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;
}
--
John
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