion, eg. trying to wait for results from a work thread in a UI event handler. Much safer to build a thread request object, load it with parameters, queue it to a work thread and exit the event handler. The thread can get the object, do work, put results back into the object and, (on Windows, anyway), PostMessage the object back. A UI message-handler can deal with the results and dispose of the object, (or recycle, reuse:). This approach means that, since the UI and worker are always operating on different data that can outlive them both, no locking and, (usually), no need to ensure that the work thread is freed when closing the app, (problems with this are ledgendary).
Rgds,
Martin
The biggest issue people face in multi threading applications are race conditions, deadlocks and not using semaphores of some sort to protect globally accessible variables.
You are facing these problems when using thread locks.
Deadlock
Priority Inversion
Convoying
“Async-signal-safety”
Kill-tolerant availability
Preemption tolerance
Overall performance
If you want to look at more advanced threading techniques you can look at the lock free threading, where many threads work on the same problem in case they are waiting.
Deadlocks, memory corruption (of shared resources) due to lack of proper synchronization, buffer overflow (even that can be occured due to memory corruption), improper usage of thread local storage are the most common things
Also it depends on under which platform and technology you're using to implement the thread. For e.g. in Microsoft Windows, if you use MFC objects, several MFC objects are not really shareable across threads because they're heavily rely on thread local storage (e.g CSocket, CWnd classes etc.)

Boost::Thread or fork() : Multithreaded HTTP Proxy

I'm testing boost::thread on a system. It happens that I needed to act as a fork(), because one thread modifies the other variables, even member variables of class
I do the project using fork() or is there some alternative still using boost::thread?
Basically I run this program in Linux and maybe FreeBSD.
It is an http proxy,accept() in main thread, and a function that accepts a class (where there is the file descriptor socket) in a secondary thread that makes the service.
Is there a better way to implement a proxy?
fork() spawns a process which have independent memory regions. Changed must be mediate through IPC.
boost::thread create a thread which can share memory.
They are not comparable.
To create thread-local storage, use boost::thread_specific_ptr.
See http://www.boost.org/doc/libs/1_42_0/doc/html/thread/thread_local_storage.html.
(You may also decorate a global variable as __thread int xyz; to make it thread-local, if the compiler and architecture can support it.)
It sounds like you are trying to allow multiple threads to alter global variables without each others' changes affecting any of the other threads. By forking, the entire memory space of your application is basically copied and each branch of the fork has its own variables and the two branches cannot communicate except through IPC.
If you want to use boost::thread, you'll have to do this copying yourself if you don't want threads to affect each other since the same memory space is common among all threads. You could just create the variables local to each thread function.
Using threads instead of forking will be much more flexible especially when you want to start letting the threads share data. If you want to have variables that all of the threads can change, they should be protected by mutex locks when being changed so that only one thread can change a variable at one time.

Thread communication theory

What is the common theory behind thread communication? I have some primitive idea about how it should work but something doesn't settle well with me. Is there a way of doing it with interrupts?
Really, it's just the same as any concurrency problem: you've got multiple threads of control, and it's indeterminate which statements on which threads get executed when. That means there are a large number of POTENTIAL execution paths through the program, and your program must be correct under all of them.
In general the place where trouble can occur is when state is shared among the threads (aka "lightweight processes" in the old days.) That happens when there are shared memory areas,
To ensure correctness, what you need to do is ensure that these data areas get updated in a way that can't cause errors. To do this, you need to identify "critical sections" of the program, where sequential operation must be guaranteed. Those can be as little as a single instruction or line of code; if the language and architecture ensure that these are atomic, that is, can't be interrupted, then you're golden.
Otherwise, you idnetify that section, and put some kind of guards onto it. The classic way is to use a semaphore, which is an atomic statement that only allows one thread of control past at a time. These were invented by Edsgar Dijkstra, and so have names that come from the Dutch, P and V. When you come to a P, only one thread can proceed; all other threads are queued and waiting until the executing thread comes to the associated V operation.
Because these primitives are a little primitive, and because the Dutch names aren't very intuitive, there have been some ther larger-scale approaches developed.
Per Brinch-Hansen invented the monitor, which is basically just a data structure that has operations which are guaranteed atomic; they can be implemented with semaphores. Monitors are pretty much what Java synchronized statements are based on; they make an object or code block have that particular behavir -- that is, only one thread can be "in" them at a time -- with simpler syntax.
There are other modeals possible. Haskell and Erlang solve the problem by being functional languages that never allow a variable to be modified once it's created; this means they naturally don't need to wory about synchronization. Some new languages, like Clojure, instead have a structure called "transactional memory", which basically means that when there is an assignment, you're guaranteed the assignment is atomic and reversible.
So that's it in a nutshell. To really learn about it, the best places to look at Operating Systems texts, like, eg, Andy Tannenbaum's text.
The two most common mechanisms for thread communication are shared state and message passing.
THe most common way for threads to communicate is via some shared data structure, typically a queue. Some threads put information into the queue while others take it out. The queue must be protected by operating system facilities such as mutexes and semaphores. Interrupts have nothing to do with it.
If you're really interested in a theory of thread communications, you may want to look into formalisms like the pi Calculus.
To communicate between threads, you'll need to use whatever mechanism is supplied by your operating system and/or runtime. Interrupts would be unusually low level, although they might be used implicitly if your threads communicate using sockets or named pipes.
A common pattern would be to implement shared state using a shared memory block, relying on an os-supplied synchronization primitive such as a mutex to spare you from busy-waiting when your read from the block. Remember that if you have threads at all, then you must have some kind of scheduler already (whether it's native from the OS or emulated in your language runtime). So this scheduler can provide synchronization objects and a "sleep" function without necessarily having to rely on hardware support.
Sockets, pipes, and shared memory work between processes too. Sometimes a runtime will give you a lighter-weight way of doing synchronization for threads within the same process. Shared memory is cheaper within a single process. And sometimes your runtime will also give you an atomic message-passing mechanism.