[Caml-list] threads, signals, and timeout
Hi, I would like to create different threads where each thread do some computation and are subject to different timeout. Without threads I usually use Unix.alarm with a SIGALARM handler that just raise a Timeout exception and everything works fine, but when I try to do something similar with threads it does not work because apparently the Unix.alarm done in one thread override the Unix.alarm done in another thread. I had a look at thread.mli but was not able to find anything related to timeout. Is there a way to have multiple timeout and multiple threads at the same time ? Here is a program that unforunately get the first timeout, but not the second :( (* ocamlc -g -thread unix.cma threads.cma signals_and_threads.ml *) exception Timeout let mytid () = let t = Thread.self () in let i = Thread.id t in i let set_timeout () = Sys.set_signal Sys.sigalrm (Sys.Signal_handle (fun _ - prerr_endline Time is up!; print_string (Printf.sprintf id: %d\n (mytid())); raise Timeout )); ignore(Unix.alarm 1); () let main = let t1 = Thread.create (fun () - set_timeout (); print_string (Printf.sprintf t1 id: %d\n (mytid())); let xs = [1;2;3] in while(true) do let _ = List.map (fun x - x + 1) xs in () done; () ) () in let t2 = Thread.create (fun () - set_timeout (); print_string (Printf.sprintf t2 id: %d\n (mytid())); let xs = [1;2;3] in while(true) do let _ = List.map (fun x - x + 1) xs in () done; () ) () in Thread.join t1; Thread.join t2; () -- Here is the output Time is up! t2 id: 2 t1 id: 1 id: 1 Thread 1 killed on uncaught exception Signals_and_threads.Timeout the program loops, meaning the second thread never received its timeout ___ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
Re: [Caml-list] threads, signals, and timeout
On Mon, Oct 26, 2009 at 11:36 AM, Till Varoquaux t...@pps.jussieu.fr wrote: You'd have the same problem in any other programming language; this is due to the underlying POSIX model. The POSIX standard mentions that alarms are global to the process [1]. There were two possible choices for alarm generation in multi-threaded applications: generation for the calling thread or generation for the process. The first option would not have been particularly useful since the alarm state is maintained on a per-process basis and the alarm that is established by the last invocation of alarm() is the only one that would be active. I've seen that that there are other C APIs for timers: setitimer, create_timer, ... are they all subject to the same limitations ? As a general matter mixing signals and threads is a thorny issue under unix; you should avoid doing so unless you have a good grasp of what is going to happen. Having a global timer thread that kills timed-out threads would be possible but seems very painful. I dont want to kill them. I want to deliver timeout exceptions to those threads, and each of those threads should be able to react differently to those timeouts. If you give us more information on what you are trying to achieve (eg your computations threads are crunshing numbers in a tight loop or you are performing io etc...) we might be able to guide you. I want to write a toy program that uses threads and timeouts in an easy way. Those threads may make heavy use of CPU or IO, I don't know. I just want an easy API for thread and timeout. Here is a quick run down of my flawed and partial understand of the world: For heavy computations threads will not help you. I am sure you are aware of this but there never is more than one ocaml thread running at the same time. To be honest, the problem I have to solve is not in OCaml, but I thought OCaml programmers would give better answers than C programmers :) I find it wierd that using timeout with process is easy, and using timeouts with threads is not. Apparently on google they say a usual technique is to have a master thread which handles all the signals, with all the other threads blocking them. But it would still require this master thread juggling with multiple timeouts requests and deliveries, and then to deliver manually some signals to the specific thread (with pthread_kill ?). I don't want to write my own scheduler. Those things should be handled at kernel level; the kernel should know about threads, and should be able to deliver signals to the thread that requested them. Maybe the new Linux thread library (NPTL) can do that ? If you can fork and collect the resulting data through pipes or shared memory you should be able to get a noticeable performance boost. There have been countless posts on the subject; I am too lazy to google them up. I/O is a very different beast and you are probably better of with an asynchronous framework. Luckily for you Ocaml has a pretty good one already available [2] Till [1]: http://www.opengroup.org/onlinepubs/95399/functions/alarm.html [2]: http://ocsigen.org/lwt/ On Mon, Oct 26, 2009 at 2:08 PM, yoann padioleau pad.a...@gmail.com wrote: Hi, I would like to create different threads where each thread do some computation and are subject to different timeout. Without threads I usually use Unix.alarm with a SIGALARM handler that just raise a Timeout exception and everything works fine, but when I try to do something similar with threads it does not work because apparently the Unix.alarm done in one thread override the Unix.alarm done in another thread. I had a look at thread.mli but was not able to find anything related to timeout. Is there a way to have multiple timeout and multiple threads at the same time ? Here is a program that unforunately get the first timeout, but not the second :( (* ocamlc -g -thread unix.cma threads.cma signals_and_threads.ml *) exception Timeout let mytid () = let t = Thread.self () in let i = Thread.id t in i let set_timeout () = Sys.set_signal Sys.sigalrm (Sys.Signal_handle (fun _ - prerr_endline Time is up!; print_string (Printf.sprintf id: %d\n (mytid())); raise Timeout )); ignore(Unix.alarm 1); () let main = let t1 = Thread.create (fun () - set_timeout (); print_string (Printf.sprintf t1 id: %d\n (mytid())); let xs = [1;2;3] in while(true) do let _ = List.map (fun x - x + 1) xs in () done; () ) () in let t2 = Thread.create (fun () - set_timeout (); print_string (Printf.sprintf t2 id: %d\n (mytid())); let xs = [1;2;3] in while(true) do let _ = List.map (fun x - x + 1) xs in () done; () ) () in Thread.join t1; Thread.join t2; () -- Here is the output Time is up! t2 id: 2 t1 id: 1 id: 1 Thread 1 killed on
Re: [Caml-list] threads, signals, and timeout
Am Montag, den 26.10.2009, 12:06 -0700 schrieb yoann padioleau: On Mon, Oct 26, 2009 at 11:36 AM, Till Varoquaux t...@pps.jussieu.fr wrote: You'd have the same problem in any other programming language; this is due to the underlying POSIX model. The POSIX standard mentions that alarms are global to the process [1]. There were two possible choices for alarm generation in multi-threaded applications: generation for the calling thread or generation for the process. The first option would not have been particularly useful since the alarm state is maintained on a per-process basis and the alarm that is established by the last invocation of alarm() is the only one that would be active. I've seen that that there are other C APIs for timers: setitimer, create_timer, ... are they all subject to the same limitations ? As a general matter mixing signals and threads is a thorny issue under unix; you should avoid doing so unless you have a good grasp of what is going to happen. Having a global timer thread that kills timed-out threads would be possible but seems very painful. I dont want to kill them. I want to deliver timeout exceptions to those threads, and each of those threads should be able to react differently to those timeouts. If you give us more information on what you are trying to achieve (eg your computations threads are crunshing numbers in a tight loop or you are performing io etc...) we might be able to guide you. I want to write a toy program that uses threads and timeouts in an easy way. Those threads may make heavy use of CPU or IO, I don't know. I just want an easy API for thread and timeout. Here is a quick run down of my flawed and partial understand of the world: For heavy computations threads will not help you. I am sure you are aware of this but there never is more than one ocaml thread running at the same time. To be honest, the problem I have to solve is not in OCaml, but I thought OCaml programmers would give better answers than C programmers :) I find it wierd that using timeout with process is easy, and using timeouts with threads is not. Apparently on google they say a usual technique is to have a master thread which handles all the signals, with all the other threads blocking them. But it would still require this master thread juggling with multiple timeouts requests and deliveries, and then to deliver manually some signals to the specific thread (with pthread_kill ?). I don't want to write my own scheduler. Those things should be handled at kernel level; the kernel should know about threads, and should be able to deliver signals to the thread that requested them. Maybe the new Linux thread library (NPTL) can do that ? Till gave already a quite good answer. I think your assumption is already wrong that using timeouts with processes is easy. It's only easy as long you don't care about race conditions and subtle problems of signals (e.g. that two pending signals are merged by the kernel). Most likely you get a program that works 99% of the time, and fails (hangs, or loops, or misses signals) 1% of the time. Signals are really low-level. Combining threads and signals leads to numerous implementation problems. Both the threads and signals API define primitives for synchronization (e.g. mutexes for threads, and masks for signals). As signals can be generated at any time, there is no control about the state of the thread-level synchronization objects: When the signal handler is entered, you don't know whether a thread (either the thread the handler is running in, or some other) has locked the mutex or not (given that the signal handler accesses some shared data). Even worse, you cannot simply wait in the handler until the mutex is free again - this could be a deadlock. Essentially, you cannot synchronize data access of a signal handler and of threads - it's conceptually impossible. Think like this: A single thread is executed sequentially, and the primitives for synchronizing several threads rely on that. Signals, however, allow it to spontaneously interrupt the sequential execution, as if one thread would be many. That breaks assumptions of multi-threading. That's the background why POSIX chose not to define a per-thread signal API. There are no per-thread timers, and you need to write your own mini scheduler. Gerd If you can fork and collect the resulting data through pipes or shared memory you should be able to get a noticeable performance boost. There have been countless posts on the subject; I am too lazy to google them up. I/O is a very different beast and you are probably better of with an asynchronous framework. Luckily for you Ocaml has a pretty good one already available [2] Till [1]: http://www.opengroup.org/onlinepubs/95399/functions/alarm.html [2]: http://ocsigen.org/lwt/ On Mon, Oct 26, 2009 at 2:08 PM, yoann padioleau pad.a...@gmail.com wrote:
Re: [Caml-list] threads, signals, and timeout
Considering that posix signals are not real-time *anyway*, using them to programme specific treatments per-thread is hmmm... say a nightmare ! Plus I don't quite see how you could eventually have a non-broken implementation. Gerd Stolpmann emphasized it if I understood well. One solution would be to use state variables to check every once in a while. Or maybe to use fairthreads instead, but I guess that the problem is actually much more complicated than just that. Well, I thought I had more interesting things to say. I was wrong, then just my two cents. Anyways, good luck! Cheers, Philippe Wang On Mon, Oct 26, 2009 at 7:08 PM, yoann padioleau pad.a...@gmail.com wrote: Hi, I would like to create different threads where each thread do some computation and are subject to different timeout. Without threads I usually use Unix.alarm with a SIGALARM handler that just raise a Timeout exception and everything works fine, but when I try to do something similar with threads it does not work because apparently the Unix.alarm done in one thread override the Unix.alarm done in another thread. I had a look at thread.mli but was not able to find anything related to timeout. Is there a way to have multiple timeout and multiple threads at the same time ? Here is a program that unforunately get the first timeout, but not the second :( (* ocamlc -g -thread unix.cma threads.cma signals_and_threads.ml *) exception Timeout let mytid () = let t = Thread.self () in let i = Thread.id t in i let set_timeout () = Sys.set_signal Sys.sigalrm (Sys.Signal_handle (fun _ - prerr_endline Time is up!; print_string (Printf.sprintf id: %d\n (mytid())); raise Timeout )); ignore(Unix.alarm 1); () let main = let t1 = Thread.create (fun () - set_timeout (); print_string (Printf.sprintf t1 id: %d\n (mytid())); let xs = [1;2;3] in while(true) do let _ = List.map (fun x - x + 1) xs in () done; () ) () in let t2 = Thread.create (fun () - set_timeout (); print_string (Printf.sprintf t2 id: %d\n (mytid())); let xs = [1;2;3] in while(true) do let _ = List.map (fun x - x + 1) xs in () done; () ) () in Thread.join t1; Thread.join t2; () -- Here is the output Time is up! t2 id: 2 t1 id: 1 id: 1 Thread 1 killed on uncaught exception Signals_and_threads.Timeout the program loops, meaning the second thread never received its timeout ___ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
