boost :: uuids :: random_generator y unicidad con varios subprocesos

Question

Cuando genero el número aleatorio con un solo subproceso, no se generan duplicados en 4M uuids pero si genero con dos subprocesos cada 1M, veo aproximadamente 16-20 duplicados. ¿Cuál podría ser la razón?

class TestUuid 
{ 
public: 
    std::string GenerateUUid(){ 
     boost::uuids::uuid uid; 
     { 
      boost::mutex::scoped_lock(m_mRandomGen); 
      uid = m_oRandomGen(); 
     } 
     std::stringstream ss; 
     ss << uid; 
     return ss.str(); 
    } 


    void TestUid(std::map<std::string, unsigned>& mUids, unsigned count){ 
    for(unsigned i = 0; i < count; ++i) { 
     std::string sUid = GenerateUUid(); 
     std::map<std::string, unsigned>::const_iterator it = mUids.find(sUid);   
     if(it == mUids.end()){ 
      mUids[sUid] = i; 
     }else { 
     std::cerr << "Duplicate uid:" << sUid << " found in thread id:" << pthread_self() << ", counter:" << i << ", earlier counter:" << it->second << ", id:" << it->first<< std::endl; 
     } 
    } 
    } 

    TestUnique() { 
    unsigned count = 4000000; 
    std::map<std::string, unsigned> uuids; 
    TestUid(uuids, count); 
    } 

    TestUniqueMultiThread() { 
    unsigned count = 1000000; 
    std::map<std::string, unsigned> mUids1; 
    boost::thread t1(boost::bind(&TestUuid::TestUid, this, mUids1, count)); 

    std::map<std::string, > Uunsignedids2; 
    boost::thread t2(boost::bind(&TestUuid::TestUid, this, mUids2, count)); 
    t1.join(); 
    t2.join(); 
    } 

private: 
    boost::mutex m_mRandonGen; 
    boost::uuids::random_generator m_oRandomGen; 

} 

int main() { 
TestUid oTest; 
oTest.TestUnique(); //work fine. no duplicate in 4M uuids 
oTest.TestUniqueMultiThread(); // around 16-20 duplicates in total 2*1M = 2M uuids 
return EXIT_SUCCESS; 
} 

a continuación es el registro.

 
Duplicate uid:9f4bfa5c-8e41-4012-ba3e-0b3e631834dc found in thread id:1103669568, counter:12016, earlier counter:12015, id:9f4bfa5c-8e41-4012-ba3e-0b3e631834dc 
Duplicate uid:0237b010-cb8f-4b89-9f47-042722902883 found in thread id:1103669568, counter:65778, earlier counter:65777, id:0237b010-cb8f-4b89-9f47-042722902883 
Duplicate uid:7a999ce7-0936-4642-b796-485334fc6ba4 found in thread id:1093179712, counter:170570, earlier counter:170568, id:7a999ce7-0936-4642-b796-485334fc6ba4 
Duplicate uid:09e1028b-5fc9-4fcd-ab70-991c02d47aec found in thread id:1093179712, counter:208740, earlier counter:208739, id:09e1028b-5fc9-4fcd-ab70-991c02d47aec 
Duplicate uid:66eb72f5-a3de-4941-8a64-6dad773f0ffb found in thread id:1093179712, counter:211449, earlier counter:211448, id:66eb72f5-a3de-4941-8a64-6dad773f0ffb 
Duplicate uid:8bccb459-1e70-4920-8486-6b0c5dcb3992 found in thread id:1093179712, counter:212972, earlier counter:212971, id:8bccb459-1e70-4920-8486-6b0c5dcb3992 
Duplicate uid:bb8109e3-6529-4122-a015-a9746900f692 found in thread id:1093179712, counter:239296, earlier counter:239295, id:bb8109e3-6529-4122-a015-a9746900f692 
Duplicate uid:a02ea282-b49b-4e4f-98a3-01406824c888 found in thread id:1103669568, counter:338582, earlier counter:338581, id:a02ea282-b49b-4e4f-98a3-01406824c888 
Duplicate uid:8bc848d7-bbe9-405c-9ef3-4d5ec312aa5e found in thread id:1093179712, counter:472035, earlier counter:472010, id:8bc848d7-bbe9-405c-9ef3-4d5ec312aa5e 
Duplicate uid:d3d8e09f-c410-4ce0-9a75-2a0c363db89c found in thread id:1093179712, counter:531441, earlier counter:531440, id:d3d8e09f-c410-4ce0-9a75-2a0c363db89c 
Duplicate uid:3130184f-345e-4d1c-bb01-d481eec29704 found in thread id:1093179712, counter:548770, earlier counter:548769, id:3130184f-345e-4d1c-bb01-d481eec29704 
Duplicate uid:29572641-2487-400a-926f-9bbf7ca176b4 found in thread id:1093179712, counter:710813, earlier counter:710811, id:29572641-2487-400a-926f-9bbf7ca176b4 
Duplicate uid:36b3567d-5f06-4c72-a395-e6f6ce056c6b found in thread id:1093179712, counter:728598, earlier counter:728597, id:36b3567d-5f06-4c72-a395-e6f6ce056c6b 
Duplicate uid:3290cb7e-2535-43bc-b53c-71ac0bc4fca1 found in thread id:1103669568, counter:846883, earlier counter:846881, id:3290cb7e-2535-43bc-b53c-71ac0bc4fca1 
Duplicate uid:59137657-2b2a-473e-b12c-1890d6058ca2 found in thread id:1093179712, counter:814812, earlier counter:814810, id:59137657-2b2a-473e-b12c-1890d6058ca2 

Answer 1

Esta es una commonerror al utilizar cerraduras RAII: se olvidó de dar a su bloqueo un nombreen la línea

 boost::mutex::scoped_lock(m_mRandomGen); 

por lo que no cerraba nada en absoluto. Cambiarlo a

 boost::mutex::scoped_lock lk(m_mRandonGen); // note the typo in mutex name 

EDIT: lo que realmente sucedió: No hubo error del compilador a pesar del error en el nombre mutex ya que la declaración

type(name); 

es la misma que

type name; 

si el nombre no ha sido declarado antes En otras palabras, ha construido por defecto un nuevo scoped_lock llamado m_mRandomGen, no asociado con un mutex.