Problem with libstdc++ in GCC 4.4.0 port to interix

[Robert Oeffner]

Hi,

Still trying to solve the problems porting libstdc++ in GCC 4.4.0 to interix 
I notice that when I compile the test code below with 
flags  -fopenmp -D_GLIBCXX_PARALLEL -O3 it works as expected at a decent 
speed. But if I omit the _GLIBCXX_PARALLEL flag then the weird behaviour 
with high CPU kernel times and slow execution occures whenever more than one 
thread is requested in the program.

Are there any libstdc++ gurus out there who knows what these symptoms might 
mean?

On a related note I see that among the source files in 
gcc-4.4.0/libstdc++-v3/config/os ctype_base is implemented for most 
platforms but interix. Does anyone know what is needed to write an interix 
implementation for ctype_base or are there a set of defaults that are being 
used?



Many thanks,

Rob



--------------------------------------------------
From: "Robert Oeffner" <robert@oeffner.net>
Sent: Wednesday, September 23, 2009 10:26 AM
To: <libstdc++@gcc.gnu.org>
Subject: Problem with libstdc++ in GCC 4.4.0 port to interix

> 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.erase() 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. 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 all execution times is just spend in user 
> mode.
>
> If there is anyone here on this list who has a good suggestion that would 
> be much 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;
> }
>
>
>