Optimize QSql of memory database in multiprocessor-thread

Question

Here's my problem, I have one sqlite memory database by using QSql. I have several threads each handling one different table of this common database. And I use Win API to make sure these threads working on different processor, like this:

SetThreadAffinityMask (hThread, processorMask);

When there's only thread handling one table, it takes 10 seconds and uses 25% of total CPU. But when there're 4 threads handling 4 different tables, it takes nearly 40 seconds and uses only 35% of total CPU. I think the reason is there's some kind of thread-safe sync in one database. But due to different thread reading or writing different table, thread-safe slows my program. How can I optimize it.

Update: The most possible reason is some kinds of lock inside of Qt or/and Sqlite 3 slows my program, so is it possible to shutdown or bypass these locks by pre-setting.

Update2: Here's an example. (Maybe a little long, sorry)

class MultiProcessorThread
{
public:
    virtual void run();
    bool start()
    {
        m_hThread = CreateThread (NULL, 0, MultiProcessorThread::ThreadFunc, this, CREATE_SUSPENDED, NULL);

        if (m_hThread != INVALID_HANDLE_VALUE)
        {
            RunningThreadCount++;
            m_ProcessorMask = 1 << ( (RunningThreadCount - 1) % ProcessorCount);
            SetThreadAffinityMask (m_hThread, m_ProcessorMask); // Make thread working on different processor
            ResumeThread (m_hThread);
            return true;
        }
        else
            return false;
    }
protected:
    static DWORD WINAPI ThreadFunc (LPVOID in);
    HANDLE m_hThread;
    DWORD_PTR m_ProcessorMask;
    static DWORD_PTR ProcessorCount;
    static DWORD_PTR RunningThreadCount;
    static DWORD_PTR GetNumCPUs();
};

DWORD_PTR MultiProcessorThread::ProcessorCount = GetNumCPUs();
DWORD_PTR MultiProcessorThread::RunningThreadCount = 0;
DWORD_PTR MultiProcessorThread::GetNumCPUs() // Get how many processors on this PC
{
    SYSTEM_INFO m_si = {0};
    GetSystemInfo (&m_si);
    return (DWORD_PTR) m_si.dwNumberOfProcessors;
}
DWORD WINAPI MultiProcessorThread::ThreadFunc (LPVOID in)
{
    static_cast<MultiProcessorThread*> (in)->run();
    return 0;
}

class Run : public MultiProcessorThread
{
public:
    void run()
    {
        int i = 0;
        QString add = "insert into %1 values(1)";
        add = add.arg (table);
        QString sel = "select a from %1 ";
        sel = sel.arg (table);
        QString del = "delete from %1 where a=1";
        del = del.arg (table);

        while (++i) // read and write database
        {
            query.exec (add);
            query.exec (sel);
            query.exec (del);
        }
    }
    QSqlQuery query;
    QString table;
};  

int main (int argc, char *argv[])
{
    QCoreApplication a (argc, argv);
    QSqlDatabase db = QSqlDatabase::addDatabase ("QSQLITE", "test"); 
    db.setDatabaseName (":memory:"); // All threads working on the same memory database.
    db.open();
    QSqlQuery q (db), q1 (db), q2 (db);
    q.exec ("create table A (a)");
    q1.exec ("create table B (a)");
    q2.exec ("create table C (a)"); // All threads working on different table.
    Run b[3];
    b[0].query = QSqlQuery (q);
    b[0].table = "A";
    b[1].query = QSqlQuery (q1);
    b[1].table = "B";
    b[2].query = QSqlQuery (q2);
    b[2].table = "C";
    b[0].start();
    b[1].start();
    b[2].start();
    return a.exec();
}

Answer 1

First of all, don't set affinity mask explicitly, windows will automatically allocate threads on the most idle cores. It's better to rely on OS to do thread distribution than your code in this case.

As far as I know, sqlite locks whole database while writing, that's why you don't get a performance boost you expected. Take a look at sqlite locking documentation http://www.sqlite.org/lockingv3.html

Answer 2

Have you measured how much time the threads are spending on the CPU as compared to, say, disk I/O throughput?

This could have nothing to do with threading and locks. It could have everything to do with Amdahl's law.

Answer 3

The Qt docs are unambiguous about this. From http://doc.qt.nokia.com/4.7/threads-modules.html#threads-and-the-sql-module:

Threads and the SQL Module

A connection can only be used from within the thread that created it. Moving connections between threads or creating queries from a different thread is not supported.

In addition, the third party libraries used by the QSqlDrivers can impose further restrictions on using the SQL Module in a multithreaded program. Consult the manual of your database client for more information

There is no way to do what you want through the Qt API.