libstdc++ and openmp problem with GCC4.4.0 port to interix

Robert Oeffner robert@oeffner.net
Sat Sep 26 10:21:00 GMT 2009


Hi,

Probably a long shot but I wonder if anyone would have a useful tip on a 
problem porting gcc4.4.0 to interix (a BSD-like OS running on top of the 
Windows kernel).

As libgomp in GCC so far isn't targeting interix I have made some changes to 
libgomp in my copy of the GCC 4.4.0 distribution.  A new source file was 
created, gcc-4.4.0/libgomp/config/posix/interix/proc.c, which is templated 
on the existing gcc-4.4.0/libgomp/config/posix/proc.c and 
gcc-4.4.0/libgomp/config/posix/mingw32/proc.c in the distribution (see 
http://www.oeffner.net/stuff/gcc-4.4.0_interix_changes.zip or 
http://www.suacommunity.com/forum/tm.aspx?m=16600 ). With this file and 
modifications to GCC configuration files in the distribution I can bootstrap 
GCC 4.4.0 to build gcc and g++ compilers on interix.

The port produces fast code for single threaded running programs. However, 
there's a major problem with OpenMP. It's something to do with libstdc++ 
that tends to go in overdrive when you request OpenMP to create more than 
one thread for the compiled program. When calling string::clear() from 
libstdc++ it somehow hogs the CPU with high kernel times and runs orders of 
magnitudes slower. The code below demonstrates the problem. It runs fast 
when using just one thread but abysmally slow when two or more threads are 
present, even though the loop doing the work is actually single threaded and 
the other threads remain idle.
Windows Taskmanager shows that execution times is roughly 50% kernel and 50% 
user time whenever you run more than one thread. Invoked with a single 
thread execution time is just spend in user mode.

As far as I know releasing and locking data objects is done by the OS on 
behalf of a programs request and it's done in kernel mode. Are there 
situations where libstdc++ may be confused about idle threads in a program 
and then do unnecessary requests for locking and releasing data objects?

If there is anyone who has a suggestion on what causes these symptoms in my 
GCC port that would be greatly appreciated.

Many thanks,

Rob


#include <iostream>
#include <omp.h>

using namespace std;

const long lmax = 50000;

int main()
{
    int nthreads = 1;
    cout<<"Enter number of OpenMP threads to create: ";
    cin >> nthreads;
    omp_set_num_threads(nthreads);

#pragma omp parallel
    {
#pragma omp single
        cout << "Doing string stuff with "<<omp_get_num_threads()<<"
thread(s)"<<endl;
    }

    time_t start, now;
    time( &start );

    string pairlbl("");

    for (long m = 0; m< lmax; m++)
    {
        if ((m % (lmax/20))==0)
        cout << "m = "<<m<<endl;

        for (int j=1;j<=2000;j++)
        {
            pairlbl.clear();
        }
    }

    time( &now);
    cout<<"\ntime= "<<difftime( now, start )<<" sec\n";

    return 0;
}

 



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