Question

Use the following code with your own timing code around the call to delete msg in main(). When running in debug mode, it is taking 473 times as long, on average, as when running without debugging. Does anyone know why this is happening? If so, is there a way that I can get this code to run much faster in debug mode?

Note: I am using Visual Studio 2008 SP 1 on a Windows 7 machine.

// This file is generated by using the Google Protocol Buffers compiler
// to compile a PropMsg.proto file (contents of that file are listed below)
#include "PropMsg.pb.h"

void RawSerializer::serialize(int i_val, PropMsg * o_msg)
{
        o_msg->set_v_int32(i_val);
}
void serialize(std::vector<int> const & i_val, PropMsg * o_msg)
{
    for (std::vector<int>::const_iterator it = i_val.begin(); it != i_val.end(); ++it) {
        PropMsg * objMsg = o_msg->add_v_var_repeated();
        serialize( * it, objMsg);
    }
}

int main()
{
    std::vector<int> testVec(100000);
    PropMsg * msg = new PropMsg;
    serialize(testVec, msg);
    delete msg; // Time this guy
}

PropMsg was created with the following .proto file definition:

option optimize_for = SPEED;
message PropMsg
{
  optional int32 v_int32 = 7;
  repeated PropMsg v_var_repeated = 101;
}

Here’s some sample test output that I got:

datatype: class std::vector<int,class std::allocator<int> >
                               num runs:                   10
                              num items:               100000
        deserializing from PropMsg time:               0.0046
            serializing to PropMsg time:               0.0426
                 reading from disk time:               0.7195
                   writing to disk time:               0.0298
              deallocating PropMsg time:                 8.99

Notice how this is NOT IO-bound.

Answer 1

STL containers in VS Debug are notoriously slow. Game programmers forums are rife with complaints about this. Often people choose alternative implementations. However, from what I’ve read, you can get a performance boost up front by disabling iterator debugging/checking:

#define _HAS_ITERATOR_DEBUGGING 0
#define _SECURE_SCL 0

Other things that can affect debug performance are excessive calls to new and delete. Memory pools can help with that. You haven’t provided details for PropMsg::add_v_var_repeated() or PropMsg::~PropMsg(), so I can’t comment. But I assume there’s a vector or other STL container inside that class?