> < ^ Date: Tue, 18 Aug 1992 10:06:35 +0200
> < ^ From: Frank Celler <frank.celler@math.rwth-aachen.de >
> ^ Subject: Re: Checkpoint

In a letter to Volkmar Felsch Mike Newman says

NM> I hope there will be some sort of checkpointing facility which will
MN> allow one to look at big presentations - this does not seem to be
MN> routinely available in GAP which somewhat surprised me.

Since we think that this a question that many other might have as
well, we post our answer in the gap-forum.

We think what you mean with checkpoints is that one can break the
execution at prearranged points and save the workspace to disk.

Of these steps the first is possible. You can break gap using "CTRL-C"
or use interactive functions like those for Tietze transformations.
The second step however does not exist, yet. What you can do is to
send your job into the background after having used "CTRL-C" and
continue the next day.

If you need to logout or want to change terminals you should install
"screen" (a PD program) which will give you pseudo terminals. Using
this program you can break gap, stay in the breakloop and detach your
terminal from the pseudo terminal. It is possible to resume execution
at exactly the same point of the calculation. Of course GAP will still
use parts of the swap space this way.

On the other hand we intend eventually to have the possibility to save
the workspace, but it is not a trivial job to implement this and there
are more urgent tasks. Also of course it is nontrivial to decide how
far the capabilities of such saving the workspace should go: for
instance one might even want to be able to continue on a different
computer (with maybe even a different architecture), but more one
requests here the more difficult is the job.

We hope the existing possibilities outlined above will do for all you

all the best
Frank and Joachim

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