fork on processes with lots of memory
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.
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.
Thanks,
Felix
PS: Please put me on Cc if you reply, I'm not subscribed to this mailing list.
Quoted 7 lines
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.
Quoted 27 lines
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.
Thanks,
Felix
PS: Please put me on Cc if you reply, I'm not subscribed to this mailing list.
+ linux-mm
Quoted 39 lines · Felix von Leitner
Quoted 7 lines
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.Quoted 27 lines
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.
Thanks,
Felix
PS: Please put me on Cc if you reply, I'm not subscribed to this mailing list.
Signature · 4 lines
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.Quoted 10 lines · Felix von Leitner
Quoted 7 lines
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.
Make sure you have >>4TB physical if you're going to fork from a process with a 4TB virtual address space. (I'm assuming it's not sparse, but all actually being used.)
Disable transparent hugepages (THP). The internal book-keeping mechanisms have been known to run amok with large RAM sizes causing severe performance issues. Maybe 4.x kernels are better, I haven't checked.
If you're using explicit hugepages and these kinds of RAM sizes, don't bother with RHEL 6 or 7 kernels -- they're broken. Vanilla 4.x kernels work.
We're also in the TB range, though not quite 4TB, and fork()ing from inside such processes definitely works for us. We do disable THP since it kills us otherwise.
Quoted 28 lines
Quoted 28 lines
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.
Thanks,
Felix
PS: Please put me on Cc if you reply, I'm not subscribed to this mailing list.
Quoted 31 lines · Felix von Leitner
Quoted 7 lines
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.Quoted 20 lines
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
Quoted 7 lines
Quoted 7 lines
Thanks,
Felix
PS: Please put me on Cc if you reply, I'm not subscribed to this mailing list.
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Quoted 26 lines · Hugh Dickins
Quoted 6 lines · Felix von Leitner
Quoted 6 lines
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 afork()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.Quoted 19 lines
I'm sorry, I meant 4 TB not 4 GB.
I'm not used to working with that kind of memory sizes.Quoted 16 lines
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.
Quoted 4 lines
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:
Quoted 10 lines
Quoted 5 lines
Quoted 5 lines
Thanks,
Felix
PS: Please put me on Cc if you reply, I'm not subscribed to this mailing list.--
To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to [email protected]. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"[email protected]"> [email protected] </a>
#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));
}