Re: [boost] [Block Pointer] benchmark

Subject: Re: [boost] [Block Pointer] benchmark
From: Frank Mori Hess (frank.hess_at_[hidden])
Date: 2011-05-26 19:44:36

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On Wednesday, May 25, 2011, Phil Bouchard wrote:
> On 5/25/2011 1:39 PM, Frank Mori Hess wrote:
> > What version of boost are you referring to?
> 
> boost_1_46_1
> 
> > make_shared used to be slow due
> > to it doing a lot of unnecessary copying of the storage area for the
> > pointee, but that should have been fixed probably a couple years back
> > now. Copying a shared_ptr is fairly slow due to the atomic reference
> > counting, but I would expect a compiler to be able to elide the copy
> > of the make_shared return value.
> 
> That optimization doesn't exist and it's worse with the Intel Compiler:

Well, I was thinking of the case where you are initializing a newly declared 
shared_ptr as opposed to re-assigning the same shared_ptr over and over 
again.  Running a trimmed down and tweaked version of your benchmark 
(attached) I get:

$ g++ -O3 -Wall  pb_benchmark.cpp -lrt
fhess_at_tailpipe:~/test$ ./a.out
new:
shared_ptr:                 17500558 ns
make:
shared_ptr:                 12208956 ns

Note, I moved the declaration of the shared_ptrs inside the loops.  Also, 
your benchmark on my machine shows a strong dependence on the order the 
tests are run.  The first run is slower, which is why the "new" result is 
before the "make" result above, to demonstrate the effect.

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/**
        @file
        benchmark.cpp

        @note
        Copyright (c) 2011 Phil Bouchard <phil_at_[hidden]>.

        Distributed under the Boost Software License, Version 1.0.

        See accompanying file LICENSE_1_0.txt or copy at
        http://www.boost.org/LICENSE_1_0.txt

        See http://www.boost.org/libs/smart_ptr/doc/index.html for documentation.
*/

#include <sys/time.h>

#include <memory>
#include <iostream>
#include <boost/shared_ptr.hpp>
#include <boost/make_shared.hpp>
#include <ctime>
#include <iomanip>
#include <limits>

using namespace std;
using namespace boost;

   
timespec diff(timespec start, timespec end);

int main(int argc, char* argv[])  
{  
        timespec ts[2];
        
        cout << "new:" << endl;

        clock_gettime(CLOCK_PROCESS_CPUTIME_ID, & ts[0]); 
        for (int i = 0; i < 100000; ++ i)
                shared_ptr<int> p(new int());
        clock_gettime(CLOCK_PROCESS_CPUTIME_ID, & ts[1]);
        cout << "shared_ptr:\t" << setw(numeric_limits<long>::digits10 + 2) << diff(ts[0], ts[1]).tv_nsec << " ns" << endl;

        cout << "make:" << endl;

        clock_gettime(CLOCK_PROCESS_CPUTIME_ID, & ts[0]); 
        for (int i = 0; i < 100000; ++ i)
                shared_ptr<int> p = make_shared<int>();
        clock_gettime(CLOCK_PROCESS_CPUTIME_ID, & ts[1]);
        cout << "shared_ptr:\t" << setw(numeric_limits<long>::digits10 + 2) << diff(ts[0], ts[1]).tv_nsec << " ns" << endl;
        
        
    return 0;  
}  

timespec diff(timespec start, timespec end)
{
        timespec temp;
        if ((end.tv_nsec-start.tv_nsec)<0) {
                temp.tv_sec = end.tv_sec-start.tv_sec-1;
                temp.tv_nsec = 1000000000+end.tv_nsec-start.tv_nsec;
        } else {
                temp.tv_sec = end.tv_sec-start.tv_sec;
                temp.tv_nsec = end.tv_nsec-start.tv_nsec;
        }
        return temp;
}

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