Re: [boost] [boost.chrono] Slow down process (if specifing c++11)

Subject: Re: [boost] [boost.chrono] Slow down process (if specifing c++11)
From: Gaetano Mendola (mendola_at_[hidden])
Date: 2013-08-27 04:07:27

When I wrote with 4.8.1 the time are more or less the same I meant:
all around 8900 ms, so basically no slow down.

I have straced it and I don't see any fork involved.

I have moved the chrono call around in the code and the times are:

============ gcc 4.6.3 ====================
as first operation in the main:  8900 ms
before the gettimeofday:         9500 ms
as last operations in the main: 10000 ms

============ gcc 4.8.1 ====================
as first operation in the main:  8900 ms
before the gettimeofday:         8900 ms
as last operations in the main:  8900 ms

============ clang 3.3 ====================
as first operation in the main:  9400 ms
before the gettimeofday:         9400 ms
as last operations in the main:  9400 ms

I'm not sure if it's worth at this stage to open an issue with GCC
people.

Regards
Gaetano Mendola

On 08/26/2013 04:29 PM, Julien Nitard wrote:
> Hi
> 
> I've seen something similar before. I'd check whether the call
> boost::chrono triggers a fork() in C++11 mode. If it does, Linux will mark
> your pages as copy-on-write and slow down the entire thing.
> 
> To make check whether that's the case move the memory allocation after the
> call to chrono : the pages allocated after fork() won't be affected.
> 
> Regards,
> 
> Julien
> 
> 
> 
> On Mon, Aug 26, 2013 at 9:54 PM, Gaetano Mendola <mendola_at_[hidden]> wrote:
> 
>> Hi have tried the following code compiling it with gcc 4.6.3,
>> with four combinations:
>>
>> 1) -O3
>> 2) -O3 (commeting out the chrono call)
>> 3) -O3 -std=c++0x
>> 4) -O3 -std=c++0x (commenting out the chrono call)
>>
>> and I'm obtaining the following runtime values:
>>
>> 1) 8921 ms
>> 2) 8915 ms
>> 3) 9400 ms
>> 4) 8933 ms
>>
>> as you can see the combination:  "c++11 and chrono call" slows down the
>> entire
>> process. I have tried the same with gcc 4.8.1 and all times are more or
>> less
>> the same.
>>
>> At this point I'm not sure if it's a problem of chrono with gcc 4.6.3 or
>> simply an issue of 4.6.3 disappeared in 4.8.1 series.
>>
>> Regards
>> Gaetano Mendola
>>
>>
>> Here the code I have used:
>>
>> #include <iostream>
>> #include <boost/chrono.hpp>
>> #include <sys/time.h>
>> #include <math.h>
>>
>>
>> int main() {
>>
>>   cpu_set_t  myAffinityMask;
>>   CPU_ZERO( &myAffinityMask );
>>   CPU_SET(0, &myAffinityMask );
>>   sched_setaffinity(0, sizeof(myAffinityMask), &myAffinityMask);
>>
>>   volatile float* myMemoryA = new float[(1<<24)];
>>   volatile float* myMemoryB = new float[(1<<24)];
>>
>>   struct timeval myStart;
>>   struct timeval myStop;
>>   struct timeval myResult;
>>
>>   boost::chrono::time_point<boost::chrono::steady_clock> t1 =
>> boost::chrono::high_resolution_clock::now();
>>
>>   gettimeofday(&myStart, 0);
>>
>>   for (size_t i = 0; i < (1<<24); ++i) {
>>     myMemoryA[i] = i;
>>     myMemoryB[i] = i+1;
>>   }
>>   delete []myMemoryA;
>>   delete []myMemoryB;
>>
>>   for (size_t j = 0; j < 100; ++j) {
>>     volatile float* myMemoryA = new float[(1<<24)];
>>     volatile float* myMemoryB = new float[(1<<24)];
>>     for (size_t i = 0; i < (1<<24); ++i) {
>>       myMemoryA[i] *= sqrtf(myMemoryB[i]);
>>     }
>>     delete []myMemoryA;
>>     delete []myMemoryB;
>>   }
>>   gettimeofday(&myStop, 0);
>>
>>   timersub(&myStop,&myStart,&myResult);
>>
>>   std::cout << "Time: " <<  myResult.tv_sec*1000 + myResult.tv_usec/1000.0
>> << std::endl;
>>   std::cout << "t1: " << t1 << std::endl;
>> }
>>
>>
>>
>>
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