Why pure computation time in parallel is longer than the serial version?

hi

2014-02-22 19:30 GMT+09:00 Xuening Zhu <puddingnnn529 at gmail.com>:

My cpu is *Intel(R) Core(TM) i5-3210M CPU @ 2.50GHz.* There are 2 physical
cores and additional 2 logical cores. The memory size is 8G. And my

Logical performance of your CPU..

use SIMD 4 FLOPSparClock x 2.5GHz x 2phisicalcore = 20GFLOPS
use AVX   8 FLOPSparClock x 2.5GHz x 2phisicalcore = 40GFLOPS
# Because a physical core is two, the computing unit is two.

amount of the operation of DGEMM is O(N^3).

I choose a 10^3 * 10^4 matrix and wants to evaluate its
multiplication(t(m)%*%m) time. I don't consider tcrossprod() because I just
want to make the computation longer. Maybe more cases can be compared later.

amount of the operation of the procession is O(N^3).
require calculation ... 2*(3e3 * 4e3 * (3e3+4e3)/2) = 84GFLOPS

use SIMD 84/20 = 4.2 sec
use AVX   84/40 = 2.1 sec

so becomes 2.1 seconds in the logical peak performance in your CPU.
Maybe, because the effective efficiency is about range 80% to 90%, it
is likely to become about 2.6 seconds normally.

   user  system elapsed
 10.164   0.512   5.549

maybe this performance of NEHALEM Core.
and hyperthread decreases the efficiency of cache in the procession.

Best Regards,

EI-JI Nakama  <nakama (a) ki.rim.or.jp>
"\u4e2d\u9593\u6804\u6cbb"  <nakama (a) ki.rim.or.jp>