From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755343AbcA1DJZ (ORCPT ); Wed, 27 Jan 2016 22:09:25 -0500 Received: from mail-pf0-f180.google.com ([209.85.192.180]:34028 "EHLO mail-pf0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755300AbcA1DJW (ORCPT ); Wed, 27 Jan 2016 22:09:22 -0500 Date: Wed, 27 Jan 2016 19:09:13 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Felix von Leitner cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: fork on processes with lots of memory In-Reply-To: <20160126162853.GA1836@qarx.de> Message-ID: References: <20160126160641.GA530@qarx.de> <20160126162853.GA1836@qarx.de> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > > > Thanks, > > > Felix > > > PS: Please put me on Cc if you reply, I'm not subscribed to this mailing > > list.