The following small test program seems to have a weird deadlock or something:
 It should keep printing the phrase "Doing stuff." forever, but it only gets
through maybe two iterations before its CPU usage does to zero and it stops
printing, at least on my computer.  Has anyone noticed any bad behavior with
std.stdio and multithreading?

import core.thread, std.stdio;

void main() {
    Thread[] myThreads;
    foreach(i; 0..4) {
        myThreads ~= new Thread( { doStuff(); });
        myThreads[$ - 1].start;
    }
}



void doStuff() {
    while(true) {
        synchronized {
            writeln("Doing stuff.");
        }
    }
}


If the writeln line is commented out, this thing keeps executing the empty loop with measurable CPU usage.

On Mon, 27 Apr 2009 18:53:04 -0400, dsimcha <dsimcha@yahoo.com> wrote:

> The following small test program seems to have a weird deadlock or something:
>  It should keep printing the phrase "Doing stuff." forever, but it only gets
> through maybe two iterations before its CPU usage does to zero and it stops
> printing, at least on my computer.  Has anyone noticed any bad behavior with
> std.stdio and multithreading?
>
> import core.thread, std.stdio;
>
> void main() {
>     Thread[] myThreads;
>     foreach(i; 0..4) {
>         myThreads ~= new Thread( { doStuff(); });
>         myThreads[$ - 1].start;
>     }
> }
>
>
>
> void doStuff() {
>     while(true) {
>         synchronized {
>             writeln("Doing stuff.");
>         }
>     }
> }
>
>
> If the writeln line is commented out, this thing keeps executing the empty
> loop with measurable CPU usage.

Shouldn't you be waiting for the threads to exit at the end of main?  I wonder if the GC has been shut down by main exiting.

-Steve

== Quote from Steven Schveighoffer (schveiguy@yahoo.com)'s article
> On Mon, 27 Apr 2009 18:53:04 -0400, dsimcha <dsimcha@yahoo.com> wrote:
> > The following small test program seems to have a weird deadlock or
> > something:
> >  It should keep printing the phrase "Doing stuff." forever, but it only
> > gets
> > through maybe two iterations before its CPU usage does to zero and it
> > stops
> > printing, at least on my computer.  Has anyone noticed any bad behavior
> > with
> > std.stdio and multithreading?
> >
> > import core.thread, std.stdio;
> >
> > void main() {
> >     Thread[] myThreads;
> >     foreach(i; 0..4) {
> >         myThreads ~= new Thread( { doStuff(); });
> >         myThreads[$ - 1].start;
> >     }
> > }
> >
> >
> >
> > void doStuff() {
> >     while(true) {
> >         synchronized {
> >             writeln("Doing stuff.");
> >         }
> >     }
> > }
> >
> >
> > If the writeln line is commented out, this thing keeps executing the
> > empty
> > loop with measurable CPU usage.
> Shouldn't you be waiting for the threads to exit at the end of main?  I
> wonder if the GC has been shut down by main exiting.
> -Steve

I guess you're right.  (This test program was written based on a larger program where this effect shows up, and forgetting that was just an oversight.)  On the other hand, it still doesn't fix the problem.  Also note, in case it's relevant, my OS is win32.

import core.thread, std.stdio;

void main() {
    Thread[] myThreads;
    foreach(i; 0..4) {
        myThreads ~= new Thread( { doStuff(); });
        myThreads[$ - 1].start;
    }
    doStuff();
}



void doStuff() {
    while(true) {
        synchronized {
            writeln("Doing stuff.");
        }
    }
}

On Tue, 28 Apr 2009 09:14:15 -0400, dsimcha <dsimcha@yahoo.com> wrote:

> == Quote from Steven Schveighoffer (schveiguy@yahoo.com)'s article
>> Shouldn't you be waiting for the threads to exit at the end of main?  I
>> wonder if the GC has been shut down by main exiting.
>> -Steve
>
> I guess you're right.  (This test program was written based on a larger program
> where this effect shows up, and forgetting that was just an oversight.)  On the
> other hand, it still doesn't fix the problem.  Also note, in case it's relevant,
> my OS is win32.
>
> import core.thread, std.stdio;
>
> void main() {
>     Thread[] myThreads;
>     foreach(i; 0..4) {
>         myThreads ~= new Thread( { doStuff(); });
>         myThreads[$ - 1].start;
>     }
>     doStuff();
> }
>
>
>
> void doStuff() {
>     while(true) {
>         synchronized {
>             writeln("Doing stuff.");
>         }
>     }
> }


Have you tried synchronizing on an actual object?  I remember some time back how Walter proposed removing synchronized as you have written it.  Not sure what happened for that.

The way you have written the code, assuming that the synchronized statement is doing what you think it's doing, the call to writeln should be completely syncrhonous, so multithreading issues or not, it should work.

-Steve

== Quote from Steven Schveighoffer (schveiguy@yahoo.com)'s article
> Have you tried synchronizing on an actual object?  I remember some time
> back how Walter proposed removing synchronized as you have written it.
> Not sure what happened for that.
> The way you have written the code, assuming that the synchronized
> statement is doing what you think it's doing, the call to writeln should
> be completely syncrhonous, so multithreading issues or not, it should work.
> -Steve

Good idea, still doesn't work.  All of the loops just die after a few iterations, leaving me at 0% CPU usage.  This happens on multiple win32 boxes.  Could someone please test this on some other OS?

import core.thread, std.stdio;

class Lock {}
Lock lock;

void main() {
    lock = new Lock;
    Thread[] myThreads;
    foreach(i; 0..4) {
        myThreads ~= new Thread( { doStuff(); });
        myThreads[$ - 1].start;
    }
    doStuff();
}



void doStuff() {
    while(true) {
        synchronized(lock) {
            writeln("Doing stuff.");
        }
    }
}

On Tue, 28 Apr 2009 09:51:54 -0400, dsimcha <dsimcha@yahoo.com> wrote:

> == Quote from Steven Schveighoffer (schveiguy@yahoo.com)'s article
>> Have you tried synchronizing on an actual object?  I remember some time
>> back how Walter proposed removing synchronized as you have written it.
>> Not sure what happened for that.
>> The way you have written the code, assuming that the synchronized
>> statement is doing what you think it's doing, the call to writeln should
>> be completely syncrhonous, so multithreading issues or not, it should work.
>> -Steve
>
> Good idea, still doesn't work.  All of the loops just die after a few iterations,
> leaving me at 0% CPU usage.  This happens on multiple win32 boxes.  Could someone
> please test this on some other OS?

Latest compiler? D1 D2?

-Steve

On Tue, 28 Apr 2009 09:59:20 -0400, Steven Schveighoffer <schveiguy@yahoo.com> wrote:

> On Tue, 28 Apr 2009 09:51:54 -0400, dsimcha <dsimcha@yahoo.com> wrote:
>
>> == Quote from Steven Schveighoffer (schveiguy@yahoo.com)'s article
>>> Have you tried synchronizing on an actual object?  I remember some time
>>> back how Walter proposed removing synchronized as you have written it.
>>> Not sure what happened for that.
>>> The way you have written the code, assuming that the synchronized
>>> statement is doing what you think it's doing, the call to writeln should
>>> be completely syncrhonous, so multithreading issues or not, it should work.
>>> -Steve
>>
>> Good idea, still doesn't work.  All of the loops just die after a few iterations,
>> leaving me at 0% CPU usage.  This happens on multiple win32 boxes.  Could someone
>> please test this on some other OS?
>
> Latest compiler? D1 D2?
>
From your code:
import core.Thread

Duh.  Must be D2 :)  Probably latest.

-steve

== Quote from Steven Schveighoffer (schveiguy@yahoo.com)'s article
> On Tue, 28 Apr 2009 09:51:54 -0400, dsimcha <dsimcha@yahoo.com> wrote:
> > == Quote from Steven Schveighoffer (schveiguy@yahoo.com)'s article
> >> Have you tried synchronizing on an actual object?  I remember some time
> >> back how Walter proposed removing synchronized as you have written it.
> >> Not sure what happened for that.
> >> The way you have written the code, assuming that the synchronized
> >> statement is doing what you think it's doing, the call to writeln should
> >> be completely syncrhonous, so multithreading issues or not, it should
> >> work.
> >> -Steve
> >
> > Good idea, still doesn't work.  All of the loops just die after a few
> > iterations,
> > leaving me at 0% CPU usage.  This happens on multiple win32 boxes.
> > Could someone
> > please test this on some other OS?
> Latest compiler? D1 D2?
> -Steve

2.029.  Thanks for your help.  I'm just trying to make sure this is a legit bug and maybe understand its underpinnings a little better before I file a bug report, because if it is a legit bug, it's a pretty serious one.

On Tue, 28 Apr 2009 10:16:21 -0400, dsimcha <dsimcha@yahoo.com> wrote:

> == Quote from Steven Schveighoffer (schveiguy@yahoo.com)'s article
>> On Tue, 28 Apr 2009 09:51:54 -0400, dsimcha <dsimcha@yahoo.com> wrote:
>> > == Quote from Steven Schveighoffer (schveiguy@yahoo.com)'s article
>> >> Have you tried synchronizing on an actual object?  I remember some  
>> time
>> >> back how Walter proposed removing synchronized as you have written  
>> it.
>> >> Not sure what happened for that.
>> >> The way you have written the code, assuming that the synchronized
>> >> statement is doing what you think it's doing, the call to writeln  
>> should
>> >> be completely syncrhonous, so multithreading issues or not, it should
>> >> work.
>> >> -Steve
>> >
>> > Good idea, still doesn't work.  All of the loops just die after a few
>> > iterations,
>> > leaving me at 0% CPU usage.  This happens on multiple win32 boxes.
>> > Could someone
>> > please test this on some other OS?
>> Latest compiler? D1 D2?
>> -Steve
>
> 2.029.  Thanks for your help.  I'm just trying to make sure this is a legit bug
> and maybe understand its underpinnings a little better before I file a bug report,
> because if it is a legit bug, it's a pretty serious one.

Yep, I confirm (on WinXP).  I get 2 or three outputs and then hang.

I also tried several different things:

Things that worked (continually output "Doing stuff."):
1. removing the synchronized(lock) statement
2. Changing the number of threads to 0
3. Changing the print line to use printf("Doing stuff.\n");

Things that still failed:
4. Changing the number of threads to 1 (which would mean 2 threads, the main thread and the sub thread).

The fact that 2 worked and 3 worked indicates to me that it's not simply a bug involving the lock mechanism, it's definitely something to do with writeln.

I'd file a bug with the latest code you posted.

I tried the original code once on Linux.  I get much more printouts than 2 or 3, but it still hangs.

So it's cross-platform.

-Steve

== Quote from Steven Schveighoffer (schveiguy@yahoo.com)'s article
> On Tue, 28 Apr 2009 10:16:21 -0400, dsimcha <dsimcha@yahoo.com> wrote:
> > == Quote from Steven Schveighoffer (schveiguy@yahoo.com)'s article
> >> On Tue, 28 Apr 2009 09:51:54 -0400, dsimcha <dsimcha@yahoo.com> wrote:
> >> > == Quote from Steven Schveighoffer (schveiguy@yahoo.com)'s article
> >> >> Have you tried synchronizing on an actual object?  I remember some
> >> time
> >> >> back how Walter proposed removing synchronized as you have written
> >> it.
> >> >> Not sure what happened for that.
> >> >> The way you have written the code, assuming that the synchronized
> >> >> statement is doing what you think it's doing, the call to writeln
> >> should
> >> >> be completely syncrhonous, so multithreading issues or not, it should
> >> >> work.
> >> >> -Steve
> >> >
> >> > Good idea, still doesn't work.  All of the loops just die after a few
> >> > iterations,
> >> > leaving me at 0% CPU usage.  This happens on multiple win32 boxes.
> >> > Could someone
> >> > please test this on some other OS?
> >> Latest compiler? D1 D2?
> >> -Steve
> >
> > 2.029.  Thanks for your help.  I'm just trying to make sure this is a
> > legit bug
> > and maybe understand its underpinnings a little better before I file a
> > bug report,
> > because if it is a legit bug, it's a pretty serious one.
> Yep, I confirm (on WinXP).  I get 2 or three outputs and then hang.
> I also tried several different things:
> Things that worked (continually output "Doing stuff."):
> 1. removing the synchronized(lock) statement
> 2. Changing the number of threads to 0
> 3. Changing the print line to use printf("Doing stuff.\n");
> Things that still failed:
> 4. Changing the number of threads to 1 (which would mean 2 threads, the
> main thread and the sub thread).
> The fact that 2 worked and 3 worked indicates to me that it's not simply a
> bug involving the lock mechanism, it's definitely something to do with
> writeln.
> I'd file a bug with the latest code you posted.
> I tried the original code once on Linux.  I get much more printouts than 2
> or 3, but it still hangs.
> So it's cross-platform.
> -Steve

Done.  http://d.puremagic.com/issues/show_bug.cgi?id=2907

Thanks for your help.