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Date: Tue, 26 Feb 2013 15:35:40 +0900 (JST)
From: MITSUNARI Shigeo <herumi AT nifty DOT com>
To: cygwin AT cygwin DOT com
Subject: wrong performance of malloc/free under multi-threading
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Hi.

I found that the performance of malloc/free is wrong under multi-threading.
The following test program reproduces the problem.

The program repeats malloc and free under multi-thread.
I measured the timing on Cygwin and Linux.

timing(sec)|      threadNum
-----------+----------+-------------
           | 1        |     2
-----------+----------+-------------
Linux      | 1.45     |     0.69
-----------+----------+-------------
Cygwin     | 2.059    |    53.165
-----------+----------+-------------

The timing under Linux seems good scale but it is very wrong under Cygwin.
Is it intentional behavior or do I use pthread in bad way?

env : Core i7-2600 + Windows 7 Ultimate SP1(64bit) + 8GiB memory

%gcc malloc-free-pthread.c -lpthread -Wall -Wextra -ansi -pedantic -O2 -m32

// results for Linux
% time ./a.out 1
threadNum=1, n=120000
begin=0, end=120000
end
1.432u 0.016s 0:01.45 99.3%     0+0k 0+0io 0pf+0w

% time ./a.out 2
threadNum=2, n=120000
begin=0, end=60000
begin=60000, end=120000
end
1.384u 0.000s 0:00.69 200.0%    0+0k 0+0io 0pf+0w

// results for Cygwin
// I stopped Anti-virus software under measuring.

$ time ./a.exe 1
threadNum=1, n=120000
begin=0, end=120000
end

real    0m2.059s
user    0m2.059s
sys     0m0.000s

$ time ./a.exe 2
threadNum=2, n=120000
begin=0, end=60000
begin=60000, end=120000
end

real    0m53.165s
user    0m11.949s
sys     0m19.812s

// Linux
% uname -a
Linux i3 3.5.0-17-generic #28-Ubuntu SMP Tue Oct 9 19:31:23 UTC 2012 x86_64 x8
6_64 x86_64 GNU/Linux
% gcc -v
4.7

// Cygwin
% uname -mrsv
CYGWIN_NT-6.1-WOW64 1.7.17(0.262/5/3) 2012-10-19 14:39 i686
% gcc -dumpversion
4.5.3

// test code
% cat malloc-free-pthread.c
#include <stdio.h>
#include <stdlib.h>
#include <memory.h>
#include <pthread.h>

void task(int idx)
{
    const int size = 100;
    int i;
    for (i = 1; i < size; i++) {
        char *p = (char*)malloc(i);
        memset(p, idx, i); // ensure to call malloc/free under optimization
        free(p);
    }
}

typedef struct {
    int begin;
    int end;
} Range;

void* run(void *arg)
{
    const Range *range = (const Range*)arg;
    int begin = range->begin;
    int end = range->end;
    printf("begin=%d, end=%d\n", begin, end);
    while (begin != end) {
        task(begin);
        begin++;
    }
    return 0;
}

#define MAX_THREAD_NUM 4

int main(int argc, char *argv[])
{
    const int threadNum = argc == 1 ? 1 : atoi(argv[1]);
    const int n = 1 * 2 * 3 * 4 * 5000;
    if (threadNum < 0 || threadNum > MAX_THREAD_NUM) {
        printf("threadNum = 0, 1, 2, 3, 4\n");
        return 1;
    }
    printf("threadNum=%d, n=%d\n", threadNum, n);
    if (threadNum == 0) {
        Range range;
        puts("no thread\n");
        range.begin = 0;
        range.end = n;
        run(&range);
    } else {
        const int dn = n / threadNum;
        Range range[MAX_THREAD_NUM];
        pthread_t pt[MAX_THREAD_NUM];
        int i;
        for (i = 0; i < threadNum; i++) {
            range[i].begin = i * dn;
            range[i].end = (i + 1) * dn;
            if (pthread_create(&pt[i], NULL, run, &range[i]) != 0) {
                printf("ERR create %d\n", i);
                return 1;
            }
        }
        for (i = 0; i < threadNum; i++) {
            if (pthread_join(pt[i], NULL) != 0) {
                printf("ERR join %d\n", i);
                return 1;
            }
        }
    }
    puts("end");
    return 0;
}
---
Yours,
 MITSUNARI Shigeo


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