Re: AIX and Interix also do early PID recycling.

[Steven W. Orr]

On 7/25/2012 9:20 AM, Michael Haubenwallner wrote:
> On 07/25/2012 09:59 AM, Michael Haubenwallner wrote:
> > On 07/25/2012 03:05 AM, Chet Ramey wrote:
> > > Bash holds on to the status of all terminated processes, not just
> > > background ones, and only checks for the presence of a newly-forked PID
> > > in that list if the list size exceeds CHILD_MAX.
>
> > The AIX 6.1 I've debugged on has:
> >    #define CHILD_MAX 128
>
> > I'm going to run this build job with 'truss -t kfork' again, to eventually find
> > some too small count of different PIDs before PID-recycling by the kernel...
>
> Tracing shows:
>
> The minimum fork count (including grand-childs to any depth) before PID 
> recycling starts
> looks like 255 (once), but usually 256 and more.
>
> However, one process does see a PID recycled after *at least* 128 forks,
> that is exactly the value of CHILD_MAX.
>
> First thought is of some off-by-one bug, but reducing js.c_childmax in jobs.c 
> (2 times)
> by one doesn't help.
>
> Investigating further... any hints what to look out for?
>
> /haubi/


One thing that just springs to mind: Under AIX, I believe (but don't take this 
as gospel) that PIDs are not strictly ascending. The low byte may increment, 
but it might be another byte in the pid_t that gets the increment, and the 
byte that increments may alternate. I don't know if anyone else does this, but 
I seem to remember that AIX does (or did).

You might write a little C program that does a loop of 1000 fork and wait 
calls. Just print out the pids that you create. Something like this...

#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>

main()
{
    pid_t pid;
    int status;
    int ii;

    for (ii = 0; ii < 1000; ii++)
    {
        if ((pid = fork()) != 0)
        {
            printf("New child:%d\n", pid);
            waitpid(pid, &status, 0);
        }
        else
            exit(0);
    }
}

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