Re: [Help-gsl] GSL 1.16 gsl_interp_accel_find not threadsafe?

[Leek, Jim]

OK, it appears that I don't actually need to pass in the accel object?  Because 
the gsl_spline* itself is const, it's only the accel object that isn't.  And 
digging around a bit, I see code like this:

 if (acc != 0)
    {
      index_a = gsl_interp_accel_find (acc, x_array, size, a);
      index_b = gsl_interp_accel_find (acc, x_array, size, b);
    }
  else
    {
      index_a = gsl_interp_bsearch (x_array, a, 0, size - 1);
      index_b = gsl_interp_bsearch (x_array, b, 0, size - 1);
    }

So, it looks like I can just pass in NULL as the accel object, and I'll be able 
to use the same gsl_spline object with multiple threads, and GSL will still 
produce that correct value (if a little more slowly.)  

That's correct? 
Thanks,
Jim
  
________________________________________
From: Patrick Alken address@hidden
Sent: Monday, March 21, 2016 1:47 PM
To: Leek, Jim; address@hidden
Subject: Re: [Help-gsl] GSL 1.16 gsl_interp_accel_find not threadsafe?

Yes these functions are thread-safe in the sense that they do not set
any global variables. So if you have 2 threads, each with its own accel,
then each thread will update its own accel object just fine, without
causing any problems due to global variables being set.

However, as you noted, when you interpolate a point, internal variables
inside the accel object are updated, so if 2 threads are accessing the
same accel object, they may try to update these internal variables at
the same time, causing a data race condition.

I would say its only safe to access the same gsl object from different
threads when you only need to pass a const * pointer to the object -
this means that no internal state variables are updated by the function
call, so read-only routines should be safe to call from any number of
threads.

But for routines which update the internal state, each thread needs its
own copy of the workspace.

Patrick

On 03/21/2016 02:37 PM, Jim Leek wrote:
> OK, that's the problem then.  Yes, I am using the same object with
> multiple threads.
>
> I had assumed that once the object was constructed, it was safe to
> read it from multiple threads.  So a very common thing I have done is:
>
> //This constructor is only run on 1 thread of course
> GSL1DFunction::GSL1DFunction(OBJECT* obj) {
>
>   ....[snip].....
>
>   acc = gsl_interp_accel_alloc ();
>   spline = gsl_spline_alloc (gsl_meth, XX.size());
>
>   gsl_spline_init (spline, &XX[0], &YY[0], XX.size());
> }
>
> //This eval function is called by many threads
> double GSL1DFunction::eval(double X) const {
>
>   return gsl_spline_eval (spline, X, acc);
> }
>
> //In this case I was calling this integration function from multiple
> threads:
> double GSL1DFunction::integ(double AA, double BB) const {
>   return gsl_spline_eval_integ (spline, AA, BB, acc);
> }
>
>
> So, I've done this a lot.  This is the first time it has caused an
> issue, but I think that is just because the loop was unusually tight
> and the integration call unusually long.  It it correct that both
> gsl_spline_eval_integ AND gsl_spline_eval are not thread-safe in this
> manner?  It would be difficult for me to make an individual object for
> each thread with OpenMP.  I will probably have to use a mutex.
>
> Thanks,
> Jim
>
> On 03/21/2016 12:45 PM, Patrick Alken wrote:
>> Hard to say without any example code, but are you trying to use the
>> same accel object from all the threads? Each thread will need its own
>> interpolator and accel workspace. From the two lines you posted, I
>> don't see any global variables or anything that would cause problems
>> as long as each thread has its own accel workspace.
>>
>> Patrick
>>
>> On 03/21/2016 12:34 PM, Leek, Jim wrote:
>>> I am using GSL in an openMP program.  It usually goes pretty well,
>>> but I recently ran a problem where I started getting random results
>>> (the deterministic problem returned wildly different results on each
>>> run.)
>>>
>>> I was finally able to get Intel Inspector to give me a clue. It says
>>> there is a data race in gsl_intep_accel_find at:
>>>
>>> Line 210,211:
>>>    a->miss_count++;
>>>    a->cache - gsl_interp_bsearch(ca, x, x_intex, len-1)
>>>
>>> The call stack is:
>>> gsl_spline_eval_integ:190
>>> gsl_intep_eval_integ:273
>>> cspline_eval_integ:403
>>> gsl_interp_accel_find:211
>>>
>>> Avoiding calling this in the threading region does seem to solve the
>>> issue.
>>>
>>> 1) Is this a known issue?
>>> 2) Is there a known way to get around it?
>>> 3) Is it already fixed in a newer version of GSL?
>>>
>>> Thanks,
>>> Jim
>>
>>
>