Re: Some experience with the igc branch

[Eli Zaretskii <eliz@gnu.org>]

> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> Cc: pipcet@protonmail.com,  ofv@wanadoo.es,  emacs-devel@gnu.org,
>   eller.helmut@gmail.com,  acorallo@gnu.org
> Date: Tue, 24 Dec 2024 13:56:18 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> From: Gerd Möllmann <gerd.moellmann@gmail.com>
> >> Cc: Eli Zaretskii <eliz@gnu.org>,  ofv@wanadoo.es,  emacs-devel@gnu.org,
> >>   eller.helmut@gmail.com,  acorallo@gnu.org
> >> Date: Tue, 24 Dec 2024 05:03:36 +0100
> >> 
> >> I'd prefer to send messages from handle_profiler_signal. Or something
> >> equivalent to sending messages.
> >
> > How would that be different?  If the messages arrive asynchronously
> > and are handled asynchronously, that's the moral equivalent of
> > signals, no? 
> 
> I'm using SIGPROF below to make it more concrete. Similar for other
> signals.
> 
> The idea is to get the backtrace in the SIGPROF handler, without
> accessing Lisp data. That can be done, as I've tried to show.
> Then place that backtrace somewhere.

Let's be more accurate: when I said "Lisp data", I actually meant any
data that is part of the Lisp machine's global state.  That's because
you cannot safely access that state while the Lisp machine runs (and
modifies the state).  You need the Lisp machine stopped in its tracks.
Agreed?

Now, with that definition, isn't specpdl stack part of "Lisp data"?
If so, and if we can safely access it from a signal handler, why do we
need to move it aside at all?  And how would the "message handler" be
different in that aspect from a signal hanlder?

> In an an actor model architecture, one would use a message that contains
> the backtrace and post it to a message board. I used that architecture
> just as an example, because I like it a lot. In the same architecture,
> typically a scheduler thread would then assign a thread to handle the
> message. The handler handling the profiler message would then do what
> record_backtrace today does after get_backtrace, i.e. count same
> backtraces. 

What is the purpose of delaying the part of record_backtrace after
get_backtrace to later?  Is the counting it does dangerous when done
from a signal handler?

> That's only one example architectures, of course. One can use something
> else, like queues that are handled by another thread, one doesn't need a
> scheduler thread, and so on, and so on. Pip's work queue is an
> example.

Doing this from another thread raises the problem I describe above: we
need the Lisp thread(s) stopped, because you cannot examine the data
of the Lisp machine while the machine is running.  And if we stop the
Lisp threads, why do we need the other thread at all?

I guess we are tossing ideas without sufficient detail, so each one
understands something different from each idea (since we have
different backgrounds and experiences).  My suggestion is that to
describe each idea in enough detail to make the design and its
implications clear to all.  A kind of DR, if you want.  Then we will
be on the same page, and can have an effective discussion of the
various ideas.