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Using ARM NEON is slower in a simple Addition task


By : Ramon Bernat
Date : September 17 2020, 12:00 AM
I hope this helps . Your test routine is completely flawed to start with:
Since all the inputs are clearly visible to the compiler at build time, the compiler will simply generate machine codes similar to the one below:
code :
A[0] = 3.0f;
A[1] = 5.0f;
A[2] = 7.0f;
A[3] = 9.0f;
void myFunc_c(float *pA, float *pB, uint32_t count)
{
    if (count == 0) return;

    do {
        *pA++ += *pB++;
    } while (--count);
}

void myFunc_neon(float *pA, float *pB, uint32_t count)
{
    float32x4_t a, b;

    count >>= 2;
    if (count == 0) return;

    do {
        a = vld1q_f32(pA);
        b = vld1q_f32(pB);

        a = vaddq_f32(a, b);

        vst1q_f32(pA, a);

        pA += 4;
        pB += 4;

    } while (--count);    
}


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Difference between 'addition' and 'pairwise addition' in Android neon intrinsics?


By : MoonB
Date : March 29 2020, 07:55 AM
I hope this helps . There is good information to be found at ARM's Information Center. The reference is for the assembly instructions, but the names are very similar to the intrinsics. Although if you are going to use NEON, you'll get better performance by just skipping straight to assembly. It's even easier to write than using intrinsics.
To summarize, pairwise addition adds pairs of elements in the same vector, then concatenates the results into a single vector. An illustration (I use 4-element vectors for ease of drawing):
code :
vector 'a'   vector 'b'
+-+-+-+-+    +-+-+-+-+
|0|1|2|3|    |4|5|6|7|
+-+-+-+-+    +-+-+-+-+
 \+/ \+/      \+/ \+/
  1   5        9   13
   \   \      /   /
      +-+-+-+--+
      |1|5|9|13|  result
      +-+-+-+--+
+-+-+-+--+
|4|6|8|10|
+-+-+-+--+

Addition of Integer 2D array elements using multi-threading in java slower than sequential addition


By : Pido Ayala
Date : March 29 2020, 07:55 AM
To fix the issue you can do There is a sizable amount of overhead when establishing threads. That said, if your sample data set is too small, the amount of time spent spinning up and tearing down the threads will be greater than the actual running time performance of your code.
Let's look at it subjectively. You have an array that contains only 200 elements. Your method's runtime is O(nm), where n is the row size, and m is the column size.
code :
The total sum calculated by sequential program is: -570429863
The total time taken by sequential program is: 3369190200
The total sum calculated by multi threaded program is: -570429863
The total time taken by multi threaded program is: 934624554

Why addition using bitwise operators in this code very slower than arithmetic addition


By : Manpreet Singh
Date : March 29 2020, 07:55 AM
hope this fix your issue Integer arithmetic is performed in hardware typically in a very small number of clock cycles.
You will not be able to get close to this performance in software. Your implementation using bitwise operations involves a function call and a loop. The bitwise operations that you perform typically cost similar numbers of clock cycles as arithmetic.

why is this simple C++ addition 6 times slower than the equivalent Java?


By : neha
Date : March 29 2020, 07:55 AM
like below fixes the issue hello stackoverflow users, this is my first question asked, so if there are any errors in my way of expressing it, please point it out, thank you , On Linux/Debian/Sid/x86-64, using OpenJDK 7 with
code :
// file test.java
class Test {
    public static void main(String[] args) {
    long start = System.nanoTime();
    long total = 0;
    for (int i = 0; i < 2147483647; i++) {
        total += i;
    }
    System.out.println(total);
    System.out.println(System.nanoTime() - start);
    }
}   
   // file test.cc
#include <iostream>
#include <chrono>

int main (int argc, char**argv) {
 using namespace std;
 auto start = chrono::high_resolution_clock::now();
 long long total = 0;
 for (int i = 0; i < 2147483647; i++)
   {
     total += i;
   }
 cout << total << endl;
 auto finish = chrono::high_resolution_clock::now();
 cout << chrono::duration_cast<chrono::nanoseconds>(finish - start).count()
      << endl;
}    
javac test.java
java Test
2305843005992468481
774937152
g++ -O2 -std=c++11 test.cc -o test-gcc
2305843005992468481
40291
2305843005992468481
5208949116
    .globl  main
    .type   main, @function
  main:
  .LFB1530:
    .cfi_startproc
    pushq   %rbx    #
    .cfi_def_cfa_offset 16
    .cfi_offset 3, -16
    call    _ZNSt6chrono3_V212system_clock3nowEv    #
    movabsq $2305843005992468481, %rsi  #,
    movl    $_ZSt4cout, %edi    #,
    movq    %rax, %rbx  #, start
    call    _ZNSo9_M_insertIxEERSoT_    #
    movq    %rax, %rdi  # D.35007,
    call    _ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_  #
    call    _ZNSt6chrono3_V212system_clock3nowEv    #
    subq    %rbx, %rax  # start, D.35008
    movl    $_ZSt4cout, %edi    #,
    movq    %rax, %rsi  # D.35008, D.35008
    call    _ZNSo9_M_insertIlEERSoT_    #
    movq    %rax, %rdi  # D.35007,
    call    _ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_  #
    xorl    %eax, %eax  #
    popq    %rbx    #
    .cfi_def_cfa_offset 8
    ret
    .cfi_endproc
  .LFE1530:
            .size   main, .-main

Addition of GROUP BY to simple query makes it 1000 slower


By : user2316025
Date : March 29 2020, 07:55 AM
hope this fix your issue The EXPLAIN for the first query shows that it does a table-scan (type=ALL) of 300K rows from employees, and for each one, does a partial primary key (type=ref) lookup to 1 row (estimated) in salaries.
code :
mysql> explain SELECT * FROM employees 
  INNER JOIN salaries ON employees.emp_no = salaries.emp_no;
+----+-------------+-----------+------+---------------+---------+---------+----------------------------+--------+-------+
| id | select_type | table     | type | possible_keys | key     | key_len | ref                        | rows   | Extra |
+----+-------------+-----------+------+---------------+---------+---------+----------------------------+--------+-------+
|  1 | SIMPLE      | employees | ALL  | PRIMARY       | NULL    | NULL    | NULL                       | 299113 | NULL  |
|  1 | SIMPLE      | salaries  | ref  | PRIMARY       | PRIMARY | 4       | employees.employees.emp_no |      1 | NULL  |
+----+-------------+-----------+------+---------------+---------+---------+----------------------------+--------+-------+
mysql> EXPLAIN SELECT employees.gender, AVG(salary) FROM employees 
  INNER JOIN salaries ON employees.emp_no = salaries.emp_no 
  GROUP BY employees.gender;
+----+-------------+-----------+------+---------------+---------+---------+----------------------------+--------+---------------------------------+
| id | select_type | table     | type | possible_keys | key     | key_len | ref                        | rows   | Extra                           |
+----+-------------+-----------+------+---------------+---------+---------+----------------------------+--------+---------------------------------+
|  1 | SIMPLE      | employees | ALL  | PRIMARY       | NULL    | NULL    | NULL                       | 299113 | Using temporary; Using filesort |
|  1 | SIMPLE      | salaries  | ref  | PRIMARY       | PRIMARY | 4       | employees.employees.emp_no |      1 | NULL                            |
+----+-------------+-----------+------+---------------+---------+---------+----------------------------+--------+---------------------------------+
mysql> alter table employees add index (gender, emp_no);
mysql> EXPLAIN SELECT employees.gender, AVG(salary) FROM employees 
  INNER JOIN salaries ON employees.emp_no = salaries.emp_no 
  GROUP BY employees.gender;
+----+-------------+-----------+-------+----------------+---------+---------+----------------------------+--------+-------------+
| id | select_type | table     | type  | possible_keys  | key     | key_len | ref                        | rows   | Extra       |
+----+-------------+-----------+-------+----------------+---------+---------+----------------------------+--------+-------------+
|  1 | SIMPLE      | employees | index | PRIMARY,gender | gender  | 5       | NULL                       | 299113 | Using index |
|  1 | SIMPLE      | salaries  | ref   | PRIMARY        | PRIMARY | 4       | employees.employees.emp_no |      1 | NULL        |
+----+-------------+-----------+-------+----------------+---------+---------+----------------------------+--------+-------------+
+--------+-------------+
| gender | AVG(salary) |
+--------+-------------+
| M      |  63838.1769 |
| F      |  63769.6032 |
+--------+-------------+
2 rows in set (1.06 sec)
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