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Date:	Tue, 21 Mar 2017 02:56:14 +0000
From:	Noah Misch <noah AT leadboat DOT com>
To:	cygwin AT cygwin DOT com
Subject:	Re: cygserver - Postgres Multiple connection Load Testing - Inifinte Loop
Message-ID:	<20170321025614.GA2100214@tornado.leadboat.com>
References:	<200408030333 DOT i733XEXn023894 AT mx3 DOT redhat DOT com> <20040803100612 DOT GP31522 AT cygbert DOT vinschen DOT de>
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On Tue, Aug 03, 2004 at 12:06:12PM +0200, Corinna Vinschen wrote:
> On Aug  2 20:33, sarbx-cygwin6344 AT mailblocks DOT com wrote:
> > This time around, cygserver does not eat CPU. But after 5 to 6 
> > concurrent
> > connections nothing seem to work, looks kind of hung. There is no 
> > activity in the Postgres
> > log file. Opening a new database connection also hangs. There is no 
> > activity on the machine.

> Any chance to create a simple testcase which uncovers that behaviour
> without involving a whole database system?

Attached test program reproduces it on Cygwin 2.7.0, Cygwin 1.7.5, and a few
intermediate versions.  The program creates sixteen processes that each
perform a tight loop over the following:

- select one of four semaphores
- reduce semaphore's value from 1 to 0 ("lock" it)
- raise semaphore's value from 0 to 1 ("unlock" it)

On GNU/Linux, AIX, and Solaris, the processes keep busy and finish one million
lock/unlock cycles apiece in a few minutes.  On Cygwin, they hang within a few
seconds and under one hundred cycles apiece.  At that point, cygserver is
unresponsive to other clients; for example, "strace /bin/true", opening a new
Cygwin terminal, "cat /proc/sysvipc/sem" and "cygserver -S" all hang.  In most
tests, cygserver was not consuming CPU while unresponsive.

Thanks,
nm

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/*
 * Demonstrate cygserver hang under concurrent sysv semaphore traffic.  Run
 * without arguments.  Output will cease within a few seconds, and cygserver
 * will be unresponsive to all clients.
 *
 * This is compatible with default cygserver settings; it uses a single
 * semaphore set of four semaphores.
 */

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>

#define SEM_KEY 0x631a2c3e
#define N_WORKER 16
#define N_SEMA (N_WORKER/4)
#define N_CYCLE 1000000

union semun
{
    int val;
    struct semid_ds *buf;
    unsigned short *array;
};

static int print_every = 1;


/* In parallel, N_WORKER processes run this function. */
static int do_worker(int ordinal, int set)
{
    int i;
    struct sembuf op;

    printf("start worker %d\n", ordinal);
    fflush(stdout);

    op.sem_flg = 0;
    for (i = 1; i <= N_CYCLE; i++)
    {
	op.sem_num = random() % N_SEMA;

	op.sem_op = -1;
	if (0 > semop(set, &op, 1))
	{
	    perror("semop");
	    return 1;
	}

	op.sem_op = 1;
	if (0 > semop(set, &op, 1))
	{
	    perror("semop");
	    return 1;
	}

	if (i % print_every == 0)
	{
	    printf("worker %d: %d cycles elapsed\n", ordinal, i);
	    fflush(stdout);
	}
    }

    return 0;
}

int main(int argc, char **argv)
{
    int status = 1, set, i, child_status;

    if (argc == 2)
	print_every = atoi(argv[1]);
    else if (argc != 1)
    {
	fprintf(stderr, "Usage: sema_parallel [print-every-N]\n");
	return status;
    }

    puts("semget");
    fflush(stdout);
    set = semget(SEM_KEY, N_SEMA, IPC_CREAT | 0600);
    if (set == -1)
    {
	perror("semget");
	return status;
    }

    puts("SETVAL");
    fflush(stdout);
    for (i = 0; i < N_SEMA; i++)
    {
	union semun s;
	s.val = 1;

	if (0 > semctl(set, i, SETVAL, s))
	{
	    perror("semctl(SETVAL)");
	    goto cleanup;
	}
    }

    for (i = 0; i < N_WORKER; i++)
    {
	pid_t pid;
	pid = fork();
	switch (pid)
	{
	    case -1:
		perror("fork");
		goto cleanup;
	    case 0:
		return do_worker(i, set);
	}
    }

    status = 0;

cleanup:
    while (wait(&child_status) != -1)
	;
    if (errno != ECHILD)
    {
	perror("wait");
	status = 1;
    }

    if (0 > semctl(set, 0, IPC_RMID))
    {
	perror("semtctl(IPC_RMID)");
	status = 1;
    }

    return status;
}


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