Signal handling from user code

[antoine-levitt]

Haven't found an open issue on this, so here goes:

I often find myself wanting to interrupt a long computation. E.g. I run a long optimization task with a large number of max iterations, but it becomes apparent that this will converge very slowly or oscillate and will never end. At that point my only option is to either come back the next day, or interrupt the computation with C-c, in which case the result is lost. What I would like is to be able to put some code in the optimizer to listen to a user input and, if raised, stop the computation at that point and return the current state, so I can use it.

I don't know if it's common practice, but what emacs does is to listen to SIGUSR2 to drop into the debugger. Can such a functionality be made available to user code in Julia? Then it could be used in packages (e.g. break in Gallium, interrupt a computation in Optim...)

[yuyichao]

Dup of #14675, SIGINT is already catchable when exit_on_sigint is set to 0.

julia/test/repl.jl

Line 27 in 876294d

ccall(:jl_exit_on_sigint, Void, (Cint,), 0)

[antoine-levitt]

That's about catching SIGINT, what I want to do is catch SIGUSR2.

[yuyichao]

Not sure what you mean but the linked issue isn't SIGINT only and

interrupt the computation with C-c, in which case the result is lost. What I would like is to be able to put some code in the optimizer to listen to a user input and, if raised, stop the computation at that point and return the current state, so I can use it.

Is doable with SIGINT.

[antoine-levitt]

The point is that SIGINT should be "stop this code now", while SIGUSR2 has more flexibility. You're right that it is covered in the linked issue though, sorry.

[yuyichao]

SIGINT should be "stop this code now"

Call it a standard abuse if you like but I think there are a lot of code out there that treat SIGINT as "do clean up (including saving data) and exit". If you really want to stop it now, there's SIGTERM and SIGKILL.

[sbromberger]

OK, so I'm still not sure how to catch a signal (whether SIGINT or something else). Could someone please provide instructions?

What I want to do is to catch a signal to print out debugging information (sort of like how dd handles SIGINFO) within a long-running function.

[iblislin]

@sbromberger

└─[iblis@abeing]% g df  | cat
diff --git a/src/signals-unix.c b/src/signals-unix.c
index 2b69f43d6a..eb54d5a051 100644
--- a/src/signals-unix.c
+++ b/src/signals-unix.c
@@ -640,6 +640,12 @@ static void *signal_listener(void *arg)
                 jl_exit_thread0(128 + sig);
             }
         }
+#ifdef SIGINFO
+        if (SIGINFO == sig)
+        {
+            jl_safe_printf("I'm fine.\n");
+        }
+#endif  /* ifdef SIGINFO */
     }
     return NULL;
 }

julia> while true                   
       end                          

load: 0.79  cmd: julia 64121 [running] 4.82r 1.13u 0.24s 10% 193020k     

signal (29): Information request    
while loading no file, in expression starting on line 0                  
anonymous at ./<missing>:0          
jl_call_fptr_internal at /usr/home/iblis/git/julia/src/./julia_internal.h:366 [inlined]                                                            
jl_call_method_internal at /usr/home/iblis/git/julia/src/./julia_internal.h:385 [inlined]                                                          
jl_toplevel_eval_flex at /usr/home/iblis/git/julia/src/toplevel.c:620    
jl_toplevel_eval_in at /usr/home/iblis/git/julia/src/builtins.c:505      
eval at ./repl/REPL.jl:3            
eval_user_input at ./repl/REPL.jl:69                                     
macro expansion at ./repl/REPL.jl:100 [inlined]                          
#1 at ./event.jl:73                 
jl_apply at /usr/home/iblis/git/julia/src/./julia.h:1447 [inlined]       
start_task at /usr/home/iblis/git/julia/src/task.c:268                   
unknown function (ip: 0xffffffffffffffff)                                
unknown function (ip: 0xffffffffffffffff)                                
Allocations: 337516 (Pool: 337422; Big: 94); GC: 0                       
I'm fine.                           
^C^C^C^C^C^CWARNING: Force throwing a SIGINT                             
^C^C^C^C^C^C^C^C^C^C^CERROR: ^C^C^C^CInterruptException:                 
Stacktrace:                         
 [1] anonymous at ./<missing>:0

happy hacking :)

[sbromberger]

@iblis17 Thanks. I'd really like to see signal handling built in to Julia, though. Especially for HPC work, there is a need for checkpointing / graceful shutdown when job resource limits expire, prior to the system forcibly terminating the job. Ideally, I'd like to be able to have in my code, "If you get a SIGUSR2 (for example), write the current output to disk and stop work". This would allow me to configure my HPC job to send SIGUSR2 60 seconds before the system kills it.

[StefanKarpinski]

It would be great to have reliable, simple signal handling in Julia to the extent that systems allow it. That's a pretty big piece of design and implementation work, however, so it will likely have to be a post-1.0 feature. Fortunately, no issue with adding such an API in 1.1 or 1.2.

[iblislin]

I think my example can be extended as a set of c api that provide custom signal handling: maybe add a c api like `register` which accepts a function pointer, so we can register a julia function to it; then invoke user defined julia function in `signals-unix.c/signal_listener`

…

On 08/23/2017 01:27 AM, Seth Bromberger wrote: @iblis17 <https://github.com/iblis17> Thanks. I'd really like to see signal handling built in to Julia, though. Especially for HPC work, there is a need for checkpointing / graceful shutdown when job resource limits expire, prior to the system forcibly terminating the job. Ideally, I'd like to be able to have in my code, "If you get a SIGUSR2 (for example), write the current output to disk and stop work". This would allow me to configure my HPC job to send SIGUSR2 60 seconds before the system kills it. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#22883 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAufF5YhZdm6bH9oSwo7D_qkarkFmFWUks5saw9wgaJpZM4Od8BS>.

-- Iblis Lin 林峻頤 GitHub: https://github.com/iblis17