Re: [Libmesh-devel] DenseMatrix::zero()

[Kirk, B. (JSC-EG311) <ben...@na...>]

Excellent.  Any objections to including <cstring> and making it std::memset
instead?


On 6/16/11 11:07 AM, "John Peterson" <jwp...@gm...> wrote:

> Hi,
> 
> We were doing some profiling here recently and were surprised when
> DenseMatrix::zero() showed up as 11% in a particular application!
> 
> (You'll have to ask Derek why he was calling DenseMatrix::zero() so
> many times ;-P)
> 
> If you replace std::fill() in this method with a call to memset (which
> is legal C++, from everything I've read, since vector is contiguous
> and bytewise zero'ing POD types gives you actual zero for those types)
> DenseMatrix::zero() goes back down into the noise.
> 
> Some results on my workstation for doubles: memset is roughly 2 orders
> of magnitude faster than std::fill when it is measurable.
> 
> N        Fill method  Memset method
> 100      1e-06s       1e-06s
> 1000     1.2e-05s     1e-06s
> 10000    0.00012s     3e-06s
> 100000   0.001201s    5.2e-05s
> 1000000  0.012151s    0.000625s
> 
> Timings are slightly faster for int/float but the trend is the same.
> 
> We've stuck with the std::fill method for std::complex for now... I
> think in 99% of implementations memset should work there too, but I'm
> not sure if the standard guarantees std::complex has to look
> essentially like
> 
> struct
> {
> T real;
> T imag;
> };
> 
> i.e. still a POD?
> 
> 
> #include <iostream>
> #include <vector>
> #include <sys/time.h> // gettimeofday
> 
> double elapsed(const timeval& tstart, const timeval& tstop)
> {
> return
>   static_cast<double>(tstop.tv_sec  - tstart.tv_sec) +
>   static_cast<double>(tstop.tv_usec - tstart.tv_usec)*1.e-6;
> }
> 
> // Timings for fill vs. memset for zero'ing vectors of POD
> int main()
> {
>   // Timing objects
>   timeval tstart, tstop;
> 
>   // vector size
>   unsigned N=1000000;
>   typedef double pod_t;
> 
>   std::vector<pod_t> v(N);
> 
>   // Fill method
>   gettimeofday (&tstart, NULL);
>   std::fill(v.begin(), v.end(), 0);
>   gettimeofday (&tstop, NULL);
> 
>   std::cout << "Fill method: " << elapsed(tstart, tstop) << "s" << std::endl;
> 
>   // Memset method
>   gettimeofday (&tstart, NULL);
>   memset(&v[0], 0, sizeof(pod_t) * v.size());
>   gettimeofday (&tstop, NULL);
> 
>   std::cout << "Memset method: " << elapsed(tstart, tstop) << "s" <<
> std::endl;
> 
>   return 0;
> }
>