Say I have a running process with 2 running threads, what happen to the threads if the process is terminated? do they get terminated as well?
Also, what happen to the threads if the process is 'losing' the CPU attention (Other process took the CPU's attention) and therefore the process is in waiting state / suspended. Do its threads keep running? or?
thanks
Yes, the threads get disposed of by the operating system if and when the process terminates and gets cleaned up.
The threads in a process are scheduled according to their priority. In most OSes that is fairly straight forward, it's a simple matter of the higher priority threads winning. Windows does something different, a process's priority is adjusted by Windows according to whether it is the foreground process. Thus the process's threads' priorities are also adjusted...
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I am working on some real-time programs that require mlock and FIFO scheduling policy for fast paths.
I am running two processes on Ubuntu 16.04 with 12 CPU cores, and I assigned the fast paths of these processes to different cores.
Process 1 starts normally and pins its fast thread to a CPU and sets the scheduling policy to FIFO on this thread.
When process 2 starts, before its fast thread is created, it tries to call mlock.
Then, process 2 is stuck.
I attached gdb to process 2, and the call stack seems to be inside the mlock function.
If I remove the FIFO setting on process 1, both processes can run normally.
My suspicion is that mlock is trying to access some kernel resources that is acquired by the fast thread of process 1.
So it is blocked and put on wait indefinitely.
Does anyone know exactly what it is waiting for?
I have observed this problem on two similar IBM servers with Ubuntu.
But, on a Supermicro machine with a Redhat Linux, this issue didn't occur.
Thanks for any hint or solution!
If you have a SCHED_FIFO process that completely occupied a CPU such that non-sched-fifo threads never get scheduled on that CPU, some in-kernel algorithms may stop working, depending on kernel version/configuration.
Try booting with rcutree.kthread_prio=N where N is larger than SCHED_FIFO priority of your thread.
Try playing with /proc/sys/kernel/sched_rt_runtime_us
Try to get in-kernel backtrace of hanged mlock() to understand where it is waiting - this may get a hint. For that, use /proc/pid/stack (if your kernel is compiled with CONFIG_STACKTRACE) or maybe 'echo t > /proc/sysrq-trigger'
After the Kernel schedules a process that has threads, How does said process schedule its own threads during its time splice?
For most modern kernels, the kernel only schedules threads, and processes are mostly just a container for the threads to execute inside (e.g. a container that contains a virtual address space, however many threads, and a few other scraps like file handles).
For some kernels (mostly very old unix kernels that existed before threads were invented) the kernel schedules processes and then a user-space library emulates threads. For this to work properly all of the "blocking" system calls (e.g. write()) have to be replaced by asynchronous system calls (e.g. aio_write()) so that other threads in the process can be given CPU time; however I wouldn't want to assume it works properly (e.g. if any thread blocks, then maybe all threads in the process block).
Also it may not work when there's multiple CPUs (kernel gives a process one CPU, but then from the kernel's perspective that process is running and can't use a second CPU). There are sophisticated work-arounds for this (to support "M:N threading") but it's just easier and better to fix the scheduler so it works with threads. Fortunately/unfortunately this didn't matter much in the early days because very few computers had more than one CPU anyway.
Lastly; it doesn't work for thread priorities - e.g. one process might keep CPU busy executing an unimportant/low priority thread while another process doesn't get that CPU time when it desperately needs it for an important/high priority thread. This occurs because no process knows about threads belonging to other processes and the kernel only knows about processes and not threads.
Of course these are also the reasons why every kernel adopted "kernel schedules threads and not processes" (and those that didn't died).
It's down to jargon definitions, but threads are simply a bunch of processes sharing an address space. Older Unixes even called them Light Weight Processes.
With that classical understanding of threads, the answer is that, these days, it's the OS that does the scheduling and each thread gets its own timeslices.
Extras
Some OSes do things to "the whole process" - e.g. Windows will give the process that has mouse focus a priority boost (all it's threads get dynamically notched up a few priority places), to make that application appear to be more sprightly (this goes back to Windows 3).
Other operating systems will increase the priority of a thread dynamically, to solve priority inversion situations. This is where a low priority thread that has control of a resource (I/O, or perhaps a semaphore) is blocking a higher priority thread from running (because the resource is not available. This is the priority inversion, and it's solved by the OS boosting the priority of the blocking thread until it gives up the required resource.
Either the kernel schedules the threads or the kernel schedules processes simulates thread by scheduling it own threads.
Usually, the process schedules its own threads using a library that sets timers. When the timer handler saves the current "thread's" registers then loads a new set of registers from another "thread."
In the synchronous/blocking model of computation we usually say that a thread of execution will wait (be blocked) while it waits for an IO task to complete.
My question is simply will this usually cause the CPU core executing the thread to be idle, or will a thread waiting on IO usually be context switched out and put into a waiting state until the IO is ready to be processed?
A CPU core is normally not dedicated to one particular thread of execution. The kernel is constantly switching processes being executed in and out of the CPU. The process currently being executed by the CPU is in the "running" state. The list of processes waiting for their turn are in a "ready" state. The kernel switches these in and out very quickly. Modern CPU features (multiple cores, simultaneous multithreading, etc.) try to increase the number of threads of execution that can be physically executed at once.
If a process is I/O blocked, the kernel will just set it aside (put it in the "waiting" state) and not even consider giving it time in the CPU. When the I/O has finished, the kernel moves the blocked process from the "waiting" state to the "ready" state so it can have its turn ("running") in the CPU.
So your blocked thread of execution blocks only that: the thread of execution. The CPU and the CPU cores continue to have other threads of execution switched in and out of them, and are not idle.
For most programming languages, used in standard ways, then the answer is that it will block your thread, but not your CPU.
You would need to explicitely reserve a CPU for a particular thread (affinity) for 1 thread to block an entire CPU. To be more explicit, see this question:
You could call the SetProcessAffinityMask on every process but yours with a mask that excludes just the core that will "belong" to your process, and use it on your process to set it to run just on this core (or, even better, SetThreadAffinityMask just on the thread that does the time-critical task).
If we assume it's not async, then I would say, in that case, your thread owning the thread would be put to the waiting queue for sure and the state would be "waiting".
Context-switching wise, IMO, it may need a little bit more explanation since the term context-switch can mean/involve many things (swapping in/out, page table updates, register updates, etc). Depending on the current state of execution, potentially, a second thread that belongs to the same process might be scheduled to run whilst the thread that was blocked on the IO operation is still waiting.
For example, then context-switching would most likely be limited to changing register values on the CPU regarding core (but potentially the owning process might even get swapped-out if there's no much memory left).
no,in java , block thread did't participate scheduling
Processes get CPU time as managed by the OS process scheduler.
Since threads run in parallel within a single process, does this mean that a process's CPU time is further distributed(sliced) among threads?
Or can the scheduler directly distribute CPU time among threads bypassing the parent process?
I suspect the answer varies with the OS. On Windows, the process is not merely bypassed, but completely ignored -- all the scheduler deals with is threads. Processes are relevant only to the degree that all non-kernel threads do have to belong to some process, and every process has to contain at least one thread.
The threads are run/scheduled by the operating system and therefore they get their own CPU time. The process CPU time is just the sum of the CPU times of all the threads in the process.
If you want your process to schedule the tasks itself, you should use fibers (Windows). These are a kind of threads but they are not scheduled by the OS. The process should handle the scheduling of fibers itself.
For Windows see http://msdn.microsoft.com/en-us/library/ms681917%28VS.85%29.aspx
I have couple of questions on threads. Could you please clarify.
Suppose process with one or multiple threads. If the process is prempted/suspended, does the threads also get preempted or does the threads continue to run?
When the suspended process rescheduled, does the process threads also gets scheduled? If the process has process has multiple threads, which threads will be rescheduled and on what basis?
if the thread in the process is running and recieves a signal(say Cntrl-C) and the default action of the signal is to terminate a process, does the running thread terminates or the parent process will also terminate? What happens to the threads if the running process terminates because of some signal?
If the thread does fork fallowed exec, does the exece'd program overlays the address space of parent process or the running thread? If it overlays the parent process what happens to threads, their data, locks they are holding and how they get scheduled once the exec'd process terminates.
Suppose process has multiple threads, how does the threads get scheduled. If one of the thread blocks on some I/O, how other threads gets scheduled. Does the threads scheduled with the parent process is running?
While the thread is running what the current kernel variable points(parent process task_stuct or threads stack_struct?
If the process with the thread is running, when the thread starts does the parent
process gets preempted and how each threads gets scheduled?
If the process running on CPU creates multiple threads, does the threads created by the parent process schedule on another CPU on multiprocessor system?
Thanks,
Ganesh
First, I should clear up some terminology that you appear to be confused about. In POSIX, a "process" is a single address space plus at least one thread of control, identified by a process ID (PID). A thread is an individually-scheduled execution context within a process.
All processes start life with just one thread, and all processes have at least one thread. Now, onto the questions:
Suppose process with one or multiple threads. If the process is prempted/suspended, does the threads also get preempted or does the threads continue to run?
Threads are scheduled independently. If a thread blocks on a function like connect(), then other threads within the process can still be scheduled.
It is also possible to request that every thread in a process be suspended, for example by sending SIGSTOP to the process.
When the suspended process rescheduled, does the process threads also gets scheduled? If the process has process has multiple threads, which threads will be rescheduled and on what basis?
This only makes sense in the context that an explicit request was made to stop the entire process. If you send the process SIGCONT to restart the process, then any of the threads which are not blocked can run. If more threads are runnable than there are processors available to run them, then it is unspecified which one(s) run first.
If the thread in the process is running and recieves a signal(say Cntrl-C) and the default action of the signal is to terminate a process, does the running thread terminates or the parent process will also terminate? What happens to the threads if the running process terminates because of some signal?
If a thread recieves a signal like SIGINT or SIGSEGV whose action is to terminate the process, then the entire process is terminated. This means that every thread in the process is unceremoniously killed.
If the thread does fork followed by exec, does the exece'd program overlays the address space of parent process or the running thread? If it overlays the parent process what happens to threads, their data, locks they are holding and how they get scheduled once the exec'd process terminates.
The fork() call creates a new process by duplicating the address space of the original process, and duplicating just the single thread that called fork() within that new address space.
If that thread in the new process calls execve(), it will replace the new, duplicated address space with the exec'd program. The original process, and all its threads, continue running normally.
Suppose process has multiple threads, how does the threads get scheduled. If one of the thread blocks on some I/O, how other threads gets scheduled. Does the threads scheduled with the parent process is running?
The threads are scheduled independently. Any of the threads that are not blocked can run.
While the thread is running what the current kernel variable points(parent process task_stuct or threads stack_struct?
Each thread has its own task_struct within the kernel. What userspace calls a "thread" is called a "process" in kernel space. Thus current always points at the task_struct corresponding to the currently executing thread (in the userspace sense of the word).
If the process with [a second] thread is running, when the thread starts does the parent process gets preempted and how each threads gets scheduled?
Presumably you mean "the process's main thread" rather than "parent process" here. As before, the threads are scheduled independently. It's unspecified whether one runs before the other - and if you have multiple CPUs, both might run simultaneously.
If the process running on CPU creates multiple threads, does the threads created by the parent process schedule on another CPU on multiprocessor system?
That's really up to the kernel, but the threads are certainly allowed to execute on other CPUs.
Depends. If a thread is preempted because the OS scheduler decides to give CPU time to some other thread, then other threads in the process will continue running. If the process is suspended (i.e. it gets the SIGSTP signal) then AFAIK all the threads will be suspended.
When a suspended process is woken up, all the threads are marked as waiting or blocked (if they are waiting e.g. on a mutex). Then the scheduler at some points run them. There is no guarantee about any specific order the threads are run after waking up the process.
The process will terminate, and with it the threads as well.
When you fork you get a new address space, so there is no "overlay". Note that fork() and the exec() family affect the entire process, not only the thread from which they where called. When you call fork() in a multi-threaded process, the child gets a copy of that process, but with only the calling thread. Then if you call exec() in one or both of the processes (presumably only in the child process, but that's up to you), then the process which calls exec() (and with it, all its threads) is replaced by the exec()'ed program.
The thread scheduling order is decided by the OS scheduler, there is no guarantee given about any particular order.
From the kernel perspective a process is an address space with one or more threads (and some other gunk). There is no concept of threads that somehow exist without a process.
There is no such thing as a process without a single thread. A "plain process" is just a process with a single thread.
Probably yes. This is determined by the OS scheduler. Note that there are API's and tools (numactl) that one can use to force some thread(s) to run on a specific CPU core.
Assuming your questions are about POSIX threads, then
1a. A process that's preempted by the O/S will have all its threads preempted.
1b. The O/S will suspend all the threads of a process that is sent a SIGSTOP.
The O/S will resume all thread of a suspended process that is sent a SIGCONT.
By default, a SIGINT will terminate all the threads in a process.
If a thread calls fork(), then all its threads are duplicated. If it then call one of the exec() functions, then all the duplicated threads disappear.
POSIX allows for user-selection of the thread scheduling algorithm.
I don't understand the question.
I don't understand the question.
How threads are mapped to CPU-s is implementation-dependent. Many implementations will try to distribute threads amongst the available CPU-s to improve performance.
The Linux kernel doesn't distinguish between threads and processes. As far as kernel is concerned, a thread is simply another process which happens to share address space with other processes. (You would call the set of "processes" (i.e. threads) which share a single address space a "process".)
So POSIX threads are scheduled exactly as full-blown processes would be. There is no difference in scheduling whether you have one process with five threads, or five separate processes.
There are kernel calls that provide fine grained control over what is shared between processes. The POSIX threads API wraps over them.