Re: Maxima and thread safety

Hi David,

I cannot reproduce your crash using `distribute_over_tranches_mt.lisp'
from Michel's github repo.

I played around with the number of threads (1, 4, 8) and your example works
fine and it clearly is using multiple threads.

Leo

Maxima-version: "branch_5_49_base_416_g162a6093a_dirty"
Maxima build date: "2026-06-08 12:29:08"
Host type: "x86_64-unknown-linux-gnu"
Lisp implementation type: "SBCL"
Lisp implementation version: "2.6.4.debian"
User dir: "/dev/null"
Temp dir: "/tmp"
Object dir: "/home/butlerlt/maxima/sandbox/git/maxima-code/binary/branch_5_49_base_416_g162a6093a_dirty/sbcl/2_6_4_debian"
Frontend: false


On Tue, Jun 09 2026, David Scherfgen via Maxima-discuss <[email protected]> wrote:

> Hi Michel,
>
> You might add this to your examples:
>
> distribute_over_tranches(
>   '(block(
>       [x_n: float(N), x_next, i],
>       for i : 1 thru 1000 do (
>           x_next: 0.5 * (x_n + N / x_n),
>           x_n: x_next
>       ),
>       x_n)),
>   N, 1000, 16);
>
> Nothing fancy, it's Newton's algorithm to compute the square root, with a
> fixed number of iterations, just for demonstration purposes.
> Note that I have done everything possible to make sure that the variables
> x_n, x_next, i are local, by using a block.
> But yet, the calculation crashes as soon as one uses more than 1 thread.
> So, the situation is even more delicate than I thought.
>
> Best regards
> David
>
> Am Di., 9. Juni 2026 um 16:45 Uhr schrieb Michel Talon <
> [email protected]>:
>
>> I have updated the README in
>>
>> https://github.com/mtalon/mtalon/tree/master/mtalon/maxima-parallel
>>
>> to reflect David insightful examples.
>>
>> Finally "distribute_over_tranches_thread.lisp" is another way to
>> achieve the same parallelism, using threads in just one Unix
>> process. Here we use the sbcl threading support. For sufficiently
>> independent computations one may expect to get correct behaviour.
>> However if threads modify the internal state of maxima chaos can
>> occur. Examples have been provided by David Scherfgen, notably:
>> distribute_over_tranches('(concat('v, i) :: i), i, 10000, 16)$
>> which under the hood affect the global variable $values, so at the end
>> one gets length($values) different from 10000. The problem is that special
>> variables with global extent are not bound in thread local storage, but
>> can be read and modified independently by all threads. But they are not
>> protected by a mutex.
>> A small lisp program which  shows the behavior of such special global variables is:
>>
>> ;;; We are in main thread
>> (defparameter *x* 0)
>> (format nil "~a" *x*)    ;;; value of global *x* in main thread
>> (let ((*x* 1))
>> a  (format t "~a ~%" *x*)  ;;; value of *x* in main thread TLS
>>   (sb-thread:make-thread (lambda ()   ;;; spawning new thread
>> 			   (format t "~a ~%" *x*) ;;; value of *x* in new thread, the global one.
>> 			   (setq *x* 3)
>> 			   (format t "~a ~%" *x*) ;;; altered value of *x* in new thread
>> 			   "Sorry! *x* in main thread is not isolated.")))
>> (format nil "~a" *x*)     ;;; The global value of *x* has been altered in main thread
>>
>> However i still believe that the threading program may be very useful to distribute a bunch of "ordinary standard"
>> computations over the many cores of modern processors.  For example running the same computation a hundred of times for
>> different values of parameters which may take hours of compute time, will be scaled down basically by the number of cores.
>> Fortunately special variables bound locally in a let form or similar  (this being called dynamically bound) get bound in TLS
>> (thread local storage) which isolates them from thread to thread, so that most parts of maxima can still work OK.
>>
>>
>> Le 07/06/2026 à 16:55, David Scherfgen a écrit :
>>
>>
>> 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:
>>
>> --
>> Michel Talon
>>
>> _______________________________________________
>> Maxima-discuss mailing list
>> [email protected]
>> https://lists.sourceforge.net/lists/listinfo/maxima-discuss
>>
>
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-- 
---
Best regards,
Dr Butler
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