parallel task in tbb

Question

I have the following function that I want to run as parallel tasks:

void WuManberFinder::find()

so I wrote the following:

void WuManberFinder::Parallel_find()
{

 tbb::task_group g;

 for(auto i = 0; i != tbb::tbb_thread::hardware_concurrency(); ++i) 
 {
  g.run(find);
 }

   g.wait();

 }

but it gives me the following errors:

wumanber.cc:139:17: error: no matching function for call to ‘tbb::task_group::run(<unresolved overloaded function type>)’
wumanber.cc:139:17: note: candidates are:
/usr/include/tbb/task_group.h:209:10: note: void tbb::task_group::run(const F&) [with F = void (thru::matching::WuManberFinder::*)()]
/usr/include/tbb/task_group.h:209:10: note:   no known conversion for argument 1 from ‘<unresolved overloaded function type>’ to ‘void (thru::matching::WuManberFinder::* const&)()’
/usr/include/tbb/task_group.h:151:10: note: template<class F> void tbb::internal::task_group_base::run(tbb::task_handle<F>&)

Here is the serial code (not complete yet):

 void WuManberFinder::find() {    
    while(m_pos <= m_text2.size()) {        
        string bgram = m_text2.substr(m_pos - m_B, m_B);
        shift_type::iterator i = m_shift.find(bgram);           
    if (i == m_shift.end())
        {// shared variable lock
        m_pos += m_lmin - m_B + 1;
        }// unlock
    else {

        if (i->second == 0) {

            vector<size_t>& list = m_hash[bgram];
            // Verify all patterns in list against the text.

            //shared variable lock
            ++m_pos;
            //unlock         

            for (size_t j = 0; j < list.size(); ++j) {
                string const& str = m_patterns[list[j]];
                m_find_pos = m_pos - str.size() - 1;
                size_t k = 0;

                for (; k < str.size(); ++k)
                    if (str[k] != m_text2[m_find_pos + k])
                        break;

                if (k == str.size()) {
                    m_find_pattern_index = list[j];
                 //  cout << "***" << m_patterns[m_find_pattern_index] <<endl;
                   // return true
                   // shared variable lock
                   m_intrusions++;
                   // unlock
                }
            }
        }
        else
           { // access m_pos shared critical section
            m_pos += i->second;
           } // access m_pos shared critical section
    }
    }

   // return false;  nothing found
     }

I know this might seem weird, but the main idea is I want multiple tasks of this type to traverse a string. The shared variable is pos. Could you give me a hint?

I don't think this is what you want to do. Please show us what exactly find does. — ronag, Apr 09 '12 at 18:58
My suspicions too. Since TBB introduced the explicit threading constructs (in 3.0?), TBB newcomers are drawn to them like moths to a flame rather than figuring out how to work within the task-parallel work-stealing framework. — timday, Apr 09 '12 at 19:08

score 3 · Answer 1 · answered Apr 09 '12 at 18:59

3

I don't think what you are doing is a good idea, though I can't explain better without knowing what find is supposed to do. To get that code working you need to do something like:

g.run(std::bind(&WuManberFinder::find, this));

answered Apr 09 '12 at 18:59

ronag

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score 3 · Answer 2 · answered Apr 09 '12 at 19:27

3

If you want to use TBB to parallelise a search, I'd suggest basing your approach on the example code for a parallel find (section 3.3.1 of the TBB tutorial) implemented using TBB's parallel_reduce. With all the mention of locks and critical sections in your posted code, you're really failing to exploit the high level TBB features which make it worth using.

answered Apr 09 '12 at 19:27

timday

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yeah but I can't partition the string – aTm Apr 09 '12 at 19:39
I must search every character as each depends on the previous – aTm Apr 09 '12 at 19:39
so what I do is a predict what shifts I could do by activating another tasks that run dependent on a shared pos – aTm Apr 09 '12 at 19:40
1

OK more complex than I assumed; personally I'd still be looking to make it work within TBB's parallel_do, pipeline or worst case task tree stuff. e.g take a look at the fibonnacci example in 11.3 in the tutorial. If you're using TBB right, you won't need any mention of locks or critical sections in your own code. – timday Apr 09 '12 at 19:49

parallel task in tbb

2 Answers2