Maxima and thread safety (was: A fix for SBCL error "thread local storage exhausted" when creating too many rules)

Following up on the discussion about Maxima's thread safety ...

I've been playing around a bit with Michel's
distribute_over_tranches_thread.lisp
<https://github.com/maxima-project-on-github/maxima-packages/blob/master/mtalon/maxima-parallel/distribute_over_tranches_thread.lisp>.
It works nicely when the evaluated expressions are free of side effects
("pure functions").

Unfortunately, Maxima has many hidden side effects that are surprising to
the average user. The following innocent looking example simply creates
Maxima variables v1, v2, v3, ..., v10000 in 16 parallel threads:

distribute_over_tranches('(concat('v, i) :: i), i, 10000, 16)$

Note: The double-colon operator :: is like :, but it evaluates its
left-hand side.

Now that looks completely safe, right? After all, each parallel evaluation
creates a different variable. But under the hood, new variables are added
to $values, and being a true global variable, this is not thread-safe.
Different threads manipulate the $values list in parallel, getting in each
other's way. After executing the above line, let's see how many variables
Maxima thinks exist:

length($values);
9544 /* oops, not 10000! (non-deterministic) */

If we start evaluating limits in parallel, it breaks Maxima completely:

distribute_over_tranches('(limit(abs(x-i)/(x-i), x, i)), i, 100, 100);

This prints lots of low-level errors, and afterwards, Maxima is in a
corrupted state where even regular, non-parallelized limits don't work any
more. This is probably caused by database facts being added by limit, which
is also not thread-safe, but I'm not sure. Integrals are equally
problematic.

These experiments confirm that while parallel evaluation of pure functions
is achievable, parallel mutation of the Maxima environment is currently
impossible without deep architectural changes, which, given the complexity
and age of the codebase, are highly challenging.

Best regards
David Scherfgen

>

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