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I've recently heard a few people say that in Linux, it is almost always better to use processes instead of threads, since Linux is very efficient in handling processes, and because there are so many problems (such as locking) associated with threads. However, I am suspicious, because it seems like threads could give a pretty big performance gain in some situations.
So my question is, when faced with a situation that threads and processes could both handle pretty well, should I use processes or threads? For example, if I were writing a web server, should I use processes or threads (or a combination)?
Linux uses a 1-1 threading model, with (to the kernel) no distinction between processes and threads -- everything is simply a runnable task. *
On Linux, the system call clone clones a task, with a configurable level of sharing, among which are:
CLONE_FILES: share the same file descriptor table (instead of creating a copy)
CLONE_PARENT: don't set up a parent-child relationship between the new task and the old (otherwise, child's getppid() = parent's getpid())
CLONE_VM: share the same memory space (instead of creating a COW copy)
fork() calls clone(least sharing) and pthread_create() calls clone(most sharing). **
forking costs a tiny bit more than pthread_createing because of copying tables and creating COW mappings for memory, but the Linux kernel developers have tried (and succeeded) at minimizing those costs.
Switching between tasks, if they share the same memory space and various tables, will be a tiny bit cheaper than if they aren't shared, because the data may already be loaded in cache. However, switching tasks is still very fast even if nothing is shared -- this is something else that Linux kernel developers try to ensure (and succeed at ensuring).
In fact, if you are on a multi-processor system, not sharing may actually be beneficial to performance: if each task is running on a different processor, synchronizing shared memory is expensive.
* Simplified. CLONE_THREAD causes signals delivery to be shared (which needs CLONE_SIGHAND, which shares the signal handler table).
** Simplified. There exist both SYS_fork and SYS_clone syscalls, but in the kernel, the sys_fork and sys_clone are both very thin wrappers around the same do_fork function, which itself is a thin wrapper around copy_process. Yes, the terms process, thread, and task are used rather interchangeably in the Linux kernel...
Linux (and indeed Unix) gives you a third option.
Option 1 - processes
Create a standalone executable which handles some part (or all parts) of your application, and invoke it separately for each process, e.g. the program runs copies of itself to delegate tasks to.
Option 2 - threads
Create a standalone executable which starts up with a single thread and create additional threads to do some tasks
Option 3 - fork
Only available under Linux/Unix, this is a bit different. A forked process really is its own process with its own address space - there is nothing that the child can do (normally) to affect its parent's or siblings address space (unlike a thread) - so you get added robustness.
However, the memory pages are not copied, they are copy-on-write, so less memory is usually used than you might imagine.
Consider a web server program which consists of two steps:
Read configuration and runtime data
Serve page requests
If you used threads, step 1 would be done once, and step 2 done in multiple threads. If you used "traditional" processes, steps 1 and 2 would need to be repeated for each process, and the memory to store the configuration and runtime data duplicated. If you used fork(), then you can do step 1 once, and then fork(), leaving the runtime data and configuration in memory, untouched, not copied.
So there are really three choices.
That depends on a lot of factors. Processes are more heavy-weight than threads, and have a higher startup and shutdown cost. Interprocess communication (IPC) is also harder and slower than interthread communication.
Conversely, processes are safer and more secure than threads, because each process runs in its own virtual address space. If one process crashes or has a buffer overrun, it does not affect any other process at all, whereas if a thread crashes, it takes down all of the other threads in the process, and if a thread has a buffer overrun, it opens up a security hole in all of the threads.
So, if your application's modules can run mostly independently with little communication, you should probably use processes if you can afford the startup and shutdown costs. The performance hit of IPC will be minimal, and you'll be slightly safer against bugs and security holes. If you need every bit of performance you can get or have a lot of shared data (such as complex data structures), go with threads.
Others have discussed the considerations.
Perhaps the important difference is that in Windows processes are heavy and expensive compared to threads, and in Linux the difference is much smaller, so the equation balances at a different point.
Once upon a time there was Unix and in this good old Unix there was lots of overhead for processes, so what some clever people did was to create threads, which would share the same address space with the parent process and they only needed a reduced context switch, which would make the context switch more efficient.
In a contemporary Linux (2.6.x) there is not much difference in performance between a context switch of a process compared to a thread (only the MMU stuff is additional for the thread).
There is the issue with the shared address space, which means that a faulty pointer in a thread can corrupt memory of the parent process or another thread within the same address space.
A process is protected by the MMU, so a faulty pointer will just cause a signal 11 and no corruption.
I would in general use processes (not much context switch overhead in Linux, but memory protection due to MMU), but pthreads if I would need a real-time scheduler class, which is a different cup of tea all together.
Why do you think threads are have such a big performance gain on Linux? Do you have any data for this, or is it just a myth?
I think everyone has done a great job responding to your question. I'm just adding more information about thread versus process in Linux to clarify and summarize some of the previous responses in context of kernel. So, my response is in regarding to kernel specific code in Linux. According to Linux Kernel documentation, there is no clear distinction between thread versus process except thread uses shared virtual address space unlike process. Also note, the Linux Kernel uses the term "task" to refer to process and thread in general.
"There are no internal structures implementing processes or threads, instead there is a struct task_struct that describe an abstract scheduling unit called task"
Also according to Linus Torvalds, you should NOT think about process versus thread at all and because it's too limiting and the only difference is COE or Context of Execution in terms of "separate the address space from the parent " or shared address space. In fact he uses a web server example to make his point here (which highly recommend reading).
Full credit to linux kernel documentation
If you want to create a pure a process as possible, you would use clone() and set all the clone flags. (Or save yourself the typing effort and call fork())
If you want to create a pure a thread as possible, you would use clone() and clear all the clone flags (Or save yourself the typing effort and call pthread_create())
There are 28 flags that dictate the level of resource sharing. This means that there are over 268 million flavours of tasks that you can create, depending on what you want to share.
This is what we mean when we say that Linux does not distinguish between a process and a thread, but rather alludes to any flow of control within a program as a task. The rationale for not distinguishing between the two is, well, not uniquely defining over 268 million flavours!
Therefore, making the "perfect decision" of whether to use a process or thread is really about deciding which of the 28 resources to clone.
How tightly coupled are your tasks?
If they can live independently of each other, then use processes. If they rely on each other, then use threads. That way you can kill and restart a bad process without interfering with the operation of the other tasks.
To complicate matters further, there is such a thing as thread-local storage, and Unix shared memory.
Thread-local storage allows each thread to have a separate instance of global objects. The only time I've used it was when constructing an emulation environment on linux/windows, for application code that ran in an RTOS. In the RTOS each task was a process with it's own address space, in the emulation environment, each task was a thread (with a shared address space). By using TLS for things like singletons, we were able to have a separate instance for each thread, just like under the 'real' RTOS environment.
Shared memory can (obviously) give you the performance benefits of having multiple processes access the same memory, but at the cost/risk of having to synchronize the processes properly. One way to do that is have one process create a data structure in shared memory, and then send a handle to that structure via traditional inter-process communication (like a named pipe).
In my recent work with LINUX is one thing to be aware of is libraries. If you are using threads make sure any libraries you may use across threads are thread-safe. This burned me a couple of times. Notably libxml2 is not thread-safe out of the box. It can be compiled with thread safe but that is not what you get with aptitude install.
I'd have to agree with what you've been hearing. When we benchmark our cluster (xhpl and such), we always get significantly better performance with processes over threads. </anecdote>
The decision between thread/process depends a little bit on what you will be using it to.
One of the benefits with a process is that it has a PID and can be killed without also terminating the parent.
For a real world example of a web server, apache 1.3 used to only support multiple processes, but in in 2.0 they added an abstraction so that you can swtch between either. Comments seems to agree that processes are more robust but threads can give a little bit better performance (except for windows where performance for processes sucks and you only want to use threads).
For most cases i would prefer processes over threads.
threads can be useful when you have a relatively smaller task (process overhead >> time taken by each divided task unit) and there is a need of memory sharing between them. Think a large array.
Also (offtopic), note that if your CPU utilization is 100 percent or close to it, there is going to be no benefit out of multithreading or processing. (in fact it will worsen)
Threads -- > Threads shares a memory space,it is an abstraction of the CPU,it is lightweight.
Processes --> Processes have their own memory space,it is an abstraction of a computer.
To parallelise task you need to abstract a CPU.
However the advantages of using a process over a thread is security,stability while a thread uses lesser memory than process and offers lesser latency.
An example in terms of web would be chrome and firefox.
In case of Chrome each tab is a new process hence memory usage of chrome is higher than firefox ,while the security and stability provided is better than firefox.
The security here provided by chrome is better,since each tab is a new process different tab cannot snoop into the memory space of a given process.
Multi-threading is for masochists. :)
If you are concerned about an environment where you are constantly creating threads/forks, perhaps like a web server handling requests, you can pre-fork processes, hundreds if necessary. Since they are Copy on Write and use the same memory until a write occurs, it's very fast. They can all block, listening on the same socket and the first one to accept an incoming TCP connection gets to run with it. With g++ you can also assign functions and variables to be closely placed in memory (hot segments) to ensure when you do write to memory, and cause an entire page to be copied at least subsequent write activity will occur on the same page. You really have to use a profiler to verify that kind of stuff but if you are concerned about performance, you should be doing that anyway.
Development time of threaded apps is 3x to 10x times longer due to the subtle interaction on shared objects, threading "gotchas" you didn't think of, and very hard to debug because you cannot reproduce thread interaction problems at will. You may have to do all sort of performance killing checks like having invariants in all your classes that are checked before and after every function and you halt the process and load the debugger if something isn't right. Most often it's embarrassing crashes that occur during production and you have to pore through a core dump trying to figure out which threads did what. Frankly, it's not worth the headache when forking processes is just as fast and implicitly thread safe unless you explicitly share something. At least with explicit sharing you know exactly where to look if a threading style problem occurs.
If performance is that important, add another computer and load balance. For the developer cost of debugging a multi-threaded app, even one written by an experienced multi-threader, you could probably buy 4 40 core Intel motherboards with 64gigs of memory each.
That being said, there are asymmetric cases where parallel processing isn't appropriate, like, you want a foreground thread to accept user input and show button presses immediately, without waiting for some clunky back end GUI to keep up. Sexy use of threads where multiprocessing isn't geometrically appropriate. Many things like that just variables or pointers. They aren't "handles" that can be shared in a fork. You have to use threads. Even if you did fork, you'd be sharing the same resource and subject to threading style issues.
If you need to share resources, you really should use threads.
Also consider the fact that context switches between threads are much less expensive than context switches between processes.
I see no reason to explicitly go with separate processes unless you have a good reason to do so (security, proven performance tests, etc...)
Related
I am beginner in this area.
I have studied fork(), vfork(), clone() and pthreads.
I have noticed that pthread_create() will create a thread, which is less overhead than creating a new process with fork(). Additionally the thread will share file descriptors, memory, etc with parent process.
But when is fork() and clone() better than pthreads? Can you please explain it to me by giving real world example?
Thanks in Advance.
clone(2) is a Linux specific syscall mostly used to implement threads (in particular, it is used for pthread_create). With various arguments, clone can also have a fork(2)-like behavior. Very few people directly use clone, using the pthread library is more portable. You probably need to directly call clone(2) syscall only if you are implementing your own thread library - a competitor to Posix-threads - and this is very tricky (in particular because locking may require using futex(2) syscall in machine-tuned assembly-coded routines, see futex(7)). You don't want to directly use clone or futex because the pthreads are much simpler to use.
(The other pthread functions require some book-keeping to be done internally in libpthread.so after a clone during a pthread_create)
As Jonathon answered, processes have their own address space and file descriptor set. And a process can execute a new executable program with the execve syscall which basically initialize the address space, the stack and registers for starting a new program (but the file descriptors may be kept, unless using close-on-exec flag, e.g. thru O_CLOEXEC for open).
On Unix-like systems, all processes (except the very first process, usuallyinit, of pid 1) are created by fork (or variants like vfork; you could, but don't want to, use clone in such way as it behaves like fork).
(technically, on Linux, there are some few weird exceptions which you can ignore, notably kernel processes or threads and some rare kernel-initiated starting of processes like /sbin/hotplug ....)
The fork and execve syscalls are central to Unix process creation (with waitpid and related syscalls).
A multi-threaded process has several threads (usually created by pthread_create) all sharing the same address space and file descriptors. You use threads when you want to work in parallel on the same data within the same address space, but then you should care about synchronization and locking. Read a pthread tutorial for more.
I suggest you to read a good Unix programming book like Advanced Unix Programming and/or the (freely available) Advanced Linux Programming
The strength and weakness of fork (and company) is that they create a new process that's a clone of the existing process.
This is a weakness because, as you pointed out, creating a new process has a fair amount of overhead. It also means communication between the processes has to be done via some "approved" channel (pipes, sockets, files, shared-memory region, etc.)
This is a strength because it provides (much) greater isolation between the parent and the child. If, for example, a child process crashes, you can kill it and start another fairly easily. By contrast, if a child thread dies, killing it is problematic at best -- it's impossible to be certain what resources that thread held exclusively, so you can't clean up after it. Likewise, since all the threads in a process share a common address space, one thread that ran into a problem could overwrite data being used by all the other threads, so just killing that one thread wouldn't necessarily be enough to clean up the mess.
In other words, using threads is a little bit of a gamble. As long as your code is all clean, you can gain some efficiency by using multiple threads in a single process. Using multiple processes adds a bit of overhead, but can make your code quite a bit more robust, because it limits the damage a single problem can cause, and makes it much easy to shut down and replace a process if it does run into a major problem.
As far as concrete examples go, Apache might be a pretty good one. It will use multiple threads per process, but to limit the damage in case of problems (among other things), it limits the number of threads per process, and can/will spawn several separate processes running concurrently as well. On a decent server you might have, for example, 8 processes with 8 threads each. The large number of threads helps it service a large number of clients in a mostly I/O bound task, and breaking it up into processes means if a problem does arise, it doesn't suddenly become completely un-responsive, and can shut down and restart a process without losing a lot.
These are totally different things. fork() creates a new process. pthread_create() creates a new thread, which runs under the context of the same process.
Thread share the same virtual address space, memory (for good or for bad), set of open file descriptors, among other things.
Processes are (essentially) totally separate from each other and cannot modify each other.
You should read this question:
What is the difference between a process and a thread?
As for an example, if I am your shell (eg. bash), when you enter a command like ls, I am going to fork() a new process, and then exec() the ls executable. (And then I wait() on the child process, but that's getting out of scope.) This happens in an entire different address space, and if ls blows up, I don't care, because I am still executing in my own process.
On the other hand, say I am a math program, and I have been asked to multiply two 100x100 matrices. We know that matrix multiplication is an Embarrassingly Parallel problem. So, I have the matrices in memory. I spawn of N threads, who each operate on the same source matrices, putting their results in the appropriate location in the result matrix. Remember, these operate in the context of the same process, so I need to make sure they are not stamping on each other's data. If N is 8 and I have an eight-core CPU, I can effectively calculate each part of the matrix simultaneously.
Process creation mechanism on unix using fork() (and family) is very efficient.
Morever , most unix system doesnot support kernel level threads i.e thread is not entity recognized by kernel. Hence thread on such system cannot get benefit of CPU scheduling at kernel level. pthread library does that which is not kerenl rather some process itself.
Also on such system pthreads are implemented using vfork() and as light weight process only.
So using threading has no point except portability on such system.
As per my understanding Sun-solaris and windows has kernel level thread and linux family doesn't support kernel threads.
with processes pipes and unix doamin sockets are very efficient IPC without synchronization issues.
I hope it clears why and when thread should be used practically.
What are the main advantages of using a model for concurrency based on processes over one
based on threads and in what contexts is the latter appropriate?
Fault-tolerance and scalability are the main advantages of using Processes vs. Threads.
A system that relies on shared memory or some other kind of technology that is only available when using threads, will be useless when you want to run the system on multiple machines. Sooner or later you will need to communicate between different processes.
When using processes you are forced to deal with communication via messages, for example, this is the way Erlang handles communication. Data is not shared, so there is no risk of data corruption.
Another advantage of processes is that they can crash and you can feel relatively safe in the knowledge that you can just restart them (even across network hosts). However, if a thread crashes, it may crash the entire process, which may bring down your entire application. To illustrate: If an Erlang process crashes, you will only lose that phone call, or that webrequest, etc. Not the whole application.
In saying all this, OS processes also have many drawbacks that can make them harder to use, like the fact that it takes forever to spawn a new process. However, Erlang has it's own notion of processes, which are extremely lightweight.
With that said, this discussion is really a topic of research. If you want to get into more of the details, you can give Joe Armstrong's paper on fault-tolerant systems]1 a read, it explains a lot about Erlang and the philosophy that drives it.
The disadvantage of using a process-based model is that it will be slower. You will have to copy data between the concurrent parts of your program.
The disadvantage of using a thread-based model is that you will probably get it wrong. It may sound mean, but it's true-- show me code based on threads and I'll show you a bug. I've found bugs in threaded code that has run "correctly" for 10 years.
The advantages of using a process-based model are numerous. The separation forces you to think in terms of protocols and formal communication patterns, which means its far more likely that you will get it right. Processes communicating with each other are easier to scale out across multiple machines. Multiple concurrent processes allows one process to crash without necessarily crashing the others.
The advantage of using a thread-based model is that it is fast.
It may be obvious which of the two I prefer, but in case it isn't: processes, every day of the week and twice on Sunday. Threads are too hard: I haven't ever met anybody who could write correct multi-threaded code; those that claim to be able to usually don't know enough about the space yet.
In this case Processes are more independent of eachother, while Threads shares some resources e.g. memory. But in a general case Threads are more light-weight than Processes.
Erlang Processes is not the same thing as OS Processes. Erlang Processes are very light-weight and Erlang can have many Erlang Processes within the same OS Thread. See Technically why is processes in Erlang more efficient than OS threads?
First and foremost, processes differ from threads mostly in the way their memory is handled:
Process = n*Thread + memory region (n>=1)
Processes have their own isolated memory.
Processes can have multiple threads.
Processes are isolated from each other on the operating system level.
Threads share their memory with their peers in the process.
(This is often undesirable. There are libraries and methods out there to remedy this, but that is usually an artificial layer over operating system threads.)
The memory thing is the most important discerning factor, as it has certain implications:
Exchanging data between processes is slower than between threads. Breaking the process isolation always requires some involvement of kernel calls and memory remapping.
Threads are more lightweight than processes. The operating system has to allocate resources and do memory management for each process.
Using processes gives you memory isolation and synchronization. Common problems with access to memory shared between threads do not concern you. Since you have to make a special effort to share data between processes, you will most likely sync automatically with that.
Using processes gives you good (or ultimate) encapsulation. Since inter process communication needs special effort, you will be forced to define a clean interface. It is a good idea to break certain parts of your application out of the main executable. Maybe you can split dependencies like that.
e.g. Process_RobotAi <-> Process_RobotControl
The AI will have vastly different dependencies compared to the control component. The interface might be simple: Process_RobotAI --DriveXY--> Process_RobotControl.
Maybe you change the robot platform. You only have to implement a new RobotControl executable with that simple interface. You don't have to touch or even recompile anything in your AI component.
It will also, for the same reasons, speed up compilation in most cases.
Edit: Just for completeness I will shamelessly add what the others have reminded me of :
A crashing process does not (necessarily) crash your whole application.
In General:
Want to create something highly concurrent or synchronuous, like an algorithm with n>>1 instances running in parallel and sharing data, use threads.
Have a system with multiple components that do not need to share data or algorithms, nor do they exchange data too often, use processes. If you use a RPC library for the inter process communication, you get a network-distributable solution at no extra cost.
1 and 2 are the extreme and no-brainer scenarios, everything in between must be decided individually.
For a good (or awesome) example of a system that uses IPC/RPC heavily, have a look at ros.
I've heard that under linux on multicore server it would be impossible to reach top performance when you have just 1 process but multiple threads because Linux have some limitations on the IO, so that 1 process with 8 threads on 8-core server might be slower than 8 processes.
Any comments? Are there other limitation which might slow the applications?
The applications is a network C++ application, serving 100s of clients, with some disk IO.
Update: I am concerned that there are some more IO-related issues other than the locking I implement myself... Aren't there any issues doing simultanious network/disk IO in several threads?
Drawbacks of Threads
Threads:
Serialize on memory operations. That is the kernel, and in turn the MMU must service operations such as mmap() that perform page allocations.
Share the same file descriptor table. There is locking involved making changes and performing lookups in this table, which stores stuff like file offsets, and other flags. Every system call made that uses this table such as open(), accept(), fcntl() must lock it to translate fd to internal file handle, and when make changes.
Share some scheduling attributes. Processes are constantly evaluated to determine the load they're putting on the system, and scheduled accordingly. Lots of threads implies a higher CPU load, which the scheduler typically dislikes, and it will increase the response time on events for that process (such as reading incoming data on a socket).
May share some writable memory. Any memory being written to by multiple threads (especially slow if it requires fancy locking), will generate all kinds of cache contention and convoying issues. For example heap operations such as malloc() and free() operate on a global data structure (that can to some degree be worked around). There are other global structures also.
Share credentials, this might be an issue for service-type processes.
Share signal handling, these will interrupt the entire process while they're handled.
Processes or Threads?
If you want to make debugging easier, use threads.
If you are on Windows, use threads. (Processes are extremely heavyweight in Windows).
If stability is a huge concern, try to use processes. (One SIGSEGV/PIPE is all it takes...).
If threads aren't available, use processes. (Not so common now, but it did happen).
If your threads share resources that can't be use from multiple processes, use threads. (Or provide an IPC mechanism to allow communicating with the "owner" thread of the resource).
If you use resources that are only available on a one-per-process basis (and you one per context), obviously use processes.
If your processing contexts share absolutely nothing (such as a socket server that spawns and forgets connections as it accept()s them), and CPU is a bottleneck, use processes and single-threaded runtimes (which are devoid of all kinds of intense locking such as on the heap and other places).
One of the biggest differences between threads and processes is this: Threads use software constructs to protect data structures, processes use hardware (which is significantly faster).
Links
pthreads(7)
About Processes and Threads (MSDN)
Threads vs. Processes
it really should make no difference but is probably about design.
A multi process app may have to do less locking but may use more memory. Sharing data between processes may be harder.
On the other hand multi process can be more robust. You can call exit() and quit the child safely mostly without affecting others.
It depends how dependent the clients are. I usually recommend the simplest solution.
Separating different parts of a program into different processes seems (to me) to make a more elegant program than just threading everything. In what scenario would it make sense to make things run on a thread vs. separating the program into different processes? When should I use a thread?
Edit
Anything on how (or if) they act differently with single-core and multi-core would also be helpful.
You'd prefer multiple threads over multiple processes for two reasons:
Inter-thread communication (sharing data etc.) is significantly simpler to program than inter-process communication.
Context switches between threads are faster than between processes. That is, it's quicker for the OS to stop one thread and start running another than do the same with two processes.
Example:
Applications with GUIs typically use one thread for the GUI and others for background computation. The spellchecker in MS Office, for example, is a separate thread from the one running the Office user interface. In such applications, using multiple processes instead would result in slower performance and code that's tough to write and maintain.
Well apart from advantages of using thread over process, like:
Advantages:
Much quicker to create a thread than
a process.
Much quicker to switch
between threads than to switch
between processes.
Threads share data
easily
Consider few disadvantages too:
No security between threads.
One thread can stomp on another thread's
data.
If one thread blocks, all
threads in task block.
As to the important part of your question "When should I use a thread?"
Well you should consider few facts that a threads should not alter the semantics of a program. They simply change the timing of operations. As a result, they are almost always used as an elegant solution to performance related problems. Here are some examples of situations where you might use threads:
Doing lengthy processing: When a windows application is calculating it cannot process any more messages. As a result, the display cannot be updated.
Doing background processing: Some
tasks may not be time critical, but
need to execute continuously.
Doing I/O work: I/O to disk or to
network can have unpredictable
delays. Threads allow you to ensure
that I/O latency does not delay
unrelated parts of your application.
I assume you already know you need a thread or a process, so I'd say the main reason to pick one over the other would be data sharing.
Use of a process means you also need Inter Process Communication (IPC) to get data in and out of the process. This is a good thing if the process is to be isolated though.
You sure don't sound like a newbie. It's an excellent observation that processes are, in many ways, more elegant. Threads are basically an optimization to avoid too many transitions or too much communication between memory spaces.
Superficially using threads may also seem like it makes your program easier to read and write, because you can share variables and memory between the threads freely. In practice, doing that requires very careful attention to avoid race conditions or deadlocks.
There are operating-system kernels (most notably L4) that try very hard to improve the efficiency of inter-process communication. For such systems one could probably make a convincing argument that threads are pointless.
I would like to answer this in a different way. "It depends on your application's working scenario and performance SLA" would be my answer.
For instance threads may be sharing the same address space and communication between threads may be faster and easier but it is also possible that under certain conditions threads deadlock and then what do you think would happen to your process.
Even if you are a programming whiz and have used all the fancy thread synchronization mechanisms to prevent deadlocks it certainly is not rocket science to see that unless a deterministic model is followed which may be the case with hard real time systems running on Real Time OSes where you have a certain degree of control over thread priorities and can expect the OS to respect these priorities it may not be the case with General Purpose OSes like Windows.
From a Design perspective too you might want to isolate your functionality into independent self contained modules where they may not really need to share the same address space or memory or even talk to each other. This is a case where processes will make sense.
Take the case of Google Chrome where multiple processes are spawned as opposed to most browsers which use a multi-threaded model.
Each tab in Chrome can be talking to a different server and rendering a different website. Imagine what would happen if one website stopped responding and if you had a thread stalled due to this, the entire browser would either slow down or come to a stop.
So Google decided to spawn multiple processes and that is why even if one tab freezes you can still continue using other tabs of your Chrome browser.
Read more about it here
and also look here
I agree to most of the answers above. But speaking from design perspective i would rather go for a thread when i want set of logically co-related operations to be carried out parallel. For example if you run a word processor there will be one thread running in foreground as an editor and other thread running in background auto saving the document at regular intervals so no one would design a process to do that auto saving task separately.
In addition to the other answers, maintaining and deploying a single process is a lot simpler than having a few executables.
One would use multiple processes/executables to provide a well-defined interface/decoupling so that one part or the other can be reused or reimplemented more easily than keeping all the functionality in one process.
Came across this post. Interesting discussion. but I felt one point is missing or indirectly pointed.
Creating a new process is costly because of all of the
data structures that must be allocated and initialized. The process is subdivided into different threads of control to achieve multithreading inside the process.
Using a thread or a process to achieve the target is based on your program usage requirements and resource utilization.
Whilst learning the "assembler language" (in linux on a x86 architecture using the GNU as assembler), one of the aha moments was the possibility of using system calls. These system calls come in very handy and are sometimes even necessary as your program runs in user-space.
However system calls are rather expensive in terms of performance as they require an interrupt (and of course a system call) which means that a context switch must be made from your current active program in user-space to the system running in kernel-space.
The point I want to make is this: I'm currently implementing a compiler (for a university project) and one of the extra features I wanted to add is the support for multi-threaded code in order to enhance the performance of the compiled program. Because some of the multi-threaded code will be automatically generated by the compiler itself, this will almost guarantee that there will be really tiny bits of multi-threaded code in it as well. In order to gain a performance win, I must be sure that using threads will make this happen.
My fear however is that, in order to use threading, I must make system calls and the necessary interrupts. The tiny little (auto-generated) threads will therefore be highly affected by the time it takes to make these system calls, which could even lead to a performance loss...
my question is therefore twofold (with an extra bonus question underneath it):
Is it possible to write assembler
code which can run multiple threads
simultaneously on multiple cores at
once, without the need of system
calls?
Will I get a performance gain if I have really tiny threads (tiny as in the total execution time of the thread), performance loss, or isn't it worth the effort at all?
My guess is that multithreaded assembler code is not possible without system calls. Even if this is the case, do you have a suggestion (or even better: some real code) for implementing threads as efficient as possible?
The short answer is that you can't. When you write assembly code it runs sequentially (or with branches) on one and only one logical (i.e. hardware) thread. If you want some of the code to execute on another logical thread (whether on the same core, on a different core on the same CPU or even on a different CPU), you need to have the OS set up the other thread's instruction pointer (CS:EIP) to point to the code you want to run. This implies using system calls to get the OS to do what you want.
User threads won't give you the threading support that you want, because they all run on the same hardware thread.
Edit: Incorporating Ira Baxter's answer with Parlanse. If you ensure that your program has a thread running in each logical thread to begin with, then you can build your own scheduler without relying on the OS. Either way, you need a scheduler to handle hopping from one thread to another. Between calls to the scheduler, there are no special assembly instructions to handle multi-threading. The scheduler itself can't rely on any special assembly, but rather on conventions between parts of the scheduler in each thread.
Either way, whether or not you use the OS, you still have to rely on some scheduler to handle cross-thread execution.
"Doctor, doctor, it hurts when I do this". Doctor: "Don't do that".
The short answer is you can do multithreaded programming without
calling expensive OS task management primitives. Simply ignore the OS for thread
scheduling operations. This means you have to write your own thread
scheduler, and simply never pass control back to the OS.
(And you have to be cleverer somehow about your thread overhead
than the pretty smart OS guys).
We chose this approach precisely because windows process/thread/
fiber calls were all too expensive to support computation
grains of a few hundred instructions.
Our PARLANSE programming langauge is a parallel programming language:
See http://www.semdesigns.com/Products/Parlanse/index.html
PARLANSE runs under Windows, offers parallel "grains" as the abstract parallelism
construct, and schedules such grains by a combination of a highly
tuned hand-written scheduler and scheduling code generated by the
PARLANSE compiler that takes into account the context of grain
to minimimze scheduling overhead. For instance, the compiler
ensures that the registers of a grain contain no information at the point
where scheduling (e.g., "wait") might be required, and thus
the scheduler code only has to save the PC and SP. In fact,
quite often the scheduler code doesnt get control at all;
a forked grain simply stores the forking PC and SP,
switches to compiler-preallocated stack and jumps to the grain
code. Completion of the grain will restart the forker.
Normally there's an interlock to synchronize grains, implemented
by the compiler using native LOCK DEC instructions that implement
what amounts to counting semaphores. Applications
can fork logically millions of grains; the scheduler limits
parent grains from generating more work if the work queues
are long enough so more work won't be helpful. The scheduler
implements work-stealing to allow work-starved CPUs to grab
ready grains form neighboring CPU work queues. This has
been implemented to handle up to 32 CPUs; but we're a bit worried
that the x86 vendors may actually swamp use with more than
that in the next few years!
PARLANSE is a mature langauge; we've been using it since 1997,
and have implemented a several-million line parallel application in it.
Implement user-mode threading.
Historically, threading models are generalised as N:M, which is to say N user-mode threads running on M kernel-model threads. Modern useage is 1:1, but it wasn't always like that and it doesn't have to be like that.
You are free to maintain in a single kernel thread an arbitrary number of user-mode threads. It's just that it's your responsibility to switch between them sufficiently often that it all looks concurrent. Your threads are of course co-operative rather than pre-emptive; you basically scatted yield() calls throughout your own code to ensure regular switching occurs.
If you want to gain performance, you'll have to leverage kernel threads. Only the kernel can help you get code running simultaneously on more than one CPU core. Unless your program is I/O bound (or performing other blocking operations), performing user-mode cooperative multithreading (also known as fibers) is not going to gain you any performance. You'll just be performing extra context switches, but the one CPU that your real thread is running will still be running at 100% either way.
System calls have gotten faster. Modern CPUs have support for the sysenter instruction, which is significantly faster than the old int instruction. See also this article for how Linux does system calls in the fastest way possible.
Make sure that the automatically-generated multithreading has the threads run for long enough that you gain performance. Don't try to parallelize short pieces of code, you'll just waste time spawning and joining threads. Also be wary of memory effects (although these are harder to measure and predict) -- if multiple threads are accessing independent data sets, they will run much faster than if they were accessing the same data repeatedly due to the cache coherency problem.
Quite a bit late now, but I was interested in this kind of topic myself.
In fact, there's nothing all that special about threads that specifically requires the kernel to intervene EXCEPT for parallelization/performance.
Obligatory BLUF:
Q1: No. At least initial system calls are necessary to create multiple kernel threads across the various CPU cores/hyper-threads.
Q2: It depends. If you create/destroy threads that perform tiny operations then you're wasting resources (the thread creation process would greatly exceed the time used by the tread before it exits). If you create N threads (where N is ~# of cores/hyper-threads on the system) and re-task them then the answer COULD be yes depending on your implementation.
Q3: You COULD optimize operation if you KNEW ahead of time a precise method of ordering operations. Specifically, you could create what amounts to a ROP-chain (or a forward call chain, but this may actually end up being more complex to implement). This ROP-chain (as executed by a thread) would continuously execute 'ret' instructions (to its own stack) where that stack is continuously prepended (or appended in the case where it rolls over to the beginning). In such a (weird!) model the scheduler keeps a pointer to each thread's 'ROP-chain end' and writes new values to it whereby the code circles through memory executing function code that ultimately results in a ret instruction. Again, this is a weird model, but is intriguing nonetheless.
Onto my 2-cents worth of content.
I recently created what effectively operate as threads in pure assembly by managing various stack regions (created via mmap) and maintaining a dedicated area to store the control/individualization information for the "threads". It is possible, although I didn't design it this way, to create a single large block of memory via mmap that I subdivide into each thread's 'private' area. Thus only a single syscall would be required (although guard pages between would be smart these would require additional syscalls).
This implementation uses only the base kernel thread created when the process spawns and there is only a single usermode thread throughout the entire execution of the program. The program updates its own state and schedules itself via an internal control structure. I/O and such are handled via blocking options when possible (to reduce complexity), but this isn't strictly required. Of course I made use of mutexes and semaphores.
To implement this system (entirely in userspace and also via non-root access if desired) the following were required:
A notion of what threads boil down to:
A stack for stack operations (kinda self explaining and obvious)
A set of instructions to execute (also obvious)
A small block of memory to hold individual register contents
What a scheduler boils down to:
A manager for a series of threads (note that processes never actually execute, just their thread(s) do) in a scheduler-specified ordered list (usually priority).
A thread context switcher:
A MACRO injected into various parts of code (I usually put these at the end of heavy-duty functions) that equates roughly to 'thread yield', which saves the thread's state and loads another thread's state.
So, it is indeed possible to (entirely in assembly and without system calls other than initial mmap and mprotect) to create usermode thread-like constructs in a non-root process.
I only added this answer because you specifically mention x86 assembly and this answer was entirely derived via a self-contained program written entirely in x86 assembly that achieves the goals (minus multi-core capabilities) of minimizing system calls and also minimizes system-side thread overhead.
System calls are not that slow now, with syscall or sysenter instead of int. Still, there will only be an overhead when you create or destroy the threads. Once they are running, there are no system calls. User mode threads will not really help you, since they only run on one core.
First you should learn how to use threads in C (pthreads, POSIX theads). On GNU/Linux you will probably want to use POSIX threads or GLib threads.
Then you can simply call the C from assembly code.
Here are some pointers:
Posix threads: link text
A tutorial where you will learn how to call C functions from assembly: link text
Butenhof's book on POSIX threads link text