[CppAD] using CppAD threadsafe

[Stefan Vigerske]

Hi,

I'm not sure whether you have actually read my mail.
I do not used OpenMP for multithreading, it is just pthreads.

But in trunk, most OpenMP specific stuff seem to be captured in 
omp_alloc. Maybe I could overwrite parts of it with my own code that 
provides the pthreads thread number?

Stefan

> There has been some recent improvements to how CppAD interacts with
> OpenMP; see
>       http://www.coin-or.org/CppAD/Doc/whats_new_11.htm
>
> I suggest using svn to download the trunk or downloading the most recent
> tarball at
>       http://www.coin-or.org/download/source/CppAD/
>
> The documentation for using CppAD with OpenMP can be found at
>       http://www.coin-or.org/CppAD/Doc/omp_parallel.xml
>
> Have you tried running the OpenMP test driver described  here
>       http://www.coin-or.org/CppAD/Doc/openmp_run.sh.xml
> Especially the AD test documented here
>       http://www.coin-or.org/CppAD/Doc/multi_newton.cpp.xml
>
> On 8/10/2011 12:44 PM, Stefan Vigerske wrote:
>> Hi,
>>
>> I try to use CppAD 20110101.0 in a multithreading environment (pthreads).
>> That is, there are several threads running in parallel, each one using
>> CppAD to compute derivatives.
>> If I use CppAD like I am used to, I get strange error messages or
>> segmentation faults.
>> Looking a bit around, it seems that CppAD stores the tapes in some
>> static global variable.
>>
>> But I saw these OpenMP stuff, so I compiled my code with -fopenmp.
>> Now I still get an error:
>> CppAD 20110101.0 error from a known source:
>> Independent: cannot create a new tape because
>> a previous tape is still active (for this thread).
>> AD<Base>::abort_recording() would abort this previous recording.
>> Error detected by false result for
>>        ADBase::tape_ptr() == CPPAD_NULL
>> at line 172 in the file
>>        lib/cppad/local/independent.hpp
>> OpenMP: thread_num = 0
>>
>> I guess this is because CppAD cannot distinguish the different threads
>> that I'm using, thus always assumes it is called from thread 0.
>> It seems to use omp_get_thread_num to get the thread number, but a
>> printf tells me that this is always 0, which isn't suprising, since the
>> program I'm using uses the pthreads library for parallelization, so how
>> should omp_get_thread_num() know?
>>
>> I also printed omp_get_max_threads() and this gives me 4, on a quad-core
>> machine. Seems ok. So I doubt that calling
>> omp_alloc::set_max_num_threads(number) would make any difference.
>> In the documentation it says something about calling
>> CppAD::omp_alloc::set_max_num_threads, but this function, or the whole
>> omp_alloc, does not seem to exists in the CppAD version that I'm using.
>>
>> Any idea how to make CppAD threadsafe?
>>
>> Stefan
>>
>
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