Question as stated above .. from the stand point of Operating System, which one is easier to create, a thread or a process?
A new thread should be faster to create than a new process.
A process is a heavy weight system structure. It has it's own virtual memory space, owns all handles (mutexes, semaphores, open files), and has protection from other processes. Cross-process communication has to go through the OS.
A thread is a "child" to a process. A thread is simply an execution context (registers, stack, and thread-local state) that can run on another hardware core or be co-scheduled on the same core as other threads within a process. Multiple threads share the resources of a single process including the address space and OS handles owned by the process.
There are structures even faster than dynamically creating threads for achieving multitasking during a programs runtime.
Some systems or code libraries support have thread pools (light-weight threads). In this case, you tell the system how many threads you want to run and it creates them up front. Then instead of creating and destroying threads (which is still a relatively slow process), you can allocate and free threads from this pool.
Job Tasking is another similar lighter weight multicore structure where you have several threads with a job queues of tasks to execute. They run the tasks in their job queues and then sleep when the queues are empty.
For both thread pools and job tasking, there is no need for thread startup / shutdown cost aside from upon creation and destruction of the global pools and queues.
Well traditionally threads are called "lightweight processes" so I guess they are easier to set up.
IIRC in Linux both forking and starting a new thread (clone(2)) are implemented deep down with a call to the same function (do_fork) and the set-up times are really comparable for decent numbers. For large numbers of forks / clones (think thousands) they start to add up.
In TLPI there is a nice comparison:
Forking 100,000 times: 22.27 seconds
Cloning 100,000 times: 2.97 seconds
In particular a really nice feature of clone is that the speed remains constant even if the size of the process cloned grows.
The real advantage of threads lies in that they don't need IPC.
A new thread is easier to create, since when a new process is created, it requires more setup than a thread, e.g. a security context, an inheritable handle, a current directory, etc.
The major difference between threads and processes is
1.Threads share the address space of the process that
created it; processes have their own address.
2.Threads have direct access to the data segment of its
process; processes have their own copy of the data segment
of the parent process.
3.Threads can directly communicate with other threads of
its process; processes must use interprocess communication
to communicate with sibling processes.
4.Threads have almost no overhead; processes have
considerable overhead.
5.New threads are easily created; new processes require
duplication of the parent process.
6.Threads can exercise considerable control over threads of
the same process; processes can only exercise control over
child processes.
7.Changes to the main thread (cancellation, priority
change, etc.) may affect the behavior of the other threads
of the process; changes to the parent process does not
affect child processes.
A thread just has to be as easy or easier to create than a process since a creating a process implies creating at least one thread to run the process code.
Rgds,
Martin
Related
I believe I have a reasonable understanding of threading from an Object Oriented perspective using the Thread class and the Runnable interface. In one of my applications there is a "download" button that allows the user to run a task in the background that takes about half an hour whilst continuing to use the VB.NET application.
However, I do not understand how Threading maps to the physical architecture of a computer. If you have a single threaded application that runs on a PC with a quadcore processor then does a .net program use all four processors?
If you have a multi threaded application (say four threads) on a quadcore processor then does each thread execute on different cores?
Do you have any control of this as a developer?
I have referenced a book I read at university called Operating System Concepts, but I have not found a specific answer.
If you have a single threaded application that runs on a PC with a quadcore processor then does a .net program use all four processors?
No, it can’t, at least not simultaneously. However, it’s theoretically possible that the operating system’s scheduler first executes your thread on one processor and later moves it to another processor. Such a scheduler is necessary to allow simultaneously running more applications / threads than there are physical processors present: execution of a thread is sliced into small pieces, which are fed to the processor(s) one after the other. Each threads gets some time slice allocated during which it can calculate before usage of the CPU switches to another thread.
Do you have any control of this as a developer?
Not directly. What you can control is the priority of your thread to make it more important to the task scheduler.
On a more general note, you should not use threads in your use-case – at least not directly. Threads are actually pretty low-level primitives. For your specific use-case, there’s a component called BackgroundWorker which abstracts many of the low-level details of thread management for you.
If you have a multi threaded application (say four threads) on a quadcore processor then does each thread execute on different cores?
Not necessarily; again, the application has next to no control over how exactly its threads are executed; the operating system however tries really hard to schedule threads “smartly”. This means that in practice, if your application has several busy threads, they are spread out as evenly as possible across the available cores. In particular, if there are no more threads than cores then each thread gets its own core.
Generally, you do not need to worry about mapping to physical architecture, .NET and the OS will do their best to maximize efficiency of your application. Just make it multi-threaded, even at the cost of being slower on a single threaded computer. You can, however, limit your maximum number of threads (if your app theoretically scales to infinity), to a number of cores, or double that. Test performance for each case, and decide which maximum works best for you.
Sometimes setting a core # can make your app's performance even worse. For example, if core #1 is currently scanning your PC for viruses, and your antivirus is single threaded. With the above scenario, assuming a quad-core PC, you do NOT want to run your 4-threaded app on a 1-per-core basis.
Having said that, if you really want to run a thread on specific core, it is possible - see this:
How to start a Thread at Specific Core
How Can I Set Processor Affinity in .NET?
Also check this out:
How to control which core a process runs on?
I searched a variety of sources but don't really understand the difference between using NSThreads and GCD. I'm completely new to the OS X platform so I might be completely misinterpreting this.
From what I read online, GCD seems to do the exact same thing as basic threads (POSIX, NSThreads etc.) while adding much more technical jargon ("blocks"). It seems to just overcomplicate the basic thread creation system (create thread, run function).
What exactly is GCD and why would it ever be preferred over traditional threading? When should traditional threads be used rather than GCD? And finally is there a reason for GCD's strange syntax? ("blocks" instead of simply calling functions).
I am on Mac OS X 10.6.8 Snow Leopard and I am not programming for iOS - I am programming for Macs. I am using Xcode 3.6.8 in Cocoa, creating a GUI application.
Advantages of Dispatch
The advantages of dispatch are mostly outlined here:
Migrating Away from Threads
The idea is that you eliminate work on your part, since the paradigm fits MOST code more easily.
It reduces the memory penalty your application pays for storing thread stacks in the application’s memory space.
It eliminates the code needed to create and configure your threads.
It eliminates the code needed to manage and schedule work on threads.
It simplifies the code you have to write.
Empirically, using GCD-type locking instead of #synchronized is about 80% faster or more, though micro-benchmarks may be deceiving. Read more here, though I think the advice to go async with writes does not apply in many cases, and it's slower (but it's asynchronous).
Advantages of Threads
Why would you continue to use Threads? From the same document:
It is important to remember that queues are not a panacea for
replacing threads. The asynchronous programming model offered by
queues is appropriate in situations where latency is not an issue.
Even though queues offer ways to configure the execution priority of
tasks in the queue, higher execution priorities do not guarantee the
execution of tasks at specific times. Therefore, threads are still a
more appropriate choice in cases where you need minimal latency, such
as in audio and video playback.
Another place where I haven't personally found an ideal solution using queues is daemon processes that need to be constantly rescheduled. Not that you cannot reschedule them, but looping within a NSThread method is simpler (I think). Edit: Now I'm convinced that even in this context, GCD-style locking would be faster, and you could also do a loop within a GCD-dispatched operation.
Blocks in Objective-C?
Blocks are really horrible in Objective-C due to the awful syntax (though Xcode can sometimes help with autocompletion, at least). If you look at blocks in Ruby (or any other language, pretty much) you'll see how simple and elegant they are for dispatching operations. I'd say that you'll get used to the Objective-C syntax, but I really think that you'll get used to copying from your examples a lot :)
You might find my examples from here to be helpful, or just distracting. Not sure.
While the answers so far are about the context of threads vs GCD inside the domain of a single application and the differences it has for programming, the reason you should always prefer GCD is because of multitasking environments (since you are on MacOSX and not iOS). Threads are ok if your application is running alone on your machine. Say, you have a video edition program and want to apply some effect to the video. The render is going to take 10 minutes on a machine with eight cores. Fine.
Now, while the video app is churning in the background, you open an image edition program and play with some high resolution image, decide to apply some special image filter and your image application being clever detects you have eight cores and starts eight threads to process the image. Nice isn't it? Except that's terrible for performance. The image edition app doesn't know anything about the video app (and vice versa) and therefore both will request their respectively optimum number of threads. And there will be pain and blood while the cores try to switch from one thread to another, because to avoid starvation the CPU will eventually let all threads run, even though in this situation it would be more optimal to run only 4 threads for the video app and 4 threads for the image app.
For a more detailed reference, take a look at http://deusty.blogspot.com/2010/11/introducing-gcd-based-cocoahttpserver.html where you can see a benchmark of an HTTP server using GCD versus thread, and see how it scales. Once you understand the problem threads have for multicore machines in multi-app environments, you will always want to use GCD, simply because threads are not always optimal, while GCD potentially can be since the OS can scale thread usage per app depending on load.
Please, remember we won't have more GHz in our machines any time soon. From now on we will only have more cores, so it's your duty to use the best tool for this environment, and that is GCD.
Blocks allow for passing a block of code to execute. Once you get past the "strange syntax", they are quite powerful.
GCD also uses queues which if used properly can help with lock free concurrency if the code executing in the separate queues are isolated. It's a simpler way to offer background and concurrency while minimizing the chance for deadlocks (if used right).
The "strange syntax" is because they chose the caret (^) because it was one of the few symbols that wasn't overloaded as an operator in C++
See:
https://developer.apple.com/library/ios/#documentation/General/Conceptual/ConcurrencyProgrammingGuide/OperationQueues/OperationQueues.html
When it comes to adding concurrency to an application, dispatch queues
provide several advantages over threads. The most direct advantage is
the simplicity of the work-queue programming model. With threads, you
have to write code both for the work you want to perform and for the
creation and management of the threads themselves. Dispatch queues let
you focus on the work you actually want to perform without having to
worry about the thread creation and management. Instead, the system
handles all of the thread creation and management for you. The
advantage is that the system is able to manage threads much more
efficiently than any single application ever could. The system can
scale the number of threads dynamically based on the available
resources and current system conditions. In addition, the system is
usually able to start running your task more quickly than you could if
you created the thread yourself.
Although you might think rewriting your code for dispatch queues would
be difficult, it is often easier to write code for dispatch queues
than it is to write code for threads. The key to writing your code is
to design tasks that are self-contained and able to run
asynchronously. (This is actually true for both threads and dispatch
queues.)
...
Although you would be right to point out that two tasks running in a
serial queue do not run concurrently, you have to remember that if two
threads take a lock at the same time, any concurrency offered by the
threads is lost or significantly reduced. More importantly, the
threaded model requires the creation of two threads, which take up
both kernel and user-space memory. Dispatch queues do not pay the same
memory penalty for their threads, and the threads they do use are kept
busy and not blocked.
GCD (Grand Central Dispatch): GCD provides and manages FIFO queues to which your application can submit tasks in the form of block objects. Work submitted to dispatch queues are executed on a pool of threads fully managed by the system. No guarantee is made as to the thread on which a task executes. Why GCD over threads :
How much work your CPU cores are doing
How many CPU cores you have.
How much threads should be spawned.
If GCD needs it can go down into the kernel and communicate about resources, thus better scheduling.
Less load on kernel and better sync with OS
GCD uses existing threads from thread pool instead of creating and then destroying.
Best advantage of the system’s hardware resources, while allowing the operating system to balance the load of all the programs currently running along with considerations like heating and battery life.
I have shared my experience with threads, operating system and GCD AT http://iosdose.com
I am aware that threads are used for multi-tasking and they are light weight. But my doubt is lets say I need a process without any multi-tasking. I just created a process. Now will the CPU associate a single thread to the process OR will it execute the process alone without need to have a thread?
Please clarify.
Regards,
Harish
Well, that depends on the OS that you're talking about but, for many, the creation of a process includes the act of creating a single thread for that process.
That thread is then free to go and create other threads belonging to the process.
It makes little sense to talk about a process with no threads since that means no code is running for that process so it can't really do anything useful. And one of the things it won't be able to do is create the first thread for that process if you want it to do any useful work :-)
As an example, in the Linux kernel, the creation of a process is little different to creating a new thread. That's because the kernel schedules threads rather than processes.
Processes are now considered to be groups of threads with the same thread group ID (TGID), that TGID being the thread ID (TID) of the first thread created for that process.
When you fork or vfork or clone (without CLONE_THREAD), you get a new thread with a new TID and the TGID is set to that TID - that's a new process.
When you clone with CLONE_THREAD, you get a new thread with a new TID but the TGID remains the same as your cloner. That's a different thread in the same process.
That's how Linux (as an example) distinguishes between processes and threads without having to make the scheduler too complicated. The scheduler can choose to ignore thread groups entirely if it wishes. It's actually incredibly clever.
To code outside the scheduler, a group of threads with the same TGID is considered a process (with the TGID being what the code outside of the scheduler sees as the process ID).
This includes both user space code and other bits of the kernel since, for example, how threads are grouped into processes has a bearing on things like signal delivery and exit codes.
A process -is- a thread.
When a process begins, it begins with a single thread.
Before the days of multi-threading, the term thread was unnecessary because you couldn't have a process with more than one thread.
Now days, you can create additional threads, and so have a process with multiple threads.
A process is also a bunch of other things - memory, stack, whathaveyou; one of the things it is, is threads. The threads share some of the other things in the process (such as memory), but have their own individual instances of others (such as stacks).
My software will simulate a few hundred hardware devices, each of which will send several thousand reports to a database server.
Trying it without threading did not give very good results, so now it's time to thread.
Since I am load testing the d/b server, some of those transactions will succeed and a few may fail. The GUI of the main program needs to reflect this. How should the threads communicate their results back to the main program? Update global variables? Send a message? Or something lese?
Now, if I update only at the end of each thread then the GUI is going to look rather boring (and I can't tell if the program hung). It might be nice to update the GUI periodically. But that might cause contention, with threads waiting for other threads to update (for instance, if I am writing to global variables, I need a mutex, which will block each thread which is waiting to write).
I'm new to threading. How is this normally done? Perhaps the main program could poll the threads, instead of the threads iforming the main program?
One way to organize this is for your threads to add messages to a thread-safe queue (e.g. a ConcurrentQueue) as they get data. To keep things simple you can have a timer thread in your UI that periodically dequeues the queued messages to a private list and then renders them. This design allows your threads to easily queue and forget messages with minimal contention, and for your UI to periodically update itself without blocking your writers too much (i.e. for only the period it takes to dequeue current messages to a private list).
Although you are attempting to simulate the load of hundreds of devices, using thread per device is not the way to model this as you can only run so many threads concurrently anyway.
What is the difference between a thread/process/task?
Process:
A process is an instance of a computer program that is being executed.
It contains the program code and its current activity.
Depending on the operating system (OS), a process may be made up of multiple threads of execution that execute instructions concurrently.
Process-based multitasking enables you to run the Java compiler at the same time that you are using a text editor.
In employing multiple processes with a single CPU,context switching between various memory context is used.
Each process has a complete set of its own variables.
Thread:
A thread is a basic unit of CPU utilization, consisting of a program counter, a stack, and a set of registers.
A thread of execution results from a fork of a computer program into two or more concurrently running tasks.
The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process. Multiple threads can exist within the same process and share resources such as memory, while different processes do not share these resources.
Example of threads in same process is automatic spell check and automatic saving of a file while writing.
Threads are basically processes that run in the same memory context.
Threads may share the same data while execution.
Thread Diagram i.e. single thread vs multiple threads
Task:
A task is a set of program instructions that are loaded in memory.
Short answer:
A thread is a scheduling concept, it's what the CPU actually 'runs' (you don't run a process). A process needs at least one thread that the CPU/OS executes.
A process is data organizational concept. Resources (e.g. memory for holding state, allowed address space, etc) are allocated for a process.
To explain on simpler terms
Process: process is the set of instruction as code which operates on related data and process has its own various state, sleeping, running, stopped etc. when program gets loaded into memory it becomes process. Each process has atleast one thread when CPU is allocated called sigled threaded program.
Thread: thread is a portion of the process. more than one thread can exist as part of process. Thread has its own program area and memory area. Multiple threads inside one process can not access each other data. Process has to handle sycnhronization of threads to achieve the desirable behaviour.
Task: Task is not widely concept used worldwide. when program instruction is loaded into memory people do call as process or task. Task and Process are synonyms nowadays.
A process invokes or initiates a program. It is an instance of a program that can be multiple and running the same application. A thread is the smallest unit of execution that lies within the process. A process can have multiple threads running. An execution of thread results in a task. Hence, in a multithreading environment, multithreading takes place.
A program in execution is known as process. A program can have any number of processes. Every process has its own address space.
Threads uses address spaces of the process. The difference between a thread and a process is, when the CPU switches from one process to another the current information needs to be saved in Process Descriptor and load the information of a new process. Switching from one thread to another is simple.
A task is simply a set of instructions loaded into the memory. Threads can themselves split themselves into two or more simultaneously running tasks.
for more Understanding refer the link: http://www.careerride.com/os-thread-process-and-task.aspx
Wikipedia sums it up quite nicely:
Threads compared with processes
Threads differ from traditional multitasking operating system processes in that:
processes are typically independent, while threads exist as
subsets of a process
processes carry considerable state information, whereas multiple
threads within a process share state
as well as memory and other resources
processes have separate address spaces, whereas threads share their
address space
processes interact only through system-provided inter-process
communication mechanisms.
Context switching between threads in the same process is
typically faster than context
switching between processes.
Systems like Windows NT and OS/2 are said to have "cheap" threads and "expensive" processes; in other operating systems there is not so great a difference except the cost of address space switch which implies a TLB flush.
Task and process are used synonymously.
from wiki clear explanation
1:1 (Kernel-level threading)
Threads created by the user are in 1-1 correspondence with schedulable entities in the kernel.[3] This is the simplest possible threading implementation. Win32 used this approach from the start. On Linux, the usual C library implements this approach (via the NPTL or older LinuxThreads). The same approach is used by Solaris, NetBSD and FreeBSD.
N:1 (User-level threading)
An N:1 model implies that all application-level threads map to a single kernel-level scheduled entity;[3] the kernel has no knowledge of the application threads. With this approach, context switching can be done very quickly and, in addition, it can be implemented even on simple kernels which do not support threading. One of the major drawbacks however is that it cannot benefit from the hardware acceleration on multi-threaded processors or multi-processor computers: there is never more than one thread being scheduled at the same time.[3] For example: If one of the threads needs to execute an I/O request, the whole process is blocked and the threading advantage cannot be utilized. The GNU Portable Threads uses User-level threading, as does State Threads.
M:N (Hybrid threading)
M:N maps some M number of application threads onto some N number of kernel entities,[3] or "virtual processors." This is a compromise between kernel-level ("1:1") and user-level ("N:1") threading. In general, "M:N" threading systems are more complex to implement than either kernel or user threads, because changes to both kernel and user-space code are required. In the M:N implementation, the threading library is responsible for scheduling user threads on the available schedulable entities; this makes context switching of threads very fast, as it avoids system calls. However, this increases complexity and the likelihood of priority inversion, as well as suboptimal scheduling without extensive (and expensive) coordination between the userland scheduler and the kernel scheduler.