Hello Sierra !
My 2 cents, have you tried to compile with -fsanitize=thread to see if
it catch the problem ?
https://github.com/google/sanitizers/wiki/ThreadSanitizerCppManual
Cheers !
On 13/5/20 14:50, Sierra Ansuas, Juan Pablo wrote:
Hello, and sorry for insisting about this issue.
The program is returning correct values as long as the call to
glp_simplex is enclosed inside anOMP CRITICAL region. This is the only
part requiring a mutex. The rest of the glp_* functions are called
concurrently by several threads with no issue at all.
When the OMP CRITICAL surrounding glp_simplex is removed, the calls to
*glp_get_row_dual* and *glp_get_col_prim* return different values.
Each thread uses a different glp_prob, which is built from scratch by
the same thread and used by the same thread.
Each thread also changes some values on the RHS and calls glp_simplex
several times. After that, each thread deletes its own GLPK problem.
What could be affecting the way glp_simplex works? Is there anything
you would suggest looking at?
I'm very grateful for the help I've received so far.
Regards,
Juan Pablo.
------------------------------------------------------------------------
*Subject:* Re: [Fwd: Possible bug in GLPK - Error detected in file
env/alloc.c at line 72]
Dear Heinrich,
I have been working on the project that I have been assigned and
applied the changes you suggested with successful results.
- glp_prob objects are created, used and deleted by the same thread
- GLPK lib compiled with reentrancy support (checking whether to
enable reentrancy support... yes)
The program is no longer crashing. /Thank you./
I have looked at the multiseed.c example. The main difference between
that example and the project that I am working on is that I build a
glp_prob from scratch, then using glp_copy_prob make as many copies as
processors are available on the machine. Each thread changes a couple
values on the right hand side and runs the simplex again, without
building the glp_prob again or starting the search from scratch.
However, the problem that I am now facing is that when several threads
are calling glp_simplex at the same time, some values appear to be
mixed up such as those returned by glp_get_row_dual. Adding a mutex
(OMP CRITICAL) that contains_only the glp_simplex call_ makes the
problem go away and no issues can be seen, the program generates the
expected output data and ends without errors.
I have looked at the error and output handling contained in the
multiseed.c example. I ended up using glp_open_tee and redirecting the
output of each thread to a separate file. No warnings were displayed.
Thank you very much for your help.
Regards,
Juan Pablo
------------------------------------------------------------------------
*From:* Heinrich Schuchardt <xypron.g...@gmx.de>
*Sent:* Thursday, March 26, 2020 6:46 PM
*To:* bug-g...@gnu.org; Sierra Ansuas, Juan Pablo
*Cc:* Andrew Makhorin
*Subject:* Re: [Fwd: Possible bug in GLPK - Error detected in file
env/alloc.c at line 72]
On 3/25/20 8:23 PM, Andrew Makhorin wrote:
> -------- Forwarded Message --------
> Date: Wed, 25 Mar 2020 18:34:56 +0000
>
>> Dear GLPK mantainer(s),
>>
>> I am having problems when trying to run the GLPK solver in a multi-
>> threaded fashion using its C API and OpenMP. While the solver does
>> appear to work, an error comes up when trying to free the memory with
>> glp_delete_prob:
>>
>> glp_free: memory allocation error
>> Error detected in file env/alloc.c at line 72
>>
>> I am no GLPK expert but I would be very grateful if you could check
>> the provided source code and determine whether this is being caused
>> by a bug or a simple beginner mistake.
>>
>> Running on an eight core processor, 8GiB ram. Stack size limit: 2GB
>> (ulimit -s 2000000), Stack size limit per thread: 200MB (export
>> OMP_STACKSIZE=200000). Provided source makes little sense but is
>> quite short and reproduces the problem every time. Compile command is
>> as follows:
>>
>> gcc test_c.c -o test_c -fopenmp -I/home/user/libglpk/glpk-
>> 4.65/include /home/user/libglpk/glpk-4.65/src/.libs/libglpk.so.40.3.0
>>
>>
>> Regards,
>>
>> Juan Pablo Sierra
Hello Juan,
Did you compile GLPK with '--enable-reentrant=yes'?
You are creating all problems in one thread and then call the solver in
separate threads. This is not supported by GLPK.
Please, have a look at:
glpk-4.65/examples/threads/multiseed.c
Here all GLPK library calls needed for each individual model are called
within the same separate thread.
Please, have a look at the error and output handling in the example too.
Best regards
Heinrich
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