Related
I want to know if a program can run two threads at the same time (that is basically what it is used for correct?). But if I were to do a system call in one function where it runs on thread A, and have some other tasks running in another function where it runs on thread B, would they both be able to run at the same time or would my second function wait until the system call finishes?
Add-on to my original question: Now would this process still be an uninterruptable process while the system call is going on? I am talking about using any system call on UNIX/LINUX.
Multi-threading and parallel processing are two completely different topics, each worthy of its own conversation, but for the sake of introduction...
Threading:
When you launch an executable, it is running in a thread within a process. When you launch another thread, call it thread 2, you now have 2 separately running execution chains (threads) within the same process. On a single core microprocessor (uP), it is possible to run multiple threads, but not in parallel. Although conceptually the threads are often said to run at the same time, they are actually running consecutively in time slices allocated and controlled by the operating system. These slices are interleaved with each other. So, the execution steps of thread 1 do not actually happen at the same time as the execution steps of thread 2. These behaviors generally extend to as many threads as you create, i.e. packets of execution chains all working within the same process and sharing time slices doled out by the operating system.
So, in your system call example, it really depends on what the system call is as to whether or not it would finish before allowing the execution steps of the other thread to proceed. Several factors play into what will happen: Is it a blocking call? Does one thread have more priority than the other. What is the duration of the time slices?
Links relevant to threading in C:
SO Example
POSIX
ANSI C
Parallel Processing:
When multi-threaded program execution occurs on a multiple core system (multiple uP, or multiple multi-core uP) threads can run concurrently, or in parallel as different threads may be split off to separate cores to share the workload. This is one example of parallel processing.
Again, conceptually, parallel processing and threading are thought to be similar in that they allow things to be done simultaneously. But that is concept only, they are really very different, in both target application and technique. Where threading is useful as a way to identify and split out an entire task within a process (eg, a TCP/IP server may launch a worker thread when a new connection is requested, then connects, and maintains that connection as long as it remains), parallel processing is typically used to send smaller components of the same task (eg. a complex set of computations that can be performed independently in separate locations) off to separate resources (cores, or uPs) to be completed simultaneously. This is where multiple core processors really make a difference. But parallel processing also takes advantage of multiple systems, popular in areas such as genetics and MMORPG gaming.
Links relevant to parallel processing in C:
OpenMP
More OpenMP (examples)
Gribble Labs - Introduction to OpenMP
CUDA Tookit from NVIDIA
Additional reading on the general topic of threading and architecture:
This summary of threading and architecture barely scratches the surface. There are many parts to the the topic. Books to address them would fill a small library, and there are thousands of links. Not surprisingly within the broader topic some concepts do not seem to follow reason. For example, it is not a given that simply having more cores will result in faster multi-threaded programs.
Yes, they would, at least potentially, run "at the same time", that's exactly what threads are for; of course there are many details, for example:
If both threads run system calls that e.g. write to the same file descriptor they might temporarily block each other.
If thread synchronisation primitives like mutexes are used then the parallel execution will be blocked.
You need a processor with at least two cores in order to have two threads truly run at the same time.
It's a very large and very complex subject.
If your computer has only a single CPU, you should know, how it can execute more than one thread at the same time.
In single-processor systems, only a single thread of execution occurs at a given instant. because Single-processor systems support logical concurrency, not physical concurrency.
On multiprocessor systems, several threads do, in fact, execute at the same time, and physical concurrency is achieved.
The important feature of multithreaded programs is that they support logical concurrency, not whether physical concurrency is actually achieved.
The basics are simple, but the details get complex real quickly.
You can break a program into multiple threads (if it makes sense to do so), and each thread will run "at its own pace", such that if one must wait for, eg, some file I/O that doesn't slow down the others.
On a single processor multiple threads are accommodated by "time slicing" the processor somehow -- either on a simple clock basis or by letting one thread run until it must wait (eg, for I/O) and then "switching" to the next thread. There is a whole art/science to doing this for maximum efficiency.
On a multi-processor (such as most modern PCs which have from 2 to 8 "cores") each thread is assigned to a separate processor, and if there are not enough processors then they are shared as in the single processor case.
The whole area of assuring "atomicity" of operations by a single thread, and assuring that threads don't somehow interfere with each other is incredibly complex. In general a there is a "kernel" or "nucleus" category of system call that will not be interrupted by another thread, but thats only a small subset of all system calls, and you have to consult the OS documentation to know which category a particular system call falls into.
They will run at the same time, for one thread is independent from another, even if you perform a system call.
It's pretty easy to test it though, you can create one thread that prints something to the console output and perform a system call at another thread, that you know will take some reasonable amount of time. You will notice that the messages will continue to be printed by the other thread.
Yes, A program can run two threads at the same time.
it is called Multi threading.
would they both be able to run at the same time or would my second function wait until the system call finishes?
They both are able to run at the same time.
if you want, you can make thread B wait until Thread A completes or reverse
Two thread can run concurrently only if it is running on multiple core processor system, but if it has only one core processor then two threads can not run concurrently. So only one thread run at a time and if it finishes its job then the next thread which is on queue take the time.
I have written a small application on go, which starts 4 threads for doing various things + one main thread. So in total there are 5 threads. But if I'll start activity monitor and monitor the process, this is what I see
First of all why 7 threads. And it is not constant. Sometimes it is 5 and other times it is 7. Also all 4 threads started by main thread ends after doing hat they are suppose to. I verify that threads end by putting a differ statement on the top of thread. Still thread count in Activity monitor stays 7.
Does anyone knows what is going on over here? Are these extra threads started by go runtime? Is there a way to find out how many threads are active my program that are started by my code and not by go runtime.
Yes they are started by the runtime, for example http://play.golang.org/p/c0cIngo_sO it will print 4 goroutines are running.
Goroutines aren't threads, 1 OS thread can handle 100s of goroutines, however if you're doing something heavy or using a blocking system call, the runtime will start a new thread to handle the other goroutines.
I suppose you mean Goroutines when you say threads.
The Go runtime transparently multiplexes lightweight Goroutines onto OS threads. That's also why you don't need to call functions like select()—that's the runtime's job.
If you spawn 7 Go routines and some of them block, the runtime might decide to terminate the idle OS threads. This is why you see less threads than Go routines.
I think you mistake Goroutines for thread.
In your go program, the thread you mean is actually goroutine ，which is a coroutine and is not a real thread , which is implemented by go's runtime(you need to know about go runtime, every go program is running on a runtime, and runtime actually use thread to implement goroutines).Diffrent goroutine may be running in the same thread, or may be not ,but you never know . You can use runtime.GOMAXPROCS for multi-core cpu .
And the threads you see in the monitor are real threads .
I want to know if a program can run two threads at the same time (that is basically what it is used for correct?). But if I were to do a system call in one function where it runs on thread A, and have some other tasks running in another function where it runs on thread B, would they both be able to run at the same time or would my second function wait until the system call finishes?
Add-on to my original question: Now would this process still be an uninterruptable process while the system call is going on? I am talking about using any system call on UNIX/LINUX.
Multi-threading and parallel processing are two completely different topics, each worthy of its own conversation, but for the sake of introduction...
Threading:
When you launch an executable, it is running in a thread within a process. When you launch another thread, call it thread 2, you now have 2 separately running execution chains (threads) within the same process. On a single core microprocessor (uP), it is possible to run multiple threads, but not in parallel. Although conceptually the threads are often said to run at the same time, they are actually running consecutively in time slices allocated and controlled by the operating system. These slices are interleaved with each other. So, the execution steps of thread 1 do not actually happen at the same time as the execution steps of thread 2. These behaviors generally extend to as many threads as you create, i.e. packets of execution chains all working within the same process and sharing time slices doled out by the operating system.
So, in your system call example, it really depends on what the system call is as to whether or not it would finish before allowing the execution steps of the other thread to proceed. Several factors play into what will happen: Is it a blocking call? Does one thread have more priority than the other. What is the duration of the time slices?
Links relevant to threading in C:
SO Example
POSIX
ANSI C
Parallel Processing:
When multi-threaded program execution occurs on a multiple core system (multiple uP, or multiple multi-core uP) threads can run concurrently, or in parallel as different threads may be split off to separate cores to share the workload. This is one example of parallel processing.
Again, conceptually, parallel processing and threading are thought to be similar in that they allow things to be done simultaneously. But that is concept only, they are really very different, in both target application and technique. Where threading is useful as a way to identify and split out an entire task within a process (eg, a TCP/IP server may launch a worker thread when a new connection is requested, then connects, and maintains that connection as long as it remains), parallel processing is typically used to send smaller components of the same task (eg. a complex set of computations that can be performed independently in separate locations) off to separate resources (cores, or uPs) to be completed simultaneously. This is where multiple core processors really make a difference. But parallel processing also takes advantage of multiple systems, popular in areas such as genetics and MMORPG gaming.
Links relevant to parallel processing in C:
OpenMP
More OpenMP (examples)
Gribble Labs - Introduction to OpenMP
CUDA Tookit from NVIDIA
Additional reading on the general topic of threading and architecture:
This summary of threading and architecture barely scratches the surface. There are many parts to the the topic. Books to address them would fill a small library, and there are thousands of links. Not surprisingly within the broader topic some concepts do not seem to follow reason. For example, it is not a given that simply having more cores will result in faster multi-threaded programs.
Yes, they would, at least potentially, run "at the same time", that's exactly what threads are for; of course there are many details, for example:
If both threads run system calls that e.g. write to the same file descriptor they might temporarily block each other.
If thread synchronisation primitives like mutexes are used then the parallel execution will be blocked.
You need a processor with at least two cores in order to have two threads truly run at the same time.
It's a very large and very complex subject.
If your computer has only a single CPU, you should know, how it can execute more than one thread at the same time.
In single-processor systems, only a single thread of execution occurs at a given instant. because Single-processor systems support logical concurrency, not physical concurrency.
On multiprocessor systems, several threads do, in fact, execute at the same time, and physical concurrency is achieved.
The important feature of multithreaded programs is that they support logical concurrency, not whether physical concurrency is actually achieved.
The basics are simple, but the details get complex real quickly.
You can break a program into multiple threads (if it makes sense to do so), and each thread will run "at its own pace", such that if one must wait for, eg, some file I/O that doesn't slow down the others.
On a single processor multiple threads are accommodated by "time slicing" the processor somehow -- either on a simple clock basis or by letting one thread run until it must wait (eg, for I/O) and then "switching" to the next thread. There is a whole art/science to doing this for maximum efficiency.
On a multi-processor (such as most modern PCs which have from 2 to 8 "cores") each thread is assigned to a separate processor, and if there are not enough processors then they are shared as in the single processor case.
The whole area of assuring "atomicity" of operations by a single thread, and assuring that threads don't somehow interfere with each other is incredibly complex. In general a there is a "kernel" or "nucleus" category of system call that will not be interrupted by another thread, but thats only a small subset of all system calls, and you have to consult the OS documentation to know which category a particular system call falls into.
They will run at the same time, for one thread is independent from another, even if you perform a system call.
It's pretty easy to test it though, you can create one thread that prints something to the console output and perform a system call at another thread, that you know will take some reasonable amount of time. You will notice that the messages will continue to be printed by the other thread.
Yes, A program can run two threads at the same time.
it is called Multi threading.
would they both be able to run at the same time or would my second function wait until the system call finishes?
They both are able to run at the same time.
if you want, you can make thread B wait until Thread A completes or reverse
Two thread can run concurrently only if it is running on multiple core processor system, but if it has only one core processor then two threads can not run concurrently. So only one thread run at a time and if it finishes its job then the next thread which is on queue take the time.
I read many answers given here for questions related to thread safety, re-entrancy, but when i think about them, some more questions came to mind, hence this question/s.
1.) I have one executable program say some *.exe. If i run this program on command prompt, and while it is executing, i run the same program on another command prompt, then in what conditions the results could be corrupted, i.e. should the code of this program be re-entrant or it should be thread safe alone?
2.) While defining re-entrancy, we say that the routine can be re-entered while it is already running, in what situations the function can be re-entered (apart from being recursive routine, i am not talking recursive execution here). There has to be some thread to execute the same code again, or how can that function be entered again?
3.) In a practical case, will two threads execute same code, i.e. perform same functionality. I thought the idea of multi-threading is to execute different functionality, concurrently(on different cores/processors).
Sorry if these queries seem different, but they all occured to me, same time when i read about the threadsafe Vs reentrant post on SO, hence i put them together.
Any pointers, reading material will be appreciated.
thanks,
-AD.
I'll try to explain these, in order:
Each program runs in its own process, and gets its own isolated memory space. You don't have to worry about thread safety in this situation. (However, if the processes are both accessing some other shared resource, such as a file, you may have different issues. For example, process 1 may "lock" the data file, preventing process 2 from being able to open it).
The idea here is that two threads may try to run the same routine at the same time. This is not always valid - it takes special care to define a class or a process in a way that multiple threads can use the same instance of the same class, or the same static function, without errors occurring. This typically requires synchronization in the class.
Two threads often execute the same code. There are two different conceptual ways to parition your work when threading. You can either think in terms of tasks - ie: one thread does task A while another does task B. Alternatively, you can think in terms of decomposing the the problem based on data. In this case, you work with a large collection, and each element is processed using the same routine, but the processing happens in parallel. For more info, you can read this blog post I wrote on Decomposition for Parallelism.
Two processes cannot share memory. So thread-safety is moot here.
Re-entrancy means that a method can be safely executed by two threads at the same time. This doesn't require recursion - threads are separate units of execution, and there is nothing keeping them both from attempting to run the same method simultaneously.
The benefits to threading can happen in two ways. One is when you perform different types of operations concurrently (like running cpu-intensive code and I/O-intensive code at the ame time). The other is when you can divide up a long-running operation among multiple processors. In this latter case, two threads may be executing the same function at the same time on different input data sets.
First of all, I strongly suggest you to look at some basic stuffs of computer system, especially how a process/thread is executing on CPU and scheduled by operating system. For example, virtual address, context switching, process/thread concepts(e.g., each thread has its own stack and register vectors while heap is shared by threads. A thread is an execution and scheduling unit, so it maintains control flow of code..) and so on. All of the questions are related to understanding how your program is actually working on CPU
1) and 2) are already answered.
3) Multithreading is just concurrent execution of any arbitrary thread. The same code can be executed by multiple threads. These threads can share some data, and even can make data races which are very hard to find. Of course, many times threads are executing separate code(we say it as thread-level parallelism).
In this context, I have used concurrent as two meaning: (a) in a single processor, multiple threads are sharing a single physical processor, but operating system gives a sort of illusion that threads are running concurrently. (b) In a multicore, yes, physically two or more threads can be executed concurrently.
Having concrete understanding of concurrent/parallel execution takes quite long time. But, you already have a solid understanding!
How do I control the number of threads that my program is working on?
I have a program that is now ready for mutithreading but one problem is that the program is extremely memory intensive and i have to limit the number of threads running so that i don't run out of ram. The main program goes through and creates a whole bunch of handles and associated threads in suspended state.
I want the program to activate a set number of threads and when one thread finishes, it will automatically unsuspended the next thread in line until all the work has been completed. How do i do this?
Someone has once mentioned something about using a thread handler, but I can't seem to find any information about how to write one or exactly how it would work.
If anyone can help, it would be greatly appreciated.
Using windows and visual c++.
Note: i don't need to worry about the traditional problems of access with the threads, each one is completely independent of each other, its more of like batch processing rather than true mutithreading of a program.
Thanks,
-Faken
Don't create threads explicitly. Create a thread pool, see Thread Pools and queue up your work using QueueUserWorkItem. The thread pool size should be determined by the number of hardware threads available (number of cores and ratio of hyperthreading) and the ratio of CPU vs. IO your work items do. By controlling the size of the thread pool you control the number of maximum concurrent threads.
A Suspended thread doesn't use CPU resources, but it still consumes memory, so you really shouldn't be creating more threads than you want to run simultaneously.
It is better to have only as many threads as your maximum number of simultaneous tasks, and to use a queue to pass units of work to the pool of worker threads.
You can give work to the standard pool of threads created by Windows using the Windows Thread Pool API.
Be aware that you will share these threads and the queue used to submit work to them with all of the code in your process. If, for some reason, you don't want to share your worker threads with other code in your process, then you can create a FIFO queue, create as many threads as you want to run simultaneously and have each of them pull work items out of the queue. If the queue is empty they will block until work items are added to the queue.
There is so much to say here.
There are a few ways
You should only create as many thread handles as you plan on running at the same time, then reuse them when they complete. (Look up thread pool).
This guarantees that you can never have too many running at the same time. This raises the question of funding out when a thread completes. You can have a callback be called just before a thread terminates where a parameter in that callback is the thread handle that just finished. Use Boost bind and boost signals for that. When the callback is called, look for another task for that thread handle and restart the thread. That way all you have to do is add to the "tasks to do" list and the callback will remove the tasks for you. No polling needed, and no worries about too many threads.