Re: fork on processes with lots of memory

[lwoodman@redhat.com]

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On 01/27/2016 10:09 PM, Hugh Dickins wrote:
> On Tue, 26 Jan 2016, Felix von Leitner wrote:
>>> Dear Linux kernel devs,
>>> I talked to someone who uses large Linux based hardware to run a
>>> process with huge memory requirements (think 4 GB), and he told me that
>>> if they do a fork() syscall on that process, the whole system comes to
>>> standstill. And not just for a second or two. He said they measured a 45
>>> minute (!) delay before the system became responsive again.
>> I'm sorry, I meant 4 TB not 4 GB.
>> I'm not used to working with that kind of memory sizes.
>>
>>> Their working theory is that all the pages need to be marked copy-on-write
>>> in both processes, and if you touch one page, a copy needs to be made,
>>> and than just takes a while if you have a billion pages.
>>> I was wondering if there is any advice for such situations from the
>>> memory management people on this list.
>>> In this case the fork was for an execve afterwards, but I was going to
>>> recommend fork to them for something else that can not be tricked around
>>> with vfork.
>>> Can anyone comment on whether the 45 minute number sounds like it could
>>> be real? When I heard it, I was flabberghasted. But the other person
>>> swore it was real. Can a fork cause this much of a delay? Is there a way
>>> to work around it?
>>> I was going to recommend the fork to create a boundary between the
>>> processes, so that you can recover from memory corruption in one
>>> process. In fact, after the fork I would want to munmap almost all of
>>> the shared pages anyway, but there is no way to tell fork that.
> You might find madvise(addr, length, MADV_DONTFORK) helpful:
> that tells fork not to duplicate the given range in the child.
>
> Hugh

I dont know exactly what program they are running but we test RHEL with 
up to 24TB
of memory and have not seen this problem.  I have mmap()'d 12TB of 
memory into a
parent process private, touched every page then forked a child which 
wrote to every
page thereby incurring tons of ZFOD and COW faults.  It takes a while to 
process the
6 billion faults but the system didnt come to a halt.  The time I do see 
significant pauses
is when we overcommit RAM and swap space and get into an OOMkill storm.

Attached is the program:

>
>>> Thanks,
>>> Felix
>>> PS: Please put me on Cc if you reply, I'm not subscribed to this mailing
>>> list.
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[-- Attachment #2: forkoff.c --]
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#include <stdlib.h>
#include <unistd.h>
#include <sys/mman.h>
#include <errno.h>
#include <stdio.h>
main(int argc,char *argv[])
{
	unsigned long siz, procs, itterations, cow;
	char	*ptr1;
	char	*i;
	int	pid, j, k, status;

	if ((argc <= 1)||(argc >4)) {
		printf("bad args, usage: forkoff <memsize-in-GB> #children #itterations cow:0|1\n");
		exit(-1);
	}
	siz = ((long)atol(argv[1])*1024*1024*1024);
	procs = atol(argv[2]);
	itterations = atol(argv[3]);
	cow = atol(argv[4]);
	printf("mmaping %ld anonymous bytes\n", siz); 
	ptr1 = (char *)mmap((void *)0,siz,PROT_READ|PROT_WRITE,MAP_ANONYMOUS|MAP_PRIVATE,-1,0);
	if (ptr1 == (char *)-1) {
		printf("ptr1 = %lx\n", ptr1);
		perror("");
	}
	if (cow) {
		printf("priming parent for child COW faults\n");
		// This will cause the ZFOD faults in the parent & COW faults in the children.
		for (i=ptr1; i<ptr1+siz-1; i+=4096)
			*i=(char)'i';
	}
	printf("forking %ld processes\n", procs);
	k = procs;
	do{
		pid = fork();
		if (pid == -1) {
			printf("fork failure\n");
			exit(-1);
		} else if (!pid) {
			printf("PID %d touching %d pages\n", getpid(), siz/4096);
			// This will ZFOD fault if the parent didnt otherwise it will COW fault.
			for (j=0; j<itterations; j++) {
				for (i=ptr1; i<ptr1+siz-1; i+=4096) {
						*i=(char)'i';
				}
			}
			printf("All done, exiting\n");
			exit(0);
		}
	
	} while(--k);

	while (procs-- && wait(&status));	
}