[GAP Forum] Run GAP command for x seconds

Thu Jan 28 10:31:28 GMT 2021

Hi,

Chris: I cannot seem to load the function IO_CallWithTimeout, or indeed IO_CallWithTimeoutList -- there seems to be no function with this name. I have IO 4.5.2 loaded.

Frank: Your code runs fine, but when I run e.g the following code
> ParTakeFirstResultByFork([function(x) local F; F := FreeGroup(x); return F; end], [[1]],rec(TimeOut :=rec(tv_sec :=2, tv_usec := 0 )));

this makes my GAP go haywire and totally breaks it, forcing me to restart it (it finishes the computation 2 seconds later, though...). It should just return FreeGroup(1), of course). I get the error message "No 1st choice method found for IO_Pickle", which, as delicious as it sounds, doesn't tell me much. Perhaps I am doing something wrong? It doesn't seem to break if I do, say, "F := x^2" instead of "F := FreeGroup(x)", so it's quite a mystery to me. "F := FreeMonoid(x)" also breaks it. The problem seems to be with calling any function inside these functions...

Here is the full error message. I am using Linux.

Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `IO_Pickle' on 2 arguments at /usr/local/gap-4.9.3/lib/methsel2.g:250 called from
IO_Pickle( j.childtoparent, ret ); at /usr/local/gap-4.9.3/pkg/io-4.5.2/gap/background.gi:110 called from
BackgroundJobByForkChild( j, fun, args ); at /usr/local/gap-4.9.3/pkg/io-4.5.2/gap/background.gi:85 called from
BackgroundJobByFork( jobs[i], args[i], rec(
    ImmediatelyTerminate := true ) ) at /usr/local/gap-4.9.3/pkg/io-4.5.2/gap/background.gi:283 called from
<function "unknown">( <arguments> )
 called from read-eval loop at *stdin*:8

Best,
Carl-Fredrik

________________________________
From: Christopher Jefferson <caj21 at st-andrews.ac.uk>
Sent: Thursday, January 28, 2021 9:28 AM
To: Frank Lübeck <frank.luebeck at math.rwth-aachen.de>; Carl-Fredrik Nyberg Brodda (MTH - Postgraduate Researcher) <C.Nyberg-Brodda at uea.ac.uk>
Cc: forum at gap-system.org <forum at gap-system.org>
Subject: RE: [GAP Forum] Run GAP command for x seconds

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Hi,

I agree with Frank. There is an alternative function (which uses the same underlying mechanism) -- IO_CallWithTimeout (and IO_CallWithTimeoutList, which just takes the function arguments as a list).

IO_CallWithTimeout(rec(seconds := 2), Intersection, AlternatingGroup(20), AlternatingGroup(20)*(1,2));

(note that IO_CallWithTimeout uses different names for the times, seconds in this case).

After about 2 seconds outputs [ false ]

Chris

-----Original Message-----
From: Frank Lübeck <frank.luebeck at math.rwth-aachen.de>
Sent: 27 January 2021 18:19
To: Carl-Fredrik Nyberg Brodda (MTH - Postgraduate Researcher) <C.Nyberg-Brodda at uea.ac.uk>
Cc: forum at gap-system.org
Subject: Re: [GAP Forum] Run GAP command for x seconds

On Wed, Jan 27, 2021 at 04:54:03PM +0000, Carl-Fredrik Nyberg Brodda (MTH - Postgraduate Researcher) wrote:
> Thank you for your response. I am having trouble getting the timeout
> to work. Do you have an example of using the timeout?
>
> I have attempted to pass the last (third, optional) argument of
> ParTakeFirstResultByFork as rec(TimeOut := rec(tv_sec :=2, tv_usec :=
> 0)), hoping for my execution to run for 2 seconds, but this does not
> work (it never times out). I have also tried setting the tv_sec record
> to be the current time (plus 10 seconds or so), but this does not time out either.

Hi,

This looks correct and it works for me, for example:

gap> Runtimes();  ParTakeFirstResultByFork([function(x) while true do
gap> od; end], [[1]],rec(TimeOut := rec(tv_sec :=2, tv_usec := 0 )));
gap> Runtimes();
rec( system_time := 90, system_time_children := 15, user_time := 1103,
  user_time_children := 3985 )
[  ]
rec( system_time := 94, system_time_children := 15, user_time := 1103,
  user_time_children := 5984 )

I'm only using Linux systems. Some functions of the IO package may not work as expected when GAP is used in Windows (but I don't know the details).

Best regards again,
  Frank Lübeck

> Best,
> Carl-Fredrik Nyberg Brodda
> ________________________________
> From: Frank Lübeck <frank.luebeck at math.rwth-aachen.de>
> Sent: Wednesday, January 27, 2021 3:53 PM
> To: Carl-Fredrik Nyberg Brodda (MTH - Postgraduate Researcher)
> <C.Nyberg-Brodda at uea.ac.uk>
> Cc: forum at gap-system.org <forum at gap-system.org>
> Subject: Re: [GAP Forum] Run GAP command for x seconds
>
> On Wed, Jan 27, 2021 at 02:53:14PM +0000, Carl-Fredrik Nyberg Brodda (MTH - Postgraduate Researcher) wrote:
> > Hi all,
> >
> > A problem which I recently stumbled upon is the following: is there
> > a way to execute a given GAP command only for a certain amount of
> > time, before stepping past the command and continuing execution?
> >
> > Let us say I have a table of finitely many finitely presented
> > groups, and wish to see whether GAP has a "naive" solution for their
> > word problem. I find a KnuthBendixRewritingSystem() of the group,
> > and then run
> > MakeConfluent() on this rewriting system. Of course, this process
> > may not terminate at all! But sometimes it does. My problem is
> > essentially that I wish to run MakeConfluent() for some amount of
> > time (say 5 seconds), and then, if the command has not terminated,
> > continue on with the next group. This avoids all decidability
> > issues, and would in other programming languages be easily implemented with a timer.
> >
> > Of course the problem is general, so the above is just a use-case.
> > Is there something that can help with this?
> >
> > Best,
> > Carl-Fredrik Nyberg Brodda
>
> Dear Forum,
>
> I have used successfully the function
>     ParTakeFirstResultByFork
> from the IO package for this kind of applications.
> It has an optional third argument which allows to specify a timeout.
>
> Best regards,
>    Frank

--
///  Dr. Frank Lübeck, Lehrstuhl für Algebra und Zahlentheorie, \\\  Pontdriesch 14/16, 52062 Aachen, Germany ///  E-mail: Frank.Luebeck at Math.RWTH-Aachen.De
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